**RHIC Multi-Year Beam Use Request**

**For Run 9 -- Run 13**

##### The STAR Collaboration {#the-star-collaboration .unnumbered}

*April 21, 2008*

![](media/image1.jpeg){width="5.31875in" height="5.258333333333334in"}

![](media/image2.png){width="5.411111111111111in"
height="1.1215277777777777in"}

1\. Executive summary [3](#executive-summary)

2\. Report on data taking in Run 8 [8](#report-on-data-taking-in-run-8)

3\. Polarized proton results and proposed running for Run 9
[12](#polarized-proton-results-and-proposed-running-for-run-9)

3.1. Report on data taking in Run 6
[12](#report-on-data-taking-in-run-6)

3.2. Recent spin physics results [13](#recent-spin-physics-results)

3.3. Ongoing studies [21](#ongoing-studies)

3.4. Polarized proton request in Runs 9 and 10
[28](#polarized-proton-request-in-runs-9-and-10)

3.5. Physics with tagged forward protons with STAR
[39](#physics-with-tagged-forward-protons-with-star)

4\. RHIC beam energy scan and search for QCD critical point
[44](#__RefHeading___Toc70426003)

4.1. Physics motivation [44](#physics-motivation)

4.2. Beam use request for Run 10 [50](#beam-use-request-for-run-10)

5\. Heavy ion collisions at 200 GeV
[53](#heavy-ion-collisions-at-200-gev)

5.1. Triggered probes and heavy flavor with heavy ions
[53](#triggered-probes-and-heavy-flavor-with-heavy-ions)

5.2. Triggered probes and heavy flavor beam use request
[62](#__RefHeading___Toc70426008)

5.3. Non-triggered datasets at 200 GeV: Recent heavy ion physics results
[64](#non-triggered-datasets-at-200-gev-recent-heavy-ion-physics-results)

5.4. Future EM probes in STAR [73](#__RefHeading___Toc70426010)

5.5. U+U collisions at RHIC [74](#__RefHeading___Toc70426011)

5.6. Non-triggered probes at 200 GeV: Au+Au beam use request for Runs 9
and 10
[75](#non-triggered-probes-at-200-gev-auau-beam-use-request-for-runs-9-and-10)

5.7. Non-triggered probes: p+p beam use request for Runs 9 and 10
[77](#non-triggered-probes-pp-beam-use-request-for-runs-9-and-10)

**RHIC Multi-Year Beam Use Request**

**For Runs 9 -- 13**

##### The STAR Collaboration {#the-star-collaboration-1 .unnumbered}

April 21, 2008

# Executive summary

####### The STAR Collaboration, in order to achieve its spin and relativistic heavy ion physics goals on a timescale consistent with intense international interest and competition in these areas, as well as to utilize RHIC beams effectively, taking full advantage of planned improvements in machine and detector capability as a function of time, makes the following five year beam use proposal: {#the-star-collaboration-in-order-to-achieve-its-spin-and-relativistic-heavy-ion-physics-goals-on-a-timescale-consistent-with-intense-international-interest-and-competition-in-these-areas-as-well-as-to-utilize-rhic-beams-effectively-taking-full-advantage-of-planned-improvements-in-machine-and-detector-capability-as-a-function-of-time-makes-the-following-five-year-beam-use-proposal .unnumbered}

Table 1: 25 cryo-weeks FY09 run, 18 weeks production with two species.

+------+---------------+--------------+------------+------------------+
| **R  | **Energy**    | **System**   | **Time**   | **Goal**         |
| un** | **(GeV)**     |              |            |                  |
+------+---------------+--------------+------------+------------------+
| 9^   | s = 200       | p p          | > 12       | 50 pb^-1^ P^4^L  |
| (1)^ |               |              | >          | 6.5 pb^-1^       |
|      |               |              | weeks^(a)^ |                  |
+------+---------------+--------------+------------+------------------+
|      | s = 500       | p p          | > 2        | Commissioning    |
|      |               |              | >          |                  |
|      |               |              | weeks^(b)^ |                  |
+------+---------------+--------------+------------+------------------+
|      | s = 200       | p p          | ½ week     | pp2pp            |
+------+---------------+--------------+------------+------------------+
|      | s~NN~ = 200   | Au + Au      | > 3 weeks  | 0.3B minbias,    |
|      |               |              |            | 0.5 nb^-1^       |
+------+---------------+--------------+------------+------------------+
|      | s~NN~ = 5     | Au + Au      | > ½        | Commisioning     |
|      |               |              | >          |                  |
|      |               |              |  week^(c)^ |                  |
+------+---------------+--------------+------------+------------------+
| 10^  | s~NN~ = 39 -- | Au + Au      | 14 weeks   | 1^st^ energy     |
| (2)^ | 6.1           |              |            | scan             |
+------+---------------+--------------+------------+------------------+
|      | s~NN~ = 5     | Au + Au      | 1          | Commisioning     |
|      |               |              | week^(d)^  |                  |
+------+---------------+--------------+------------+------------------+
|      | s~NN~ = 200   | Au + Au      | 2 weeks    | 200M central     |
+------+---------------+--------------+------------+------------------+
|      | s~NN~ = 200   | Au + Au      | 1          | 50M central      |
|      |               |              | week^(e)^  |                  |
+------+---------------+--------------+------------+------------------+
|      | s = 200       | p p          | ½ week     | pp2pp            |
+------+---------------+--------------+------------+------------------+
|      | s = 500 or    | p p or p p   | 4 ½ weeks  | Spin studies     |
|      | 200           |              |            |                  |
+------+---------------+--------------+------------+------------------+
| 11^  | s = 200       | p p or p p   | 6 weeks    | 20-30 pb^-1^     |
| (2)^ |               |              |            |                  |
+------+---------------+--------------+------------+------------------+
|      | s = 500       | p p or p p   | 15 weeks   | 150 pb^-1^       |
+------+---------------+--------------+------------+------------------+
|      | s~NN~ = 200   | U + U        | 2          | Commissioning    |
|      |               |              | weeks^(f)^ |                  |
+------+---------------+--------------+------------+------------------+
| 12^  | s~NN~ = 200   | Au + Au      | 12 weeks   | 0.5B minbias, 5  |
| (3)^ |               |              |            | nb^-1^           |
+------+---------------+--------------+------------+------------------+
|      | s~NN~ = 39 -- | Au + Au      | 13 weeks   | 2^nd^ energy     |
|      | 5             |              |            | scan             |
+------+---------------+--------------+------------+------------------+
| 13^  | s = 200       | p p or p p   | 13 weeks   | 2B minbias, 100  |
| (3)^ |               |              |            | pb^-1^           |
+------+---------------+--------------+------------+------------------+
|      | s = 500       | p p or p p   | 12 weeks   | 300 pb^-1^       |
+------+---------------+--------------+------------+------------------+

\(1\) 25 cryo-weeks, 18 weeks production with two species

\(2\) 30 cryo weeks, 23 weeks production with two species.

\(3\) 30 cryo weeks, 25 weeks production with one species.

\(a\) 60% or higher polarization at both yellow and blue rings is
needed. Spin flipper development desirable.

\(b\) Contingent on C-AD achieving efficiencies in accelerator and beam
set-up.

\(c\) C-AD low energy test, contingent on achieving goals with
transverse stochastic cooling.

\(d\) C-AD test for higher luminosity at the lowest energy.

\(e\) photon converter HBT test program.

\(f\) HFT fingers prototyping.

Table 2: 16 cryo-weeks, FY09 run, 11 weeks for production with one
species.

+----+---------------+--------------+--------------+-----------------+
| *  | **Energy      | **System**   | **Time**     | **Goal**        |
| *R | (GeV)**       |              |              |                 |
| un |               |              |              |                 |
| ** |               |              |              |                 |
+----+---------------+--------------+--------------+-----------------+
| 9^ | s = 200       | p p          | > 11         | 50 pb^-1^,      |
| (1 |               |              | > weeks^(a)^ | P^4^L 6.5       |
| )^ |               |              |              | pb^-1^          |
+----+---------------+--------------+--------------+-----------------+
|    | s = 200       | pp           | ½ week^(b)^  | pp2pp           |
+----+---------------+--------------+--------------+-----------------+
| 1  | s~NN~ = 39 -- | Au + Au      | 12 weeks     | 1^st^ energy    |
| 0^ | 6.1           |              |              | scan            |
| (2 |               |              |              |                 |
| )^ |               |              |              |                 |
+----+---------------+--------------+--------------+-----------------+
|    | s~NN~ = 5     | Au + Au      | 1 week^(c)^  | Commissioning   |
+----+---------------+--------------+--------------+-----------------+
|    | s~NN~ = 200   | Au + Au      | 3 ½ weeks    | 0.5B events,    |
|    |               |              |              | 0.5 nb^-1^      |
+----+---------------+--------------+--------------+-----------------+
|    | s~NN~ = 200   | Au + Au      | 1 week^(d)^  | 50M central     |
+----+---------------+--------------+--------------+-----------------+
|    | s = 500       | p p or p p   | 5 weeks^(e)^ | Commissioning   |
+----+---------------+--------------+--------------+-----------------+
|    | s = 200       | p p or p p   | ½ week       | pp2pp           |
+----+---------------+--------------+--------------+-----------------+
| 1  | s = 200       | p p or p p   | 6 weeks      | 20-30 pb^-1^    |
| 1^ |               |              |              |                 |
| (2 |               |              |              |                 |
| )^ |               |              |              |                 |
+----+---------------+--------------+--------------+-----------------+
|    | s = 500       | p p or p p   | 15 weeks     | 150 pb^-1^      |
+----+---------------+--------------+--------------+-----------------+
|    | s~NN~ = 200   | U + U        | 2 weeks^(f)^ | Commissioning   |
+----+---------------+--------------+--------------+-----------------+
| 1  | s~NN~ = 200   | Au + Au      | 12 weeks     | 0.5B minbias, 5 |
| 2^ |               |              |              | nb^-1^          |
| (3 |               |              |              |                 |
| )^ |               |              |              |                 |
+----+---------------+--------------+--------------+-----------------+
|    | s~NN~ = 39 -  | Au + Au      | 13 weeks     | 2^nd^ energy    |
|    | 5             |              |              | scan            |
+----+---------------+--------------+--------------+-----------------+
| 1  | s = 200       | p p or p p   | 13 weeks     | 2B minbias, 100 |
| 3^ |               |              |              | pb^-1^          |
| (3 |               |              |              |                 |
| )^ |               |              |              |                 |
+----+---------------+--------------+--------------+-----------------+
|    | s = 500       | p p or p p   | 12 weeks     | 300 pb^-1^      |
+----+---------------+--------------+--------------+-----------------+

\(1\) 16 cryo weeks, 11 weeks production with one species.

\(2\) 30 cryo weeks, 23 weeks production with two species.

\(3\) 30 cryo weeks, 25 weeks production with one species.

\(a\) 60% or higher polarization at both yellow and blue rings is
needed. Spin flipper development desirable.

\(b\) Contingent on achieving goals with longitudinal running

\(c\) C-AD test for higher luminosity at the lowest energy

\(d\) photon converter HBT test program

\(e\) Contingent on C-AD achieving efficiencies in accelerator and beam
set-up

\(f\) HFT fingers prototyping

+------------+------------------+--------------------------------------+
| *          | **Completion**   | **Key physics measurements**         |
| *Upgrade** |                  |                                      |
+------------+------------------+--------------------------------------+
| FMS        | Fall 2007        | (a)Transverse asymmetry at forward   |
|            |                  | rapidity                             |
|            | Ready for Run 08 |                                      |
|            |                  | \(b\) CGC                            |
+------------+------------------+--------------------------------------+
| TPC DAQ    | Summer 2008      | Large data set, minimal dead time    |
|            |                  |                                      |
| (DAQ1000)  | Ready for Run 09 |                                      |
+------------+------------------+--------------------------------------+
| MRPC TOF   | Summer 2009      | Full PID in full azimuthal           |
|            |                  | acceptance                           |
|            | Full TOF ready   |                                      |
|            | for Run 10       |                                      |
+------------+------------------+--------------------------------------+
| FGT        | Summer 2010      | Forward W for flavor separated quark |
|            |                  | polarization                         |
|            | Ready for Run 11 |                                      |
+------------+------------------+--------------------------------------+
| HFT        | Summer 2011      | \(a\) Precision hadronic ID for      |
|            |                  | Charm and Bottom hadrons             |
|            | Ready for Run 12 |                                      |
|            |                  | \(b\) Charm and Bottom hadron energy |
|            |                  | loss and flow                        |
+------------+------------------+--------------------------------------+

Table 3: Timeline for STAR major upgrade and key physics measurements.

In this proposed physics-driven plan, the STAR Collaboration intends to
make the most efficient use of RHIC beam time and upgrades in order to
make timely progress in determining the properties of the new state of
matter produced at RHIC, to map the x dependence of the gluon
polarization in the proton, G(x), and to perform a definitive search for
the phase boundary and the existence and location, if it exists, of the
QCD critical point.

The primary goals of the proposed program are:

**[Run 9#]{.underline}**: 25 cryo-weeks, 18 weeks for physics production
with two species:

a)  Initial map at s = 200 GeV of the x dependence of gluon polarization
    in the proton, G(x)

b)  C-AD commissioning of s = 500 GeV pp collisions to study the
    environment at the the new energy with polarized beams

c)  Qualitative advance in the study of transversely polarized pp
    elastic scattering

d)  C-AD development of transverse stochastic cooling for 200 GeV Au+Au
    collisions

**[Run 9##]{.underline}**: 16 cryo-weeks, 11 weeks for physics
production with one species::

a)  Initial map at s = 200 GeV of the x dependence of gluon polarization
    in the proton, G(x)

**[Run 10#]{.underline}**: 30 cryo-weeks, 23 weeks for physics
production:

a)  Search for the existence and location of the QCD Critical point.
    First energy scan from s~NN~ = 39 to 6.1 GeV Au+Au collisions,
    combined with C-AD test to increase luminosity for s~NN~ = 5 GeV
    Au+Au collisions

b)  200 GeV Au+Au low material run with full Time-of-Flight

c)  First measurement of photon HBT with 10% absorberr

d)  Complete the pp elastic scattering program with longitudinally
    polarized collisions

e)  Further spin studies, guided by progress achieved in Run 9

**[Run 10##]{.underline}**: 30 cryo-weeks, 23 weeks for physics
production:

a)  Search for the existence and location of the QCD Critical point.
    First energy scan from s~NN~ = 39 to 6.1 GeV Au+Au collisions,
    combined with C-AD test to increase luminosity for s~NN~ = 5 GeV
    Au+Au collisions

b)  C-AD development of transverse stochastic cooling for 200 GeV Au+Au
    collisions, combined with low material run with full Time-of-Flight

c)  First measurement of photon HBT with 10% absorber

d)  Qualitative advance in the study of polarized pp elastic scattering
    with pp2pp program

e)  C-AD commissioning of s = 500 GeV pp collisions to study the
    environment at the new energy with polarized beams

**[Run 11]{.underline}**: 30 cryo-weeks, 23 weeks for physics
production:

a)  Complete the map at s = 200 GeV of the x dependence of gluon
    polarization in the proton, G(x); starts on any transverse spin
    milestones.

b)  First measurement of flavor dependence of sea quark / anti-quark
    polarization in the proton at s = 500 GeV

c)  HFT prototyping in s~NN~ = 200 GeV U+U collisions

**[Run 12]{.underline}**: 30 cryo-weeks, 25 weeks for physics
production:

a)  First HFT physics measurements of charm hadron v~2~(p~T~) and
    R~CP~(p~T~) in s~NN~ = 200 GeV Au+Au collisions

b)  Second energy-scan measurements in the search for the QCD critical
    point. Accelerator development is crucial for the measurement at
    s~NN~ = 5 GeV

**[Run 13]{.underline}**: 30 cryo-weeks, 25 weeks for physics
production:

a)  HFT physics reference measurement of charm hadron spectra in s = 200
    GeV pp collisions; complete any remaining sqrt(s) = 200 GeV spin
    milestones.

b)  Measurement of the x dependence of gluon polarization and W
    production at s = 500 GeV

#: Corresponding to a relatively long run in FY09: 25 cryo-week

##: Corresponding to a short run in FY09: 16 cryo-week

In the period from Run 9 to 10, our top priorities are:

\(1\) Measurements using longitudinally polarized pp collisions at s =
200 GeV, allowing a significant map of the *x*-dependence of the gluon
polarization from approximately 0.03 \< *x* \< 0.3. Smaller values of
*x* will subsequently be probed in 500 GeV running. Contingent on C-AD
achieving efficiencies in accelerator and beam set-up, we propose
commissioning time dedicated to preparing for a future s = 500 GeV
polarized proton run.

\(2\) Energy Scan in Au+Au collisions starting at s~NN~ = 39 -- 6.1\*
GeV. (\* The lowest collision energy at this period is determined by the
collision rates. We plan to scan lower energies later, when luminosities
are improved significantly.) This energy region corresponds to a range
in baryon chemical potential from 130 - 400 MeV and freeze-out
temperature from 170 -- 120 MeV in the QCD phase diagram.

\(3\) Development of transverse stochastic cooling for Au beams at
s~NN~=200 GeV, which is critical for the future of the RHIC program. We
propose to test it in Run 9 if possible. However, if the run is only 16
weeks long, the overhead involved is too large to permit two species. In
that case, the transverse stochastic cooling test should occur during
Run 10.

The DAQ1000 upgrade to the TPC readout electronics will be complete in
Run 9, and the full barrel of the MRPC Time-of-Flight upgrade will be
complete in Run 10. Coupled with DAQ1000, the possibility of on-line
tracking is under consideration in the collaboration.

In the period from Run 11 to 13, the highest priorities will be to:

\(1\) Complete the minimum goal of 50 pb^-1^ sampled in longitudinal
spin running to map the x-dependence of G at s = 200 GeV no later than
Run 11;

\(2\) Continue the energy scan program with the beam energy down to
s~NN~ = 5 GeV, the most crucial step in the searching for the QCD
critical point;

\(3\) Make significant measurements of the flavor-identified q and q-bar
contributions to the spin of the proton via the longitudinal-spin
asymmetry of W production at s = 500 GeV, using the FGT upgrade;

\(4\) Start the physics program with heavy flavor hadrons, enabled by
the HFT upgrade, and the program with the high luminosities of RHIC II,
enabled by stochastic cooling in Au+Au collisions at 200 GeV and
sufficient reference data in p+p at 200 GeV;

\(5\) Search for evidence of repulsive color-charge interactions in the
Sivers effect in correlations between forward photons and mid-rapidity
jets.

In this period, both the upgrades FGT, for W measurements, and the HFT,
for Charm and Bottom hadrons, will be completed

# Report on data taking in Run 8

A primary scientific goal for STAR during Run 8 was to determine whether
or not gluon saturation is the origin of the particle suppression that
has been observed in d+Au in the forward region. The primary spin goal
during the pp part of the run stated in the Run 8 STAR Beam Use Request
was to obtain the first significant measurement of the *x* dependence of
the gluon polarization in the proton. The severely curtailed duration of
Run 8 made it impossible to accomplish this spin goal. Therefore, STAR
chose to pursue one of the other goals described in the Run 8 Beam Use
Request, extending the *x~F~* and *p~T~* range of the transverse
single-spin asymmetries that we have observed in forward π^0^ production
and embarking on direct photon transverse-spin asymmetry measurements in
the forward direction. Other goals that STAR hoped to accomplish
included performing non-photonic electron measurements in both p+p and
d+Au systems with very little material in the center of the STAR
detector and measuring Upsilon and *D*-meson yields in d+Au collisions.
Finally, STAR hoped to record a few thousand good events during the
brief low-energy Au+Au test run.

The data acquisition targets for Run 8 are shown in Table 4. As seen in
the Table, the goals for the d+Au run were all met, as was that for the
Au+Au low-energy test. STAR also did well on the luminosity-based goals
for the p+p run, but fell well short of the transverse polarization
figure-of-merit goal.

Table 4: Data acquisition goals for Run 8.

+--------------+------------------+----------------+------------------+
| **System**   | **Trigger**      | **Goal**       | **Acquired**     |
+--------------+------------------+----------------+------------------+
| d+Au         | FMS              | Original: 60   | 49 nb^-1^        |
|              |                  | nb^-1^         |                  |
|              |                  |                |                  |
|              |                  | Reduced: 30    |                  |
|              |                  | nb^-1^         |                  |
+--------------+------------------+----------------+------------------+
| d+Au         | BEMC High Tower  | 30 nb^-1^      | 36 nb^-1^        |
+--------------+------------------+----------------+------------------+
| d+Au         | Minimum bias     | 30 M usable    | 46 M usable      |
|              |                  | events         | events           |
+--------------+------------------+----------------+------------------+
| p+p          | FMS integrated   | 9 pb^-1^       | 7.8 pb^-1^       |
|              | luminosity       |                |                  |
+--------------+------------------+----------------+------------------+
| p+p          | FMS integrated   | 3.8 pb^-1^     | 1.6 pb^-1^       |
|              | figure-of-merit  |                |                  |
|              | (*P^2^L*)        |                |                  |
+--------------+------------------+----------------+------------------+
| p+p          | BEMC High Tower  | 4.5 pb^-1^     | 3.1 pb^-1^       |
+--------------+------------------+----------------+------------------+
| Low-energy   | Minimum bias     | Few thousand   | Few thousand     |
| test         |                  | good events    | good events      |
|              |                  |                |                  |
| Au+Au        |                  |                |                  |
+--------------+------------------+----------------+------------------+

STAR approached the original goal that we set for integrated sampled
luminosity with the FMS during the d+Au run and exceeded the reduced
goal that was established when it was decided that the d+Au run would
end \~10 days sooner than had been expected. These data will be analyzed
in conjunction with the FMS data that were recorded under identical
conditions during the p+p run to provide an in-depth search for forward
mono-jets in d+Au, as predicted in the Color Glass Condensate model.

STAR exceeded the goals that we set for d+Au BEMC high-tower triggered
events. They will be used to perform a measurement of the non-photonic
electron spectrum in d+Au with much reduced background from conversion
electrons when compared to Run 3. The high-tower triggered events will
also provide a measurement of the Upsilon yield in d+Au with statistics
that are comparable to the measurements STAR has performed in p+p and
Au+Au. In p+p, STAR sampled 3.1 pb^-1^ with our BEMC high-tower triggers
or 69% of our goal. This overall slow detector luminosity goal was
missed due to a priority decision to record data in p+p with
significantly lower prescale factors at any given *p~T~* than were used
in d+Au enhancing those specific triggers. Hence, the event yields that
were recorded during the d+Au and p+p runs are comparable. The d+Au and
p+p data sets should help to resolve the current difference between STAR
and PHENIX regarding the magnitude of the open charm cross section.

STAR exceeded the goal that we set for usable minimum bias events in
d+Au. They will be used to measure the *D*-meson spectrum, thereby
providing an important reference for the measurements that are currently
underway in Cu+Cu (Run 5) and Au+Au (Run 7). The same events will
provide reference data for a broad range of correlation measurements
that are underway in heavy-ion collisions.

The only major shortfall during Run 8 was that the integrated transverse
figure-of-merit for the FMS was only 1.6 pb^-1^ or 43% of our goal. This
number has been calculated using the blue beam polarization values as
reported by the CNI polarimeter, scaled down by a factor of 1.14 to
account for the calibration of the CNI asymmetries that was reported to
the RHIC Spin Collaboration by the polarized jet group. The blue beam
polarization is relevant for FMS measurements. These data will permit a
significant extension of the *x~F~* and *p~T~* range of the STAR π^0^
forward single-spin asymmetries, but that extension will fall well short
of what we had intended to achieve. The data will also permit studies of
direct photon detection with the FMS, but the low figure-of-merit means
that direct photon transverse-spin asymmetry measurements probably will
not be practical. The integrated figure-of-merit would have been even
smaller if calculated using the yellow beam polarizations, as
appropriate for measurements with the Forward Pion Detector on the east
end of STAR.

The Forward Meson Spectrometer played a central role in STAR's data
acquisition goals during Run 8. The FMS is a new Pb-glass
electromagnetic calorimeter that spans the pseudorapidity interval 2.5
\< \< 4.0 and the full azimuth. Consequently, it is sensitive to a broad
mass range of particles that decay to γγ or *e*^+^*e*^--^ and are
produced at large Feynman *x* in ion collisions at RHIC. Large *x*~F~
physics at RHIC energies remains a frontier, with little known. Earlier
RHIC runs have clearly established striking transverse spin effects, and
particle suppression in heavy ion collisions that may signal the onset
of gluon saturation. There is intrinsic interest in large-*x*~F~ J/
production. Clear understanding of the large pair mass region is also a
requirement of a future measurement of transverse single spin
asymmetries for the Drell Yan production of dilepton pairs.

The preliminary results of FMS are shown in Figure 2-1. The analysis is
applied to events where the total energy deposited in the FMS is
Etot\>20 GeV and 20 \< *p*z,tot \< 25 GeV/c. The ^0^ peak is being used
to calibrate the energy scales of individual FMS cells. The tail of the
pair mass distribution extends to large mass. Peak positions, widths and
signal-to-background ratios will be strongly influenced by the ongoing
calibration efforts.

  --------------------------------------------------- ------------------------------------
  ![](media/image3.png){width="2.745138888888889in"   Figure 2‑1: Clustering and
  height="2.8604166666666666in"}                      shower-shape analysis that were
                                                      developed for the forward peon
                                                      detector have been adapted and
                                                      applied to Run 8 data from the
                                                      Forward Meson Spectrometer. (Inset)
                                                      Mass distribution from all pairs
                                                      after clustering and shower shape
                                                      analysis

  --------------------------------------------------- ------------------------------------

The last two days of Run 8 were devoted to a low-energy Au+Au test at
√*s~NN~* = 9 GeV. Previously, it had been determined that STAR recorded
essentially no real collisions during the short Au+Au low-energy test in
Run 7. The goal for this test run was to observe real collisions to
establish the RHIC luminosity scaling at low √*s~NN~* and to collect a
few thousand good events. Several different triggers were tried.
Preliminary analysis indicates that we succeeded in recording at least a
few thousand good events, but the precise number is not yet known.

In addition to our data acquisition goals, STAR was very successful
during Run 8 in commissioning new detectors for the future. One entire
sector of the TPC (out of 24) was instrumented with DAQ1000 electronics
prior to Run 8. They were commissioned early in the run, then used in
normal data-taking through the last 6 weeks of the d+Au run and all of
p+p. During p+p, the DAQ1000 sector was read out at a typical rate of
250 Hz. A special test demonstrated operation at 1000 Hz with only 5-7%
dead time, meeting specifications. Preliminary analysis of the data also
indicates that DAQ1000 is achieving good *dE*/*dx*-based particle
identification, as illustrated in Figure 2-2 and Figure 2-3. The entire
DAQ1000 system will be installed before Run 9.

Five Time-of-Flight trays, instrumented with the final TOF electronics,
were installed outside the DAQ1000 sector of the TPC. The TOF system was
commissioned during the d+Au run. It was then included in the normal
data stream during the p+p run. A total of 78 M minimum bias p+p events
were recorded with TOF+DAQ1000. Figure 2-3 shows a preliminary analysis
of a small fraction of the data. Very good π-*K*-*p* separation is seen,
which implies that both the DAQ1000 tracking and the TOF time
measurements are performing as expected. At least half of the TOF trays
will be installed before Run 9, and the entire system will be in place
for Run 10.

+-----------------------------------+----------------------------------+
| ![](media/image4.                 | ![](media/image5.p               |
| png){width="3.0861111111111112in" | ng){width="3.0791666666666666in" |
| height="2.2243055555555555in"}    | height="2.126388888888889in"}    |
|                                   |                                  |
| Figure 2‑2: Preliminary results   | Figure 2‑3: Preliminary results  |
| for TPC dE/dx vs. momentum        | for particle identification with |
| measured with the DAQ1000 sector  | TOF combined with DAQ1000        |
| during the p+p run.               | tracking. The solid lines show   |
|                                   | the expected locations for the   |
|                                   | pion, kaon, and proton bands.    |
+-----------------------------------+----------------------------------+

The Collider-Accelerator Division installed the pp2pp Roman pot systems
east and west downstream of STAR prior to Run 8. Over the course of Run
8, pp2pp was integrated into the STAR trigger and data acquisition
systems. The Roman pots were then inserted into the beam pipes and used
to trigger STAR during the last 2 hours of the p+p run. The trigger
rates in pp2pp were as expected, and no impact was seen on the
background levels in the STAR mid-rapidity detectors.

# Polarized proton results and proposed running for Run 9

## Report on data taking in Run 6

STAR sampled a total luminosity of approximately 8.5 pb^-1^ with
longitudinal beam polarization and approximately 3.4 pb^-1^ with slow
detectors and 6.8pb^-1^ with fast detectors with transverse beam
polarization at s = 200 GeV during Run 6. In addition, at the end of Run
6, STAR collected a total of 0.1 pb^-1^ at s = 62 GeV using transversely
polarized beams. The longitudinal data sample was split between an
initial sample of 2.1 pb^-1^ during the beginning of Run 6 and 6.4
pb^-1^ at the end of the 200 GeV data taking period. These two samples
were separated by data taken with transversely polarized beams.
Beginning with the start of the transverse running period at s = 200
GeV, the average beam polarization in both RHIC beams was approximately
60%. The longitudinal data sample taken during Run 6 was the last data
sample taken up to this point at longitudinal beam polarization. The
original beam use request for Run 8 asked for a longitudinal data sample
taken over 12 weeks with an anticipated total recorded luminosity of
approximately 30pb^-1^. Due to budget shortcomings, this data sample was
not taken.

During Run 6, significant improvements in RHIC's capability as a
polarized proton-proton collider as well as in STAR detector acceptance
and trigger capability resulted in a dramatic change in the quality,
magnitude, and richness of the spin data acquired. The simultaneous
improvements in integrated luminosity and polarization in 2006 yielded
at least a factor of 4 gain in the figure of merit $\text{LP}^{4}$
compared to 2005 for the measurement of any double spin asymmetry while
allowing for significant running time with longitudinal polarization.

The result of these advances in the quality of the beam delivered and
STAR's ability to mine the collisions was a Run 6 data set that far
surpassed all previous polarized proton runs in terms of physics
content. The combination of extended calorimetric coverage, reduction of
the background trigger rate, and more intelligent triggering decisions
increased the size the recorded di-jet sample by almost an
order-of-magnitude per unit integrated luminosity compared to 2005. From
these data, quantitative estimates of the magnitudes of the gluon and
quark Sivers functions in several kinematic regions have been extracted
and published. These data were compared to existing results from
semi-inclusive DIS

[^1]^,^[^2] and lead to an important understanding of the initial and
final state interactions necessary to generate a Siver's affect. Data
taken with the FPD have allowed STAR to map out the *x~F~* and *p~T~*
dependences of the large neutral pion spin asymmetries observed earlier
in STAR at high rapidity, recently submitted for publication, and to
search for jets and direct photons in this regime. The longitudinally
polarized runs have extended the existing STAR results on *A~LL~* for
inclusive jets and neutral pions to higher *p~T~*, and, due to
improvements in triggering and acceptance, they have provided the first
substantial sample of di-jets. In particular *A~LL~* preliminary results
for inclusive jets have been released and already included in a new
global analysis demonstrating important new constraints on G. In
addition the Run 6 data is crucial to the development of algorithms for
identifying direct photons in the EMC's. Further progress on the Run 6
published results, preliminary results and ongoing analyses will be
discussed in the next sections.

## Recent spin physics results

### STAR transverse spin program 

The study of transverse spin effects, both theoretically and
experimentally, has received a great deal of attention in recent years.
The ultimate goal of this effort is to extract transversity distribution
functions and determine possible orbital angular momentum effects.

The first measurement of a transverse single-spin asymmetry, A~N~, for
forward neutral pion production at 3.3 \< \< 4.1 by the STAR
collaboration[^3] was found to increase with x~F~ and to be similar in
magnitude to the measurement of A~N~ performed by the E704 experiment at
$\sqrt{s}$=20 GeV[^4]. The STAR collaboration has performed
cross-section measurements for forward neutral pion production at =3.3,
3.8 and 4.0, which are in good agreement with next-to-leading order
(NLO) perturbative QCD calculations[^5]. This provides an important
basis for the interpretation of the sizable measured asymmetries at
forward rapidity. Several models beyond a conventional collinear
perturbative QCD approach are able to account for the measured
asymmetries. The essential idea is an extension to pQCD that introduces
transverse momentum dependence (TMD) correlated with spin degrees of
freedom. These models are based on three main correlation concepts: A
correlation of the parton intrinsic transverse momentum k~T~ and the
proton spin in the initial state ('Sivers effect')[^6], a correlation
between the quark spin and the hadron k~T~ in the final state ('Collins
effect')[^7] and a higher twist correlation in the initial and final
state[^8]^,^[^9]. Spin-correlated TMD distribution functions ('Sivers
effect'), in conjunction with initial- and final-state color-charge
interactions, can explain large A~N~ effects. These functions describe
parton orbital motion within the proton, and are therefore important to
gain further insight into the spin structure of the proton. The STAR
collaboration performed an upgrade of the Forward Pion Detector (FPD)
for Run 6 as a prototype for the Forward Meson Spectrometer (FMS). The
main goal is to disentangle different mechanisms responsible for the
observed large transverse single-spin asymmetries in the forward
direction.

New results for forward neutral pion production at higher precision are
shown in Figure 3-4, which have recently been submitted for publication
to Physical Review Letters[^10]. These results are in good agreement
with previous results, but have higher precision. The measured asymmetry
A~N~ is found to be consistent with zero at negative x~F~ and grows from
0 at x \~ 0.2-0.3 up to 0.1 at x~F~ \~ 0.6. The figure also shows the
results of two calculations at different values[^11],[^12]. The measured
asymmetries are precise enough to allow for a quantitative comparison
with theory predictions. The measured x~F~ dependence in A~N~ is found
to be in fair agreement with both a collinear twist-3 calculation and a
calculation assuming factorization that attributes the spin effects to
spin-correlated intrinsic transverse momentum of the quarks within the
proton. The extended kinematic region in x~F~ and p~T~ and the increase
in the available data sample from Run 6 allow a mapping of A~N~ in x~F~
and p~T~ for the first time. Figure 3-4 (right) shows A~N~ as a function
of p~T~ in fine bins of x~F~. The current measured results for A~N~ do
not support a decreasing behavior of A~N~ with p~T~ in all x~F~ bins as
expected by various theoretical models, in particular those shown in the
figure. Future measurements capable of disentangling TMD fragmentation
and distribution function contributions to neutral pion spin effects,
and measurements of A~N~ for real and virtual photon production
sensitive to only Sivers contributions, are required to definitely
establish if partonic motion is the correct explanation of these
effects.

![](media/image6.png){width="2.6166666666666667in"
height="2.1145833333333335in"}
![](media/image7.png){width="2.327777777777778in"
height="2.0277777777777777in"}

Figure 3‑4: (Left) AN as a function of x~F~ for p+p →π^0^+X from Run 6.
The error bars represent statistical errors only. Systematic
uncertainties are indicated by the bands at the bottom of the plots. The
solid blue line represents the result of a twist-3 calculationError:
Reference source not found; the dashed red line is the result of a
calculation based on the Sivers effectError: Reference source not found.
(Right) A~N~ as a function of p~T~ for p+p→^0^+X from Run 6 in bins of
x~F~ in comparison to a calculation based on the Sivers effectError:
Reference source not found (dashed red line) and twist-3
calculationError: Reference source not found (solid blue line).

The role of orbital angular momentum as represented by Siver's functions
was also investigated and published as a Phys. Rev. Letter[^13] in the
past year. One possible manifestation of orbital angular momentum
effects could be realized through a non-zero Sivers type correlation of
the parton intrinsic transverse momentum k~T~ and the proton spin in the
initial state. The HERMES collaboration recently reported a non-zero
Sivers function in semi-inclusive DIS for^+^ production[^14]. The Sivers
function for ^-^ production has been found to be consistent with zero.
This led to the interpretation that the Sivers functions are opposite in
sign and different in magnitude for u quarks compared to d quarks. The
study of Sivers type correlations in polarized proton-proton collisions
has been considered in ref.[^15]. The theoretical expectation is that
the Sivers effect would be reflected in a spin-dependent side-ways boost
observed as a corresponding spin-dependent asymmetry in the back-to-back
di-jet opening angle. A measurement of the correlation between the spin
direction and di-jet bi-sector direction was performed to search for
this effect. A dedicated Level 2 di-jet trigger, utilizing the STAR
Barrel and Endcap Electromagnetic Calorimeters (BEMC and EEMC), was
implemented during Run 6 to provide a sufficient sample of di-jet events
for the study.

![](media/image8.png){width="4.302777777777778in"
height="3.6326388888888888in"}

Figure 3‑5: Transverse single-spin asymmetries for AN+z (left) and AN-z
(right) as a function of ~1~+~2~ for di-jet production at
$\sqrt{s} = \text{200}$ GeV from Run 6 in comparison to theoretical
calculations^2,16^ . (a,b): Fraction of the calculated di-jet cross
section with a quark (gluon) from the +z (-z) beam. (c,d): Unweighted
asymmetries compared with pQCD calculations for two models of quark
Sivers functions fitted to SDIS results. (e,f): Asymmetries for weighted
yields, compared with calculations based on twist-3 quark-gluon
correlations.

Figure 3-5 shows the measured transverse single-spin asymmetry, A~N~,
for di-jet production as functionError: Reference source not found of
~1~+~2~. The region of large ~1~+~2~ values emphasizes quark (gluon)
Sivers effects for +z (--z). In both cases, the measured transverse
single-spin asymmetries are found to be consistent with zero. The
measured results are compared to predictions based on u and d quark
Sivers functions extracted from measured HERMES semi-inclusive DIS data,
which assume a zero gluon-type Sivers functionError: Reference source
not found^,Error:\ Reference\ source\ not\ found^. pQCD calculations can
reconcile these results with sizeable SSA observed for forward hadron
production in pp and for semi-inclusive deep inelastic scattering via
canceling contributions from u and d quarks and from initial- and
final-state interactions. These data constrain unified theoretical
accounts for transverse SSA in hard pQCD processes, and their connection
to parton orbital momentum.

STAR longitudinal spin program

The longitudinal STAR spin physics program profits enormously from the
unique capabilities of the STAR experiment for large acceptance jet
production, identified hadron production and photon production[^16]. The
measurement of the gluon polarization through inclusive measurements
such as jet production and ^0^ production has so far been the prime
focus of the physics analysis program of the Runs 3, 4, 5, and 6 data
samples. The sensitivity of these inclusive measurements to the
underlying gluon polarization in high-energy polarized proton-proton
collisions has been discussed in detail[^17]. Inclusive hadron
production and jet production are strongly affected by the relative
contributions from quark-quark, quark-gluon and gluon-gluon
subprocesses. The low p~T~ region is dominated by gluon-gluon
scattering, while at high p~T~ the quark-gluon contribution starts to
become important. As a result, the sign of A~LL~ in this high p~T~
region indicates the sign of the gluon polarization. The fact that the
inclusive photon channel is dominated by quark-gluon scattering results
in a strong sensitivity to the underlying gluon polarization, despite
the small production cross-section. Throughout the following discussion,
four gluon polarization scenarios have been used as input to NLO
perturbative QCD calculations of A~LL~. The GRSV standard case refers to
the best global analysis fit to polarized DIS data[^18]. The case for a
vanishing gluon polarization (GRSV-ZERO) and the case of a maximally
positive (GRSV-MAX) or negative (GRSV-MIN) gluon polarization have been
also considered. In addition, the global analysis result by the group of
Gehrmann and Sterling (GS) has been used, specifically the set C of
polarized parton distribution functions. In this set, the polarized
gluon distribution function has a node in the measured kinematic region
being negative at large x and positive at low x values, which results
overall in a larger first moment value compared to the GRSV-STD
scenario.

The first longitudinal double-spin asymmetry measurement for inclusive
jet production and the associated inclusive jet cross-section
measurement at mid-rapidity has been published[^19] based on data taken
during Runs 3 and 4. The measured asymmetries are consistent with NLO
perturbative QCD calculations based on DIS polarized lepton-nucleon
data, and disfavor a large positive value of the gluon polarization in
the proton. In addition, the STAR collaboration has recently submitted
the Run 5 longitudinal double-spin asymmetry measurement to Physical
Review Letters[^20]. The recently released Run 6 measurement of the
longitudinal double-spin asymmetry provides a substantial improvement
over previous measurements and clearly rules out extreme gluon
polarization scenarios, suggesting that the gluon polarization in the
measured kinematic region is rather small. Furthermore, an update of the
preliminary result on neutral pion production based on Run 6 data has
recently been released. The (unpolarized) cross-section measurements of
inclusive jet and neutral pion production support applicability of
perturbative QCD.

The following measurements of A~LL~ from Run 5 at at $\sqrt{s}$=200GeV
for identified hadron production and^,^nd inclusive jet productionError:
Reference source not found are based on an average beam polarization of
approximately 50% and a data sample of approximately 3 pb^-1^. The
overall normalization uncertainty due to conservative error estimates on
the preliminary polarization values amounts to \~40%. All A~LL~ analyses
presented below make use of the STAR BEMC and EEMC system at the trigger
and reconstruction level. A high-tower (HT1 / HT2) trigger is based on
an energy threshold for a single tower (x = 0.05 x 0.5) whereas a
jet-patch (JP1 / JP2) trigger is based on an energy threshold for a
group of towers over a region in and of x = 1.0x 1.0. Both triggers are
taken in coincidence with a minimum-bias condition using the STAR
Beam-Beam Counter.

  --------------------------------------------------- --------------------------------
  ![](media/image9.png){width="3.001388888888889in"   Figure 3‑6: Longitudinal
  height="2.0652777777777778in"}                      double-spin asymmetry A~LL~ for
                                                      neutral pion production at
                                                      $\sqrt{s} = \text{200}$GeV as a
                                                      function of p~T~ for Runs 5 and
                                                      6 in comparison to several gluon
                                                      polarization scenarios.

  --------------------------------------------------- --------------------------------

6 shows the measured longitudinal double-spin asymmetry A~LL~ for
neutral pion production as a function of p~T~ together with different
gluon polarization scenarios as described above. The NLO calculations of
A~LL~ make use of the recently released set of fragmentation functions
based on a global analysis of ee, ep and pp data sets[^21]. The error
bars include statistical uncertainties only. The systematic error band
includes contributions from the neutral pion yield extraction and
background subtraction, possible non-longitudinal spin contributions and
the relative luminosity uncertainty. The Run 5 data sample is based on a
restricted pseudo-rapidity region of the STAR BEMC. The data from Run 6
includes the full acceptance of the STAR BEMC. This analysis allowed an
extension of the p~T~ region towards larger values. The 2006 results
exclude a large positive gluon polarization scenario.

In addition to the first neutral pion analysis of A~LL~ at mid-rapidity,
the STAR collaboration has presented preliminary results for neutral
pion production using the STAR EEMC in the pseudo-rapidity acceptance
region of 1.086 \< \<2.0 from Run 5[^22] and Run 6[^23]. The data are
consistent with zero over the p~T~ range of 3 \< p~T~ \< 9 GeV/c. The
systematic uncertainties during Run 5 were comparable to the statistical
uncertainties and dominated by beam induced background. These background
contributions were observed to be suppressed by a factor 20 during Run
6, profiting from the installation of shielding to reduce beam induced
background between the runs. This resulted in an improved measurement
for A~LL~ in the STAR EEMC acceptance region based on data taken during
Run 6, which is shown in Figure 3-7. The EEMC based analysis provides an
important baseline measurement for future prompt photon measurements in
the STAR EEMC acceptance region.

The STAR collaboration has also presented the first measurement of the
longitudinal double-spin asymmetry A~LL~ for inclusive charged pion
production from Run 5. The asymmetries are calculated over the
transverse momentum region 2 \< p~T~ \< 10 GeV/c and compared to several
gluon polarization scenarios as described earlier. This analysis is
unique in that the difference A~LL~() - A~LL~(^+^) tracks the sign of
the gluon polarization, due to the opposite signs of the polarized
distribution functions for up and down quarks. The STAR TPC offers
robust reconstruction and identification of charged pions over the
transverse momentum range 2 \< p~T~ \< 10 GeV/c. Particle identification
in the TPC is accomplished using measurements of ionization energy loss
of TPC hits. Figure 3-8 shows preliminary results during Run 5 for
charged pion production. The measured asymmetries are compared to
theoretical predictions for A~LL~ based on different gluon polarization
scenarios. The fragmentation functions for ^+^ and ^-^ are based on the
KKP fragmentation functions[^24]. This first measurement of A~LL~ for
charged pion production disfavors a large gluon polarization scenario.
Several systematic checks have been performed. The leading systematic
uncertainty is due to the bias introduced by the trigger used for this
analysis. This trigger is based on a jet patch trigger, which introduces
a bias towards jets with a large fraction of neutral energy. The impact
of this trigger on the charge pion asymmetry analysis has been estimated
using a MC sample. This uncertainty amounts to approximately 5 x 10^-3^,
which is comparable to the statistical uncertainty of the first p~T~
bin. In addition, asymmetries were calculated for trigger-jets in
comparison to away-side jets. Both asymmetries have been found to be
consistent within statistical uncertainties.

![](media/image10.png){width="3.9097222222222223in"
height="2.651388888888889in"}

Figure 3‑7: Double spin asymmetry for inclusive neutral pions in the
EEMC region

(1 \< \<2) from Run 6.

A first measurement of the longitudinal spin transfer D~LL~ in inclusive
( → p^-^) and ( → p^+^) production in polarized proton-proton collisions
at a center-of-mass energy of 200 GeV has also been carried out by STAR
using data collected in Run-5. The data have 5-10% precision and cover
transverse momenta (p~T~) in the range of 1 to 4 GeV/c with a
longitudinal momentum fraction of x~F~ = 7.5 x 10^-3^. At improved
precision and larger p~T~ the measurement of D~LL~ for inclusive and
![](media/image12.pct){width="0.16666666666666666in"
height="0.16666666666666666in"} production is sensitive to strange
(anti) quark polarization[^25] and polarized fragmentation
functions[^26].

![](media/image13.pct){width="5.829861111111111in"
height="2.1770833333333335in"}

Figure 3‑8: Longitudinal double-spin asymmetry ALL for charged pion
production at $\sqrt{s} = \text{200}$GeV as a function of pT (-1 \< η
\< 1) for Run 5 in comparison to several gluon polarization scenarios.

The STAR result for Run 5 of the measurement of the longitudinal
double-spin asymmetry A~LL~ for inclusive jet productionError: Reference
source not found is shown in Figure 3-9 as a function of p~T~ for 5 \<
p~T~ \< 30 GeV/c in comparison to several gluon polarization scenarios
as described earlier. Jets are reconstructed using a midpoint con
clustering algorithm using a cone radius of 0.4. This algorithm is fed
in the case of STAR by reconstructed electromagnetic clusters in the
STAR BEMC and tracks from the STAR TPC. Various systematic effects have
been studied. The dominant contributions to the systematic uncertainties
result from trigger bias and jet reconstruction bias, relative
luminosity monitoring and backgrounds. The 2005 A~LL~ inclusive jet
measurement is found to be in good agreement with the previous 2003/2004
measurements. The current measurement extends the p~T~ region to larger
values, where the quark-gluon contribution starts to become important.
This analysis rules out most GRSV fits with G much larger than the GRSV
std. solution. The most recent A~LL~ result for inclusive jet production
is shown in Figure 3-10 which provides the most precise measurement to
date to constrain the gluon polarization of the proton at RHIC. This
analysis is based on a luminosity of 4.7pb^-1^ and an average
polarization of approximately 60% which resulted together with the full
STAR BEMC acceptance in a factor of 3-4 improvement in statistical
precision for p~T~ \> 13GeV/c compared to the Run 5 jet result.

Taking all current PHENIX and STAR A~LL~ measurements together in
comparison to different NLO perturbative QCD predictions for A~LL~
yields a consistent picture which clearly rules out extreme gluon
polarization scenarios and suggests that the gluon polarization on
average in the measured kinematic region is rather small.

The first global analysis, incorporating polarized semi-inclusive DIS,
polarized inclusive DIS and polarized proton-proton, data has recently
been released[^27]. Figure 3-11 shows a comparison of the STAR A~LL~
Runs 5 and 6 jet results in comparison to a calculation of A~LL~ using
the extracted polarized gluon distribution function including the jet
results from STAR. Good agreement is found between those measurements
and the calculated A~LL~ value. The yellow band indicates a 2% variation
of the ^2^ and provides a reasonable measure of the experimental
uncertainties. Figure 3-12 shows the extracted gluon polarization as a
function of x at Q^2^ =1 0GeV^2^. The extracted gluon polarization is
found to be rather small in the measured kinematic region with a node at
an x value of approximately 0.1. The impact of the STAR jet result is
shown in the bottom plot of Figure 3-12 showing the ^2^ variation as a
function of the truncated first moment. The STAR jet results provided a
substantial impact constraining the gluon polarization of the proton.

+-----------------------------------+----------------------------------+
| ![](media/image14                 | ![](media/image15.               |
| .png){width="2.957638888888889in" | png){width="2.967361111111111in" |
| height="2.0409722222222224in"}    | height="2.01875in"}              |
|                                   |                                  |
| Figure 3‑9: Longitudinal          | Figure 3‑10: Longitudinal        |
| double-spin asymmetry A~LL~ for   | double-spin asymmetry A~LL~ for  |
| inclusive jet production          | inclusive jet production at      |
| $\sqrt{s} = \text{200}$GeV as a   | $\sqrt{s} = \text{200}$GeV as a  |
| function of p~T~ (0.2 \< \< 0.8)  | function of p~T~ (-0.7 \< \<     |
| for Run 5 in comparison to        | 0.9) for Run 6 in comparison to  |
| several gluon polarization        | several gluon polarization       |
| scenarios. The error bars are     | scenarios.                       |
| statistical. The gray bands show  |                                  |
| the systematic uncertainties in   |                                  |
| the measured A~LL~ and jet ~pT~.  |                                  |
+-----------------------------------+----------------------------------+
|                                   |                                  |
+-----------------------------------+----------------------------------+

+------------------------------------+---------------------------------+
| ![](media/image16                  | Figure 3‑11: Longitudinal       |
| .png){width="2.8152777777777778in" | double-spin asymmetry ALL for   |
| height="2.691666666666667in"}      | inclusive jet production *at*   |
|                                    | $\sqrt{s} = \text{200}$GeV from |
|                                    | STAR based on data taken during |
|                                    | Run 5 and Run 6 in comparison   |
|                                    | to a calculation of A~LL~ using |
|                                    | a polarized gluon distribution  |
|                                    | extracted from a global         |
|                                    | analysisError: Reference source |
|                                    | not found which includes the    |
|                                    | STAR jet result.                |
|                                    |                                 |
|                                    | *Error: Reference source not    |
|                                    | found*                          |
+------------------------------------+---------------------------------+

![](media/image17.png){width="1.9965277777777777in"
height="1.8520833333333333in"}![](media/image18.png){width="3.7680555555555557in"
height="2.0868055555555554in"}

Figure 3‑12: Gluon polarization as a function of x for Q2=10GeV2 for
three different global analyses. The solid line indicates the recent
global fit result by DSSVError: Reference source not found including the
STAR jet data (left). ^2^ variation as a function of the truncated first
moment for the total ^2^ and the individual experiment contributions
indicating a strong impact of the STAR jet result to constrain the
polarized gluon distribution function (right).

## Ongoing studies

As the luminosity of RHIC grows, the already large STAR detector
acceptance will allow measurements of jet-jet, hadron-hadron and
photon-jet correlations. At leading order (LO) the hard scattering
sub-process kinematics can be calculated on an event-by-event basis.
This of course then leads to addressing the need to constrain the
x-dependence of the polarized gluon distribution function. Such LO
extractions must be followed up with a full global analysis
incorporating correlation measurements beyond inclusive measurements,
which has been the prime focus of the STAR longitudinal spin program so
far. The theoretical framework to take advantage of those correlation
measurements in a global analysis does exist. The reconstruction of
di-jets and photon final states will play a critical role. The following
section provides a brief overview of ongoing efforts towards a first
di-jet and photon result from STAR.

### Di-Jet analysis 

Employing two-body kinematics of the underlying hard sub-process in
di-jet production, the invariant mass *M* and rapidity *y* can be
related to the x-values of both partons in the initial state:
$M = \sqrt{x_{1}x_{2}s}$ and *y* = (~3~+~4~)/2 =
$(1/2)\text{ln}\left( x_{1}/x_{2} \right)$.

Figure 3-13 shows a comparison of data taken during Run 5 and a PYTHIA
MC simulation for a di-jet event sample showing distributions of the
invariant mass *M*, the rapidity *y*, and the cosine of the partonic
center-of-mass angle ^\*^. Good agreement is found between data and MC
in all three cases. The overall normalization is performed with respect
to the invariant mass distribution, i.e. with only one normalization
factor for all distributions shown. The Run 5 data sample is restricted
to the pseudo-rapidity range of 0.2\<\<0.8. The full STAR BEMC together
with the STAR EEMC and the STAR FMS will allow a substantial extension
of the overall kinematic coverage and points to a key strength of STAR
to cover a wide kinematic region, selecting both symmetric partonic
collisions at mid-rapidity at larger x values as well as asymmetric
collisions to extend the kinematic region towards lower x values
including in particular the STAR EEMC and STAR FMS in combination with
the STAR BEMC coverage. The current di-jet analysis of Runs 5 and 6 is
proceeding well.

![](media/image19.pct){width="5.428472222222222in"
height="3.464583333333333in"}

Figure 3‑13: Comparison of data taken during Run 5 and a PYTHIA MC
simulation for a di-jet event sample.

Photon analysis

The crucial experimental issue for the -jet program is to cleanly
identify the direct photons in the presence of a much higher flux of
neutral pions, and to do so with good efficiency. As can be seen in
Figure 3-14. The yield of inclusive neutral pions is a factor of \~10
larger than that of photons at p~T~ =10 GeV and gets larger as one goes
lower.

  ----------------------------------------------------- ------------------------------
  ![](media/image20.png){width="3.1243055555555554in"   Figure 3‑14: Inclusive γ/π^0^
  height="2.4743055555555555in"}                        ratio as a function of p~T~.

  ----------------------------------------------------- ------------------------------

Initial simulations[^28] have shown that the signal to background could
be reduced to better than 1:1 by the combination of photon isolation
cuts and selection on the shower shape in the shower maximum detector
(SMD) for -jet coincident events. Such an analysis also identifies a
background sample for subtraction. Over the past year, among the primary
efforts of the photon reconstruction group has been repeating these and
similar studies in the full Monte Carlo detector model of STAR and
analyzing the sample data sets taken in the 2006 run. These studies hope
to verify the early simulations and provide yield information from the
data. In addition we are exploring the use of other observables in the
calorimeter, e.g., pre and post-shower detectors, to see whether the
analysis might be pushed to lower p~T~ given the expected integrated
luminosity. Ultimately a composite analysis involving all observables
and algorithms incorporated into linear discriminate or higher order
analysis will likely be used and initial investigations along these
lines have been developed.

Full STAR simulations have been carried out to explore the SMD shower
shape cuts. The purpose is to try and select the single shower of a
direct photon from the double shower of a ^0^ (or other meson). Shown in
Figure 3-15 is a shower shape analysis with full PYTHIA simulations of
direct -jet events (upper left) and ^0^'s from hadronic events (upper
right).

![](media/image21.png){width="2.9243055555555557in"
height="2.782638888888889in"}![](media/image22.png){width="2.9291666666666667in"
height="2.782638888888889in"}

![](media/image23.png){width="2.8131944444444446in"
height="2.7375in"}![](media/image24.png){width="2.9291666666666667in"
height="2.782638888888889in"}

Figure 3‑15: Shower shape analysis (described in the text) of Monte
Carlo simulations of the full STAR detector using Pythia generated -jet
(upper left) and hadronic backgrounds (upper right) are shown. In the
lower left panel is shown the "distance" of events from the black curve
separating direct gamma and background events in the upper panels. The
signal efficiency and background rejection factor can then be extracted
by considering sliding a vertical line along the x axis thus producing
the 9-11 GeV plot at the lower right (included here is also a curve for
the 7-9 GeV partonic p~T~ bin for comparison).

In this analysis a candidate peak is identified and a range of strips
around it are fit with a canonical line shape. The maximum residual left
or right of the peak in each of the 2 planes is summed to plot along the
x-axis. An average of the peak areas is used to plot along the y axis. A
nominal cut separating enriched direct photon vs. background events is
shown as a black curve in the upper two figures. In the lower left panel
we show the distance away from the cut line for the (solid red) and ^0^/
(dashed blue) events. By considering the number of events to the left or
right of a "vertical line" in the lower left panel, and sliding it back
and forth we can generate a plot of the efficiency of 's surviving this
cut vs. the rejection factor for ^0^'s as shown in the lower right
panel. For the -jet partonic 9-11 GeV partonic p~T~ bin comparison about
2/3^rds^ of the background can be eliminated with this cut with about
90% of the 's surviving. For the 7-9 GeV bin these efficiency/rejection
values are slightly reduced but still quite encouraging. Evaluation of
effects of other backgrounds - neutral (neutrons and K^0^~L~'s) and
charged hadrons, and from \>2 photons, has begun. Similar shower shape
analyses involving a chi-squared peak fitting approach are also under
investigation.

The production of the above plot required some significant advances in
the understanding of the calorimeters. In particular the transverse
shower shapes in both BSMD and ESMD are not well reproduced by the Monte
Carlo simulations. We find that the data shapes are somewhat wider and
the fluctuations in tails of the shapes are greater than predicted by
our GEANT model of the STAR detector. Monte Carlo and data from 2006
have been extensively studied in the EEMC. The deficiency in the Monte
Carlo simulations has been addressed by selecting samples of isolated
gammas. One such sample was generated by selecting identified 's
decaying to two 's where, due to the large mass, the 's are well
separated in the SMD. Useful events were checked to be well isolated
from other particles in the detector. These isolated 's from 's were
then used to define a more representative canonical line shape and also
loaded in a library and used in the Monte Carlo to replace the generated
shape with more realistic ones.

The shapes of the showers at the SMD from Run 6 data have been studied
extensively by making full use of the EEMC's various detector
subcomponents as shown for example in Figure 3-16. The information
available is displayed on the right where in the lower 6 panels the
energy deposited in vs. is displayed for the tower (full energy
deposition) and the energy deposited in the first two layers (preshower
1 & 2) as well as in the last layer (post shower) and SMD layers is
shown. The shower shape for an isolated with highly selective cuts is
shown in the upper two panels of the right side.

![](media/image25.png){width="6.002777777777778in"
height="5.381944444444445in"}

Figure 3‑16: Shower shape analysis and example signals for candidate
event in the EEMC. The lower 6 panels on the right show the signals
available from the calorimeter as distributed in vs.. The "tower" signal
measures the full energy of the shower for the event (p~T~ \~ 12 GeV).
The preshower detectors, measuring the energy deposited in the first two
layers at the front of the calorimeter, here register little energy. The
postshower detector measures the remaining energy at the back of the
calorimeter (after 21 radiation lengths). The two panels in the upper
right show the profile of what appears to be an electromagnetic shower.
These profiles are analyzed against a canonical shape in a sided
residual analysis described in the text. The upper left panel shows this
analysis for an enriched sample of 's, vetoed by both preshower layers
and the lower left for a sample dominated by hadronic background where
both preshowers are required to fire.

The full set of information as described above can be used for checking
isolation of the signal under study as well as the shower development
through the calorimeter. As an example, the pre-shower detectors were
used to select an enriched sample of isolated 's for study from a sample
already in coincidence with an away-side jet and the candidate
containing 90% of the energy in an isolation cone of 0.7 radius in x .
There is approximately 1 radiation length of material in front of the
calorimeter in 2006. In addition the first layer presents about 1
radiation length more. Thus by insisting that there is no signal in
either pre-shower detector one enhances isolated 's over the pairs one
expects from ^0^'s. This sample was then compared to the sample
generated from 's discussed above. In studying the systematics of these
two samples we learned the following about the calorimeter: 1) While the
width difference between data and Monte Carlo is not fully understood an
insufficient amount of material in the model upstream of the calorimeter
is suspected. In addition, there are indications that the generated
width may be too narrow. We expect data from the 2008 run with limited
material near the beam in STAR to provide important information on these
questions. 2) We find the width of the distribution depends on the
location of the SMD in the detector stack. The SMD planes are in one of
3 depths in the calorimeter (3 positions are shared by two planes and a
blank alternating in adjacent sectors) and the width of the distribution
detected is wider the deeper the SMD is in the detector stack. 3) The
shape of the SMD distributions also depend on the depth at which
conversion takes place, with those photons converting after the 2^nd^
pre-shower having narrower shapes than those converting before it.
Likewise photons converting after the 1^st^ pre-shower detector have
narrower shapes than those entering the calorimeter. These effects are
now all modeled in a data driven way by the modified Monte Carlo and
reflected in Figure 3-15 showing the shower shape analysis for the Monte
Carlo. In Figure 3-16 in the left panels we see the shower shape
analysis for data. The analysis is performed with the same data driven
shape and in both cases we see that the presumed locus is centered on
the vertical axis indicating that the shape on average well represents
both the data and the Monte Carlo. The reasonable agreement between
simulations for these crucial shower shape parameters at the SMD should
greatly improve the reliability of estimates for efficiencies and
backgrounds (purity).

In addition, we have learned to generate subsamples of the data with
well controlled relationships with regard to their effect on single and
double samples that can be used to check derived estimates of the
efficiency and background. As one example, events that convert in the
first Pb layer of the calorimeter, as evidenced by a veto in the first
preshower and energy in the 2^nd^, can be used for a statistical
separation of gamma's from ^0^'s and direct comparison to event-by-event
separation. For the former, initial results indicate rough agreement of
data yields and sample behavior as expected from Pythia and other
detector data response.

Taken together this then provides most of the final tools necessary to
complete an initial full analysis of -jet events from the 2006 run. By
coupling the shower shape cuts described above with a selection of
coincident away-side jets using the jet finder (instrumental to our
inclusive and di-jet results) as well as an isolation cut around the
candidate ,we are selecting a strongly enhanced sample of direct -jet
events. This analysis procedure has been an integral part of the effort
above and is well underway. At the present, we are working toward the
first full analysis results of the 2006 data and Monte Carlo to provide
estimated yields, efficiencies and purities necessary to calculate the
statistical and systematic errors from background subtraction in a given
data set.

## Polarized proton request in Runs 9 and 10 

The STAR collaboration is requesting a polarized proton-proton run in
Run 9 and/or Run 10. The primary goal is to collect a data sample at
longitudinal polarization of 50pb^-1^ at 60% beam polarization, which
translates into a Figure-of-Merrit (FOM) of 6.5pb^-1^. The detailed
breakdown is shown in the executive summary for two different running
scenarios of 25 and 16 cryo-weeks. Once this goal is met and sufficient
time remains a switch should occur to transverse polarization to collect
at least 6pb^-1^ during Run 9. The physics justification for both the
longitudinal and transverse spin program is discussed in the following
two sections. It is also important that development for 500 GeV
polarized pp running occur in the next two years in order to be prepared
for the first long production run in 2011. We thus reserve time for this
development in the first two years of this beam use proposal under
either scenario.

### Longitudinal spin program

The primary goal of Run 9 is to advance our understanding of the gluon
polarization based on three key measurements:

-   Di-jet production over a wide kinematic region profiting from the
    large calorimetry coverage in STAR

-   Precision inclusive measurements at large transverse momentum, in
    particular for the case of inclusive jet production, which had been
    so far the flagship of the STAR longitudinal spin program

-   Embark on prompt photon measurements and photon-jet measurements

Di-jet production is a key element of the STAR longitudinal spin
program. As discussed in the previous section, di-jet production
provides sensitivity to the underlying partonic kinematics beyond
inclusive measurements, which simply integrate over the measured
kinematic region. The recent global analysis by DSSV^i^ suggest a gluon
polarization with a node around x=0.1 with a positive value for x\>0.1
and a negative value for x\<0.1. It would be extremely important to
enhance those Bjorken-x regions through specific kinematic cuts. Di-jet
(and gamma-jet) production in STAR does indeed allow such e selection.
Figure 3-17 shows the longitudinal double-spin asymmetry A~LL~ for
di-jet production as a function of the invariant mass M for different
topological combinations of the STAR BEMC and STAR EEMC region. The
projected uncertainties are shown for a luminosity of 50pb^-1^ and a
beam polarization of 60% including detector effects. Those projected
uncertainties are compared to a LO evaluation of A~LL~. A full NLO
calculation is in preparation. The A~LL~ curves at LO are shown for
GRSV-STD and GRSV-ZERO, which have been both introduced in the
longitudinal result section previously. In addition to those parton
distribution functions, LO calculations based on GS-C and GRSV-m0.3 have
been performed, where the latter refers to the case of a GRSV-type gluon
polarization with a first moment of -0.3 at a scale of 1GeV^2^. The
projected uncertainties demonstrate the excellent discrimination that
will be possible amongst different models that are fully compatible with
the current RHIC data, such as GRSV-ZERO, GRSV-m0.3, and GS-C.

The di-jet data will provide access to Δ*g*(*x*) over a broad range of
*x* values. In a leading-order picture, east barrel -- east barrel, east
barrel -- endcap, and west barrel -- west barrel di-jet events will
sample 0.1 \<\~ *x*~2~ \<\~ 0.2 in collisions where *x*~1~ is
approximately 3*x*~2~. West barrel -- endcap events will sample 0.04
\<\~ *x*~2~ \< 0.1 in collisions where *x*~1~ is nearly 7*x*~2~. In
these cases, the asymmetry is dominated by the scattering of highly
polarized quarks off lower momentum gluons. This will provide a mapping
of Δ*g*(*x*) over the region where the DSSV fit expects there to be a
node. The east barrel -- west barrel combination will provide
complementary information about symmetric partonic collisions with
*x*~1~ and *x*~2~ in the range 0.1 to 0.4, while the endcap -- endcap
region will involve collisions of very high *x* quarks with very low *x*
gluons. Di-jet production in Run 9 with an anticipated luminosity of
50pb^-1^ and a beam polarization of 60% would allow a substantial
improvement of our understanding of the gluon polarization.

![](media/image26.png){width="5.877777777777778in"
height="3.7736111111111112in"}

Figure 3‑17: Longitudinal double-spin asymmetry A~LL~ for di-jet
production as a function of the invariant mass M for different
topological combinations of the STAR BEMC and STAR EEMC region. The
projected uncertainties are shown for a luminosity of 50pb^-1^ and a
beam polarization of 60%. Those projected uncertainties are compared to
a LO evaluation of A~LL~.

The potential for inclusive jet production for Run 9 can be understood
in the context of the recent global analysis by DSSVError: Reference
source not found. Figure 3-18 shows the gluon polarization as a function
of x includin the inclusive jet result from STAR and the neutral pion
result from PHENIX for Run 6 (top) and the projected performance for Run
9 (bottom) based on a data sample of 50pb^-1^ and a beam polarization of
60%. The uncertainty is substantially reduced which is shown as a
variation of the total ^2^ by 1 unit in green and by a 2% variation of
the total ^2^ in yellow. Figure 3-19 shows the individual contributions
to the total χ^2^ with the projected Run 9 results. This underlines the
clear potential of the inclusive jet program of STAR in Run 9 to further
reduce the actual uncertainty on the extracted gluon polarization. In
practice, the gluon polarization will be even more precisely determined
with the Run 9 results than illustrated in Figure 3-19 and Figure 3-22
because these projections do not include the additional knowledge that
will be gained from the di-jet study described above.

In addition to the proposed measurements of inclusive jets and di-jets,
we propose to make significant photon-jet coincidence measurements at √s
= 200 GeV and eventually √s = 500 GeV. These measurements are sensitive
mostly to quark-gluon scattering and thus allow the rather direct
determination of the polarized gluon distribution ∆G as a function of
gluon fractional momentum x.

+---------------------------------+------------------------------------+
| Figure 3‑18: Gluon polarization | ![](media/image28                  |
| as a function of x for the      | .pct){width="3.0805555555555557in" |
| inclusive jet result from STAR  | height="3.3270833333333334in"}     |
| and the neutral pion result     |                                    |
| from PHENIX for Run 6 (top) and | Figure 3‑19: ^2^ as function of    |
| the projected performance for   | the truncated first moment (0.05,  |
| Run 9 (bottom) based on a data  | 0.2) including the inclusive jet   |
| sample of 50pb^-1^ and a beam   | result from STAR and the neutral   |
| polarization of 60%.            | pion result from PHENIX for Run 6  |
|                                 | (top) and the projected            |
| ![](medi                        | performance for Run 9 (bottom)     |
| a/image27.pct){width="2.8375in" | based on a data sample of 50pb^-1^ |
| height="2.8625in"}              | and a beam polarization of 60%.    |
+---------------------------------+------------------------------------+

At the time of the writing of this beam use proposal, we have not yet
completed a full evaluation of the sensitivities that STAR photon-jet
measurements will yield.  We have estimated sensitivities in the
acceptance of the barrel and endcap calorimeters at the PYTHIA event
generator level for the nominal integrated luminosity of 50 pb^-1^ and
60% beam polarization in Run 9.  These projected sensitivities are shown
in Figure 3-20, together with current world data from high-p~T~ hadron
(pair) and open charm production in fixed-target polarized
lepton-nucleon experiments by the COMPASS, HERMES, and SMC
collaborations.  In addition representative polarized gluon
distributions are shown at the scales typical of the STAR proposed
measurements, µ^2^ ≈ 100 GeV^2^, and of the lepton-nucleon measurements
of high p~T~ hadron(s),  µ^2^ ≈ 3 GeV^2^. The scale, µ^2^ ≈ 13 GeV^2^,
of the COMPASS open charm measurement is in between the shown scales.
Note that these COMPASS, HERMES, and SMC results were not included in
the recent DSSV fit because corresponding NLO calculations are not yet
available. In contrast, an NLO framework for photon+jet measurements
already exists. We expect that the integrated luminosity of Run 9 will
allow STAR to make a credible start on photon-jet measurements. These
initial measurements at √s = 200 GeV are expected to probe resolved
gluon momenta fractions down to x ≈ 0.03. In the region of kinematic
overlap with existing data at higher x they probe at complementary
harder perturbative scales. The accumulation of integrated luminosity at
√s = 200 GeV and √s = 500 GeV beyond Run 9 is expected to improve the
precision and extend the kinematic coverage of these measurements to x ≈
0.01.

![](media/image29.pct){width="4.21875in" height="3.259027777777778in"}

Figure 3‑20: Projected uncertainties for g/g as a function of x for
photon-jet measurements at √s = 200 GeV for an integrated luminosity of
50pb^-1^ and 60% beam polarization in Run 9. Experimental efficiencies
are not yet taken into account. Also shown are current world data from
high-p~T~ hadron (pair) and open charm production in fixed-target
polarized lepton-nucleon experiments by the COMPASS, HERMES, and SMC
collaborations.

### Transverse spin program

The window on parton orbital motion at RHIC corresponds to the Sivers
mechanism[^29] as an explanation of transverse single spin asymmetries
(SSA) for pion production at large Feynman *x* (*x*~F~). Transverse SSA
is suppressed in hard scattering in vector gauge theories in the
massless limit[^30], and the light quark sector of QCD is very nearly in
that limit. The Sivers mechanism was proposed to explain striking
experimentally observed spin effects in polarized proton collisions at
low collision energy (*s* \< 20 GeV)[^31] that could not be generated in
a naive leading-twist collinear pQCD description.

Many things have changed in the past six years. For example, there was a
theoretical proofError: Reference source not found that (naive) time
reversal is violated by the Sivers mechanism, since it requires a
(naive) T-odd spin and transverse momentum dependent distribution
function, called the Sivers function. Along with this proof, another
mechanism for explaining transverse SSA in p+p collisions producing
pions was suggested by theoristsError: Reference source not found. In
this mechanism, transversely polarized quarks must be present in a
transversely polarized proton. The Bjorken-*x* dependence of the
transverse spin asymmetry distribution is given by the transversity
function. Moments of the transversity distribution have recently been
computed in lattice QCD. The zeroth moment of transversity is the tensor
charge of the proton, relevant to estimates of the electric dipole
moment of the neutron, and so determination of the transversity
distribution is of great importance. Transverse momentum (*k*~T~) for
hadrons produced in the fragmentation of a transversely polarized quark
in the final state would have a sinusoidal variation on the azimuthal
orientation of ***k*~T~** in the Collins mechanism.

A theoretical proof that the Sivers function is zero has since been
withdrawn[^32] since final-state interactions[^33], that are required by
gauge invariance, spoil naive time reversal arguments that potentially
could suppress the Sivers mechanism. Furthermore, a specific example was
provided explicitly demonstrating how the Sivers mechanism produces a
transverse SSA, thereby overcoming the naive time reversal arguments and
supplying the requisite phase factor leading to the amplitude
interference as needed for a transverse SSA.

These theoretical developments over the past six years have coincided
with important experimental developments summarized below.

(1) Transverse SSA has been measured in semi-inclusive deep inelastic
    scattering (SIDIS) from a transversely polarized proton target by
    the HERMES collaboration[^34]. A separation of the Collins and
    Sivers mechanisms from this data has been made.

(2) Zero asymmetries are observed in SIDIS from a transversely polarized
    deuterium target by the COMPASS collaboration[^35]. The COMPASS
    results are consistent with present theoretical understanding that
    the Sivers functions for up and down quarks are opposite in sign and
    cancel for the isospin zero deuterium target. We eagerly await the
    results from COMPASS for transverse SSA from a transversely
    polarized proton target, although the Bjorken *x* range extends to
    much smaller values than probed by HERMES, where the spin effects
    may be smaller.

(3) Concurrent with the SIDIS measurements are measurements at RHIC that
    demonstrate that transverse SSA that are observed for pion
    production at *s* \< 20 GeV are present at *s* =200
    GeV[^36]^,Error:\ Reference\ source\ not\ found^. This is
    significant because measured cross sections at *s* =200 GeV are
    consistent with next-to-leading order pQCD calculationsError:
    Reference source not found, meaning that theories that aim at a
    universal understanding of transverse SSA in hard scattering
    processes are supported by a good understanding of the spin averaged
    observables. This is not the case for pion production at lower *s*,
    where observed cross sections are progressively larger than pQCD
    calculations as *s* decreases, with very pronounced enhancements
    relative to theory in the forward direction[^37].

(4) Finally, experimental observation of the Collins fragmentation
    function has been completed in (unpolarized) *e*^+^*e*^--^
    collisions by the Belle collaboration[^38]. The Collins function
    corresponds to a harmonic dependence on the azimuthal angle relative
    to the thrust axis for fragmentation products from a transversely
    polarized quark.

The final-state interactions that spoil naive time-reversal symmetry for
the Sivers mechanism have a simple intuitive explanation in SIDIS[^39].
The absorption of a virtual photon by the current quark leads to a
separation of equal and opposite colors, since the proton target is
color neutral. Given this color separation, after the virtual photon is
absorbed, the current quark does not propagate in free space, but rather
propagates in the color field generated by the spectator quarks. The
phase factor required for transverse SSA naturally arises in this
mechanism, as in the case of Coulomb wave functions for the QED analogy
of deep inelastic scattering on a hydrogen atom. This phase, as well as
a spin- and *k*~T~-dependent distribution function, is a requirement for
the Sivers mechanism. It is clear that the final-state interaction is
attractive in the case of SIDIS, because the proton is initially color
neutral.

Model calculations have suggested that the *x*~F~ dependence of
transverse SSA for p+p + *X* at *s*=200 GeV can be explained by the
Sivers functionError: Reference source not found extracted from the
HERMES data^v^. Current theory does not reliably predict the *p*~T~
dependence at fixed *x*~F~ for transverse SSA for pion production. A
resolution to this may be in sight, invoking contributions from both
Collins and Sivers mechanisms to the hadroproduction transverse SSA.
First attempts at direct separation of these contributions will be made
from data obtained with the FMS during RHIC Run 8.

If the Sivers function appears for both SIDIS and polarized proton
collisions, then we can conclude that partons have orbital motion within
the spinning proton. This is a significant development that requires
further experimental and theoretical work to establish.

Further theoretical work has led to an even more interesting consequence
of the Sivers mechanism. Namely, the color structure associated with
present theoretical understanding of the Drell-Yan (DY) production of
dilepton pairs, leads to the expectation that the Sivers function for
transverse SSA in DY is exactly opposite in sign from the Sivers
function that is extracted from SIDIS. This non-universality arises
because the attractive color interactions in SIDIS are replaced by
repulsive color interactions between like colors in DY. Experimental
observation of this sign change would identify fundamental aspects of
color charge interactions.

The RHIC experiments would need upgrades to complete a transverse SSA
measurement for the DY process. For example, tracking upstream of the
FMS is needed to discriminate like-sign from unlike-sign lepton pairs.
At PHENIX, additional acceptance is required to allow isolation cuts to
be included in identification to reduce backgrounds from open heavy
flavor production. Furthermore, the integrated luminosity requirements
mean that this as a future effort, following sustained luminosity
improvements for polarized proton collisions.

In the meantime, further theoretical work has proceeded. Specifically,
there is a prediction that repulsive color charge interactions will
produce a change in sign for forward photon +away-side jet in polarized
proton collisions at *s*=200 GeV[^40]. There are two implications of
this prediction:

(1) there is partonic orbital motion within the spinning proton,
    observable in a process dependent way in both SIDIS and polarized
    proton collisions;

(2) the sign difference between SIDIS and polarized proton collisions
    demonstrates the reality of attractive and repulsive color charge
    interactions.

The addition of the FMS prior to the start of Run 8, and the away-side
jet detection capabilities in the central region at STAR, uniquely
positions the STAR collaboration to complete this measurement. It should
be mentioned that the simple explanations, based on abelian (QED)
analogs, of why there is a sign change in forward photon + away-side jet
is not as intuitive as for transverse SSA for the DY process.
Observation of the predicted sign change would bolster the case for a
future transverse spin DY measurement at RHIC.

  ---------------------------------------------------- ----------------------------------
  ![](media/image30.png){width="3.001388888888889in"   Figure 3‑21: A schematic of the
  height="3.0625in"}                                   STAR forward meson spectrometer as
                                                       seen from the interaction point.
                                                       The Blue beam penetrates into the
                                                       page at the center of the
                                                       matrices. The yellow shaded area
                                                       represents a conservative fiducial
                                                       volume used for detection of
                                                       direct candidates, with the
                                                       remainder of the FMS used as a
                                                       veto of photons arising primarily
                                                       from decay.

  ---------------------------------------------------- ----------------------------------

The STAR FMS (Figure 3-21) would be used for detecting direct photons at
large rapidity. This device was built for studies of possible gluon
saturation phenomena at RHIC. Run 8 results will address this question.
Adaptation of cluster algorithms and shower shape analysis used for the
STAR forward pion detector has been completed and applied to data from
the FMS, after tuning the algorithms on full PYTHIA/GEANT simulations.
Although final resolutions, energy scales and peak-to-background ratios
await completion of offline calibration, first reconstructions of FMS
data look quite promising, as shown by diphoton invariant mass
distributions in Figure 3-22.

The FMS is a hermetic electromagnetic calorimeter, built from finely
segmented lead-glass detectors, positioned west of the interaction point
at STAR. It views particles produced in collisions through large holes
in the poletips of the STAR magnet. The integrated number of radiation
lengths between the calorimeter and the interaction point is dominated
by crossing the beryllium beam pipe. The FMS is an outgrowth of the
forward pion detector, corresponding to modular electromagnetic
calorimeters built from finely segmented lead-glass detectors.
Consequently, extensive experience with event reconstruction for both
simulation and data exists from the FPD project, and can be carried over
to the FMS. In general, full PYTHIA/GEANT simulations accurately account
for what has been observed in the FPD. Single beam backgrounds are
minimal, as confirmed in Run 8 by long dwell time vernier scans, due to
shielding provided by the cryostats of the RHIC magnets. Comparison
between simulation and data for the FMS awaits final calibrations.

For direct photon detection with a large-area hermetic electromagnetic
calorimeter, the most effective method is to eliminate other sources of
photons by direct detection. These other photons sources arise primarily
from the decay of andneutral mesons. Multiple aspects make this feasible
with the FMS. First, because the calorimeter is at large rapidity, the
typical photon energies are large (E\> 35 GeV) and the hadronic response
of the FMS is small. Direct simulation of the detector response via
GSTAR simulations employing the GEISHA hadronic simulation, applied to
full PYTHIA events of collisions have demonstrated that the hadronic
response of the lead glass detectors is minimal for energy depositions
in excess of 25 GeV. Second, the large Lorentz factors result in
significant spatial correlations for the decay products from andmesons.
For example, when there is a candidate direct photon having E= 25 GeV,
95% of the phase space for all ^^decays that could potentially be the
source of this energetic photon are contained in a circle of radius 23
cm (approximately four large cells of the FMS) around the 25-GeV photon
at the face of the FMS. Finally, andmesons are most typically produced
via fragmentation in p+p collisions at RHIC energies. This means that
other hadrons share the energy of the fragmenting quark or gluon,
softening the decay photon spectrum. The daughter photons from p^^ and
h decay then further divide this energy. In contrast, direct photons are
produced in the hard scattering process. Their energy is not shared via
fragmentation nor is it shared by particle decay. The energy where
direct photon probabilities exceed decay photon probabilities is \~50
GeV for \<\> \~ 3.2, with no other conditions on the event. The
andbackgrounds can be reduced, and this energy can be lowered to \~25
GeV, by subdivision of the FMS into two volumes. The fiducial volume,
used for identifying direct photon candidates, is a small annular region
within the outer calorimeter. The remainder of the FMS is used as a veto
to establish that other photons from the decay of neutral mesons are not
present. The effectiveness of the discrimination between direct and
decay photons is demonstrated via full collision event simulations in
Figure 3-23.

Restricting the measurement of the forward photon to E\>35 GeV at \<\> =
3.2 produces a signal:background ratio of 2.1. Lower thresholds result
in larger contamination, and so are undesirable. Substantial gains in
yield can be realized by small changes in the fiducial volume towards
smaller distance from the beam because of the very strong power law
dependence on *p*~T~,. A detailed optimization of signal magnitude and
signal:background ratio still must be completed and simulation of the
weighted azimuthal moments is still required.

  ---------------------------------------------------- ---------------------------------
  ![](media/image31.png){width="3.009027777777778in"   Figure 3‑22: First
  height="3.1631944444444446in"}                       reconstructions of events from
                                                       the STAR Forward Meson
                                                       Spectrometer. Clustering and
                                                       shower shape analysis codes used
                                                       for the forward pion detector
                                                       were adapted to the FMS. Although
                                                       final calibrations and efficiency
                                                       corrections are still required,
                                                       clear evidence for events is seen
                                                       from the diphoton invariant mass
                                                       distribution for both the large
                                                       cell outer calorimeter and the
                                                       small cell inner calorimeter (see
                                                       Figure 3-21).

  ---------------------------------------------------- ---------------------------------

  ----------------------------------------------------- --------------------------------------
  ![](media/image33.png){width="2.6638888888888888in"   Figure 3‑23: Simulation of inclusive
  height="2.720833333333333in"}                         forward photon spectrum. Direct photon
                                                        candidates are detected in a small
                                                        fiducial volume within the FMS as
                                                        shown by the shaded region in Figure
                                                        3-21. Isolation of radius,
                                                        R=(^2^+^2^)^½^, is then required
                                                        around the photon candidate, where and
                                                        refer to angular differences between
                                                        near-side correlations with the direct
                                                        photon candidate and potentially other
                                                        particles.

  ----------------------------------------------------- --------------------------------------

Fragmentation photons are also a background for inclusive direct photon
measurements that would require a careful study of jet asymmetries for
forward jets that contain a fragmentation photon. The prediction for the
sign change for transverse SSA is for a forward photon + recoil jet. The
isolated fragmentation photon will have its momentum balanced by two
jets, rather than one, as expected by the dominant *qg* Compton diagram.
Consequently, the fragmentation photon contribution will be spread
throughout a =~jet~distribution, whereas the signal of interest is on
the sides of the peak in the distribution, in the vicinity 0. More
detailed simulations to quantify this argument are in progress.

An estimate of the luminosity necessary for this measurement has been
made based on simulations of the distribution (Figure 3-24) for p+p
g(forward)+jet at *s*=200 GeV. The simulations use a simple clustering
algorithm to define the midrapidity jet, requiring a seed with
transverse energy of at least 0.5 GeV. A cone radius, R = (^2^ + ^2^)^½^
= 0.7 is used in the clustering. Here, h refer to the deviation of a
hadron from the thrust axis of the reconstructed jet. Jet
reconstruction, even at *p*~T,jet~ \> 2 GeV/c, is found robust by
comparison to recoil parton kinematics and by comparison to a leading
particle analysis. The result is that 10^3^ useable forward photon + jet
coincidences are expected in a 30 pb^-1^ data sample with 60% beam
polarization. The (forward)+jet yield is sensitive to the jet-finding
parameters and the fiducial volume requirement for the forward photon. A
full optimization of the event selection to test the theoretical
prediction is required, and possibly could result in robust sensitivity
to the predicted sign change (\>4) at smaller figure of merit. An
initial sample of transverse spin (forward)+jet of 6 pb^--1^ recorded
with beam polarization of 60% would be used to fine tune the figure of
merit requirements for the measurement.

  ---------------------------------------------------- ----------------------
  ![](media/image34.png){width="3.248611111111111in"   Figure 3‑24:
  height="3.248611111111111in"}                        Simulation of forward
                                                       photon + recoil jet
                                                       for p+p collisions at
                                                       s=200 GeV.

  ---------------------------------------------------- ----------------------

The goal is to start this unique measurement in Run 9 with a
continuation beyond to collect a total integrated luminosity of 30pb^-1^
at transverse beam polarization.

### Polarized proton collisions at 500 GeV

Achieving the goals in 2011 of taking a data set for W production with
physics impact cannot be met without further beam development at 500
GeV. In addition to important work on the CAD side of operations there
is development at the experiments necessary as well. In particular we
need to learn in a run before 2011 whether existing local polarimetry
will work or not. This is essential for tuning of the spin rotators. In
addition the higher luminosities will provide new challenges to
luminosity monitoring. These issues need to be addressed in development
time before 2011 to allow for modifications of existing equipment or
implementation of new techniques for the production run.

We hope that this can begin during Run 9.  However, if Run 9 is
significantly shorter than 25 weeks, this will need to be postponed
until Run 10 because progress on g(x) is a higher priority in the near
term.

**3.4.4 Polarized proton request in Runs 11-13**

Beginning with year 11 it is expected that the focus of the STAR spin
program will switch to W production with √s =500 GeV polarized pp
collisions. This channel has long been identified as a means to measure
the polarization of sea quarks[^41]^,^[^42]. It can provide direct
access to the difference between the polarizations of u-bar and d-bar
quarks, a quantity sensitive to underlying mechanisms which produce the
polarized sea[^43]. In STAR, W detection will be primarily through W^-^
e^-^ and to e ^+^ for the W^+^. Charge sign discrimination is essential
to separate the u-bar and d-bar contributions. Construction of a Forward
GEM Tracker to provide charge sign discrimination in the forward
direction, 1 \< \< 2, has recently been approved at STAR and is expected
to be installed for Run 11. In Figure 3-25 we show predictions for
various underlying quark and anti-quark distributions based on GRSV-STD,
GRSV-VAL[^44] and GS-A[^45] calculated with RHICBOS[^46], a program that
incorporates an NLO calculation of spin observables in W production.
GRSV-VAL considers a flavor asymmetric scenario of Du and Dd. It is
qualitatively similar to the sea quark distributions in DSSV. In
contrast, GRSV-STD is based on a flavor symmetric description. Since the
cross section for W production is small, a large integrated luminosity
is required. The calculations shows that 400pb^-1^ provides significant
discrimination between these models setting the scale for goals in the
√s =500 GeV spin program.

An extensive effort to simulate electron - hadron discrimination in the
forward direction covered by the EEMC are well underway. This analysis
makes use of Pythia and GEANT simulations in the full STAR software
framework. This effort is quite CPU intensive and special algorithms
have been developed to preselect events capable of depositing
significant energy in the EEMC. Still it has required the use of a Tier2
grid facility to generate sufficient hadronic background samples for the
studies. It is expected that results from this effort including
isolation cuts, awayside veto cuts and cuts based on the full
segmentation of the EEMC will be released by the time of the PAC.

As the W program is carried out, the spin-dependent gluon distribution
of the proton, ΔG(x), will be measured as well. Again, di-jets and
γ-jets will be the most effective at providing a map of ΔG(x) while
inclusive jets and other channels will provide the first results. A
major portion of the available running time in Run 11 is assigned to
collecting a substantial data set at 500 GeV. The listed goal of 150
pb^-1^ is estimated from current projections for Run 9 at 200 GeV and
scaled by the increase in √s, i.e. by 2.5. A second running period in
Run 13 is assumed to benefit from CAD upgrades and luminosity
development which will enable a substantial step in integrated
luminosity and physics reach for the W program.

However, given the amount of running time available in the preceding two
years, it is expected that our 200 GeV longitudinal goal of 50pb^-1^
(6.5pb^-1^ F.O.M.) for mapping ΔG(x), and transverse measurements in
-jet Sivers (30pb^-1^) for exploring the color sign dependence of the
strong force will require additional running time. For this reason a
portion of Run 11 is set aside for these goals. In Run 13 there will
need to be a substantial run with 200 GeV pp for comparison data for the
previous year's Au-Au data with the newly installed HFT. This will
provide an opportunity for the spin program to complete its 200GeV goals
and hopefully extend its reach for rare probes like gamma-jet and heavy
flavor.

![](media/image35.pct){width="3.5416666666666665in"
height="3.6222222222222222in"}

Figure 3‑25: A NLO prediction for the single spin parity violating
asymmetry for W production in proton - proton collisions at √s =500 GeV
from RHICBOSError: Reference source not found for various underlying
quark and anti-quark distributions.

## Physics with tagged forward protons with STAR 

By using the polarized proton beam, the STAR detector, and the pp2pp
experiment's Roman Pots [^47]^,^[^48]^,^ [^49], installed and debugged
during Run 8, we propose to further extend our scientific program into
the non-perturbative regime of QCD. Very forward protons are tagged
using the Roman Pot detectors, thus selecting processes in which the
proton stays intact and the exchange has quantum numbers of the vacuum
i.e. the so-called Pomeron (IP) exchange process. In these processes the
probability of measuring reactions where colorless gluonic matter
dominates the exchange is enhanced. The use of polarized proton beams,
unique at RHIC, will allow for the exploration of the unknown spin
dependence of diffraction including both elastic and inelastic
processes.

**Physics Motivation:** There are two main reactions that can be studied
with tagged forward protons: Central Production (CP) in double Pomeron
exchange (DPE); and polarized proton-proton elastic scattering, see 1.

Figure 3‑26: Central Production diagram (left) and Elastic Scattering
diagram (right).

**Central production of glueballs:** The DPE processes are commonly
characterized by using variables t, and M~X~, where t is the square of
the four-momentum transfer between the incoming and outgoing protons,
p/p is the momentum fraction carried off by the Pomeron and M~X~ is
invariant mass of the system produced. In the case of double Pomeron
exchange, separate t and variables exist for each proton-Pomeron vertex.

Tagging and measuring forward protons is important since it removes the
ambiguity of a (complementary) rapidity gap tag, which has a background
due to the low multiplicity of diffractive events, and allows the full
characterization of the event in terms of t, and M~X~.

The idea that the production of glueballs is enhanced in the central
region in the process pp pM~X~p was first proposed byError: Reference
source not found and was demonstrated experimentallyError: Reference
source not found. The crucial argument here is that the pattern of
resonances produced in the central region, where both forward protons
are tagged, depends on the vector difference of the transverse momentum
of the final state protons k~T1~, k~T2~, with dP~T~ k~T1~ k~T2~. The
so-called dP~T~ filter argument is that when dP~T~ is large ( ~QCD~) qq
states are prominent and when dP~T~ is small the surviving resonances
include glueball candidates.^,Error:\ Reference\ source\ not\ found^. In
quantifying the limit the WA102 experimentError: Reference source not
found found \"small\" dP~T~ \< 0.2 GeV and \"big\" dP~T~ \> 0.5 GeV.

The geometrical acceptance of our setup for both Singly Diffractive
Dissociation (SDD) and DPE processes has been studied. Protons were
generated with t and uniformly distributed in the regions 0.003\<
\|t\|\< 0.04 and 0.005 \< \< 0.05 respectively. It was further assumed
that the Roman Pots (RPs) were at least 12 of the beam size at the
detection point. 2 shows the geometrical acceptance as function of
M**~X~**, red dots are for the actual location of the RPs.

These simulations showed that there is good acceptance for measuring
inelastic diffraction DPE with ^\*^=20m. Thus, a large data sample of
diffractive states can be obtained and analyzed as function of
diffractive mass and t (d^2^σ/dM~X~^2^dt) for central production.

![](media/image38.png){width="3.071527777777778in"
height="1.801388888888889in"}

Figure 3‑27: Mass geometrical acceptance for DPE processes, red dots are
the simulation with final location of the RPs.

**Elastic Scattering**: The entire energy range of this study has been
inaccessible to proton-proton (elastic) scattering in the past. Hence
new results of high quality will be produced.

The measurement of the differential pp elastic cross section d/dt over
the extended t-range will include the region at the lower \|t\| that is
particularly sensitive to the -parameter. Hence we will measure: 1) the
-parameter, 2) the nuclear slope parameter b in a combined fit to the
differential cross section possible, and 3) the total cross section
**~tot~**.

An asymptotic difference between the differential and total cross
sections for pp and pp, if present, could be explained by a contribution
of the Odderon to the scattering amplitude. The absence of an Odderon
contribution would lead to identical cross sections, approaching each
other roughly as s^1/2^. Our measurement of the total cross section,
**~tot~**, will probe the prevalent assumption that the cross sections
for pp and pp scattering are asymptotically identical.

By measuring spin related asymmetries one will be able to determine
elastic scattering at the amplitude level [^50]^,^[^51]^,^[^52]. The
availability of longitudinal polarization at STAR would allow measuring
A~LL~ in addition to A~NN~, A~SS~, and A~N~ resulting in a significant
improvement of our physics capabilities. Full azimuthal coverage of the
elastic events that we are planning to have will assure high efficiency
and small errors of the measurements.

One of the physics motivations to measure A~N~ is the possibility of the
rise with energy of ratio of spin-flip to spin-nonflip amplitudes. In
other words, it may occur that the small contribution from hadronic
spin-flip to the spin single-spin asymmetry, measured with a polarized
jet target at 100 GeV/c, could increase at s=200 GeV. This will help to
address the long standing problem of the energy dependence of the spin
flip amplitude, which is best answered experimentally.

Reaching such a small \|t\|-value allows the measurement of the single
spin analyzing power A**~N~** close to its maximum at \|t\| =0.0024
(GeV/c)^2^, where A**~max~** = 0.04, at s = 200 GeV. The A**~N~** and
its t-dependence in the covered range are sensitive to a possible
contribution of the single spin-flip amplitude, **~5~**, from the
interference between the hadronic spin-flip amplitude with the
electromagnetic non-flip amplitude.

**Forward Tagged Proton Beam Use Request Runs 9 and 10**: To maximize
physics output from the forward tagged physics running we are requesting
a special period of 3 ½ days of beam time in Run 9 with transverse
polarization and a separate special 3 ½ days of beam in Run 10 with
longitudinal polarization. This would allow us take sizeable data
samples with both transverse and longitudinal polarization respectively.
The main reason for such runs is that in order to reach the t and values
needed for both diffractive and elastic data, beam scraping and special
optics are needed. Our tests in Run 8 showed that both STAR and PHENIX
could take data during our dedicated run.

We shall use elastic and inelastic trigger bits (ET, IT) based on
scintillation counters in the Roman Pots. The CP trigger is from the
Central Trigger Barrel and inelastic trigger. The physics reach can be
expanded to study diffraction by using FMS and BBC triggers also. Some
studies and optimization of those triggers will be required.

Using the capacity of existing power supplies optics of ^\*^ = 20 m at s
= 200 GeV can be produced allowing the t coverage with 100% acceptance
for elastic scattering for 0.003 \< \|t\| \< 0.024 (GeV/c)^2^. The setup
time of the ^\*^=20 m optics is estimated by the C-AD to be 12 - 24
hours. Giving us 40 hrs data taking with transverse polarization during
the 3½ day run. We are projecting higher efficiency as the lifetime of
the beam is longer for our stores and we estimate that no more than two
stores are needed for our data taking.

Given the achieved performance of RHIC, luminosity of 1.810^31^
cm^-2^sec^-1^, and making the correction for scraping, which is needed
to get close to the beam, and ^\*^=20m, luminosity at STAR of 310^29^
cm^-2^sec^-1^ for our special run can be obtained. The luminosity at
PHENIX will be affected by the scraping only, thus lowered by about
factor 1.3^2^ = 1.69, resulting in luminosity 1.110^31^ cm^-2^sec^-1^,
scaled from 1.810^31^ cm^-2^sec^-1^. These estimates are based on RHIC
projections of luminosity and beam polarization.

With the above conditions about 2010^6^ elastic events for each
polarization could be collected. An estimated error on the slope
parameter is b=0.31 (GeV/c)^-2^ and on the ratio of real to imaginary
part =0.01, which is comparable to the existing measurements from the pp
and pp data. The ~tot~ = 2-3 mb, where the biggest contribution is from
the error on luminosity measurement. This measurement of the total cross
section, **~tot~** at the highest possible energy will probe the
prevalent assumption that the cross sections for pp and pp scattering
are asymptotically identical.

Our data sample of elastic events will allow statistically significant
measurements of analyzing power for spin dependent processes. In four t
subintervals we shall have 510^6^ events in each resulting in
corresponding errors An=0.0017, A~nn~=A~ss~=0.003.

With the expected luminosity we can also collect about 450,000,
triggered DPE events, for which the proton momentum is reconstructed for
each beam polarization. One assumes a 10barn cross section within our
acceptance for the DPE process, where it is required that two RPs on
each side are used allowing reconstruction of the outgoing proton
momentum. Number of events for which only one proton tag is used is
factor of 4-5 higher or about 2.210^6^ triggered DPE Central production
events for each polarization.

[]{#__RefHeading___Toc70426003 .anchor}RHIC beam energy scan and search
for QCD critical point

## Physics motivation

The RHIC PAC has given initial approval to a program of studying Au+Au
collisions at energies well below those already explored. The primary
goal of this program is to expand the study of the phase diagram of QCD.
One particularly exciting possibility is the discovery of a critical
point where the transition between the quark gluon plasma (QGP) and a
hadron gas changes from a smooth cross-over to a first order phase
transition. A detailed energy scan can also investigate how the
properties of the created matter evolve as the initial conditions are
varied.

A schematic QCD phase diagram is sketched in 1. New phases of nuclear
matter can be accessed by varying the temperature (*T*) and baryon
chemical potential (~B~). The QCD phase boundary, indicated by the solid
and dashed curves, separates matter in the form of a hadron gar from the
QGP. An importanl goal of nuclear physics is to map out the phase
boundaries and understand the evolution of associated physical
processes. Different regions of the phase diagram can be studied by
varying the center of mass energy of the colliding nuclei. Collisions
with higher energies lead to lower ~B~ values. As the beam energy is
decreased, the created matter enters regions of larger ~B~. 2 shows the
region of the phase diagram that can be accessed at the RHIC facility
along with freeze-out data from previous heavy-ius measurements. Model
simulations suggest that the nuclear matter is initially created at a
point near the top of the shaded area. As the matter expands and cools,
it evolves towards the phase transition and eventually to the freeze-out
point. Experimental values for the freeze-out point are shown as open
circles.

  ----------------------------------------------------- ---------------------------
  ![](media/image39.png){width="3.3027777777777776in"   Figure 4‑28: **Schematic of
  height="2.654166666666667in"}                         QCD phase diagram of
                                                        nuclear matter in terms of
                                                        the temperature (T) and
                                                        chemical potential (~B~).
                                                        The possible location of
                                                        the Critical Point is
                                                        indicated*.***

  ----------------------------------------------------- ---------------------------

**Critical Point Search:** One of the important motivations for the RHIC
energy scan is to search for the QCD critical point, an important
landmark on the phase diagram. Model calculations suggest that at large
chemical potentials, the boundary between hadron gas and the QGP is a
first-order phase transition. Lattice QCD (LQCD) calculations show that
at zero baryon chemical potential, the transition is a smooth crossover.
The point where the transition changes from an abrupt first order phase
transition to a smooth crossover is called a critical point.

For technical reasons, LQCD calculations become unreliable at
non-vanishing baryon chemical potential. Various methods have been
devoloped to circumvent these problems but many uncertainties remain.
Available LQCD calculations at non-zero baryon chemical potential
suggest that as the baryon chemical potential increases, the
fluctuations on the cross-over line increase dramatically, suggesting
the existence of a Critical Point in the phase diagram. The Critical
Point is obtained on a lattice using several methods and several
different assumptions regarding the physical volume of the lattice, the
lattice spacing, the number of quark flavors, as well as the chosen
values for quark mass. None of the computations performed so far could
satisfy all the required conditions and thus gave varying results for
the location of the Critical Point. Figure 4-29 shows the QCD phase
diagram with locations of three different estimations for the Critical
Point. As seen in the figure, the different lattice predictions of the
Critical Point location vary considerably, but all lie well within the
region that will be explored in the RHIC Beam Energy Scan.

  ----------------------------------------------------- -----------------------
  ![](media/image40.png){width="3.6798611111111112in"   Figure 4‑29: **The
  height="3.170138888888889in"}                         phase diagram of
                                                        nuclear matter in terms
                                                        of the temperature (T)
                                                        and chemical potential
                                                        (~B~). Possible
                                                        locations of the
                                                        Critical Point,
                                                        estimated by three
                                                        different lattice
                                                        calculations, are
                                                        indicated. The shaded
                                                        area shows the
                                                        parameter space
                                                        accessible at RHIC;
                                                        most of this region has
                                                        yet to be explored, as
                                                        indicated by the blue
                                                        boundary line.**

  ----------------------------------------------------- -----------------------

Verifying the existence of a Critical Point and fixing its location
through experimental measurements would be a major step forward in the
effort to determine the properties of QCD at high temperature and
density. Specifically, discovery of the Critical Point would identify a
crucial landmark in the QCD phase diagram, key to understanding the
phases of QCD and their quasi-particles.

LQCD calculations confirm that, at the Critical Point, the baryon number
susceptibility, the net charge susceptibility, and the specific heat
diverge. In the vicinity of the Critical Point, critical fluctuations
occur in parameters such as the baryon number density, quark number, and
charge. These fluctuations should be reflected in experimental
observables such as hadronic yields and source size parameters, elliptic
flow, fluctuations in net charge, \<p~T~\>, and particle ratios such as
K/, p/, etc. The scale dependence of the fluctuations in these
quantities may reveal the source of the signal.

The experimental search for the Critical Point must cover the range of
temperatures and baryon chemical potentials suggested by the LQCD
calculations. This can be achieved at RHIC with a beam energy scan. The
experiment should be capable of making comprehensive measurements of all
the signals related to the onset of deconfinement. As an example, let us
consider one of the signals, the ratio of K^+^ to ^+^ and its comparison
with the K^-^ to ^-^ ratio, as shown in Figure 4-30. The K^+^/^+^ ratio
shows a characteristic dramatic structure at a center-of-mass energy
near 8 GeV, whereas the K^-^/^-^ ratio shows a smooth behavior as a
function of the beam energy. The newly analyzed data from STAR at 19.6
GeV fits the systematic trend quite well. The RHIC energy scan includes
a detailed study of this region with improved statistical and systematic
errors, as well as extending the measurements somewhat farther below the
observed structure.

  -------------------------------------------- --------------------------
  ![](media/image41.jpeg){width="3.5in"        Figure 4‑30: **Beam energy
  height="2.654861111111111in"}                dependence of the ratios
                                               of K^+^/^+^ and
                                               K^-^/^-^.**

  -------------------------------------------- --------------------------

The STAR detector, as a consequence of its large, uniform acceptance,
and excellent PID capability with the inclusion of the new TOF barrel,
has superb capability for carrying out a search for the signatures
outlined above, and for providing definitive results on the existence
and location of the Critical Point. Detailed analyses from the STAR
experiment have shown that large acceptance with full azimuthal coverage
is essential for fluctuation measurements such as \<p~T~\> fluctuations.
The utility of such measures has already been demonstrated at available
RHIC energies where the variance excess for \<p~T~\> fluctuations in
terms of \<Δp~Ti~ Δp~Tj~\> has been inverted into p~T~ angular
correlations, which are then used to characterize elliptic flow and the
near- and away-side medium response (4). These have proved to be an
important tool for understanding the role of elliptic flow, mini-jets,
and medium response in producing the \<p~T~\> fluctuations, forming an
important baseline for the QCD critical point at lower energy as well.

![](media/image42.png){width="4.981944444444444in"
height="1.9659722222222222in"}

Figure 4‑31: **The variance excess from mean p~T~ fluctuations can be
inverted to study the role of elliptic flow, mini-jets, and medium
response to the correlations observed in Au+Au collisions**.

  ----------------------------------------------------- ----------------------------------------------------
  ![](media/image43.png){width="3.0944444444444446in"   ![](media/image44.png){width="3.188888888888889in"
  height="2.6631944444444446in"}                        height="2.660416666666667in"}

  Figure 4‑32: **A study, based on 100,000 simulated    
  events, of the statistical and systematic             
  uncertainties with and without the PID capability of  
  the STAR TOF. The left panel shows the capability for 
  kaon identification with the inclusion of the         
  Time-of-Flight detector. The right panel shows the    
  beam energy dependence of the dynamic fluctuations    
  associated with the ratio of kaons to pions. With the 
  inclusion of the Time-of-Flight detector, the         
  estimated error is reduced by a large factor.**       
  ----------------------------------------------------- ----------------------------------------------------

As shown in 5, it is essential to have comprehensive particle
identification capability within STAR's acceptance to minimize the
systematic error in searching for critical fluctuations, which might
indicate the approach to the critical point. Systematic errors dominate
the ultimate precision. As seen in5Figure 4-32, based on simulation of
100,000 events at 40A GeV, the increase in the statistical error without
the full-acceptance STAR TOF is very significant. A misidentification of
only 1%, leading to a swapping of pions for kaons, reduces the width of
the observed K/ fluctuation distribution by 10%. A misidentification of
2% leads to a reduction in width of 20%. The conclusion from our
simulations is that to optimize the potential to discover the QCD
Critical Point, a large acceptance with 2 azimuthal coverage and
excellent PID capability, which will be provided by the full-acceptance
TOF barrel, is essentia

**Energy Dependence of Near-side Correlations:** STAR measurements of
two-particle correlations either with or without minimum *p*~T~ cuts can
be decomposed into a few analytic structures, each related to different
mechanisms such as elliptic flow, HBT, and jets or mini-jets. This
decomposition reveals a near-side correlation structure that extends
over a large range in pseudo-rapidity, . This large correlation is
unique to A+A collisions and shows an abrupt variation with collision
centrality. Figure 4-33 shows the correlation for d+Au and Au+Au
collisions formed with intermediate-*p*~T~ particle pairs. The
long-range correlation is only present in the Au+Au data.

![](media/image45.png){width="2.75in"
height="1.9152777777777779in"}![](media/image47.png){width="2.533333333333333in"
height="2.0in"} ![](media/image48.png){width="2.765972222222222in"
height="1.9319444444444445in"}

Figure 4‑33: Correlations of intermediate-p~T~ particles in d+Au (left)
and Au+Au (right) collisions at √s~NN~ = 200 GeV.

The source of these correlations is under debate, but their extent to
large values of indicates that they must arise earlier in the collision
evolution, before the different regions become causally disconnected.
Figure 4-34 shows the amplitude, width, and volume of the near-side
correlation in Au+Au collisions formed with all unique pairs of charged
particles.

![](media/image49.png){width="5.78125in" height="1.6979166666666667in"}

Figure 4‑34: From left to right: amplitude, pseudorapidity width, and
volume of the same-side Gaussian correlation component versus transverse
particle density. Binary scaling predictions of minijet correlations in
proton-proton collisions are indicated by the blue dashed curves.

Data from two collision energies are shown in 7. For both energies, the
abrupt transitions of amplitude and width occur at the same value of
tranverse density. This is a non-trivial observation since these
correspond to different *N*~part~ or *N*~binary~ values, indicating that
*N*~part~ and *N*~binary~ are not relevant to the physical mechanism
causing the abrupt change. Finding the correct scaling quantity can help
to elucidate the mechanism responsible for the development of the
correlation structure. With the two energies studied, there are still at
least two possible scaling quanties consistent with the data; transverse
density and Bjorken energy density. These variables differ by constants
and a factor of *m*~T~. By carrying out a similar analysis at different
beam energies corresponding to significantly different *m*~T~ values, we
can better establish what mechanism leads to the abrupt transition in
the peak width and volume. For example, is this related to the
liberation of quark and gluon degrees of freedom at a particular energy
density, or is it due to multiple scattering of minimum-bias partons?
This is one example of an area in which the RHIC beam energy scan can
provide important new insights into questions raised by studies at
higher energies. It is estimated that these studies require 5-10 0
million events at several center of mass energies between 10 and \~40
GeV.

**Proven Performance at Low Energies:** An engineering study was
undertaken by C-AD and the RHIC experiments at the end of the most
recent run (Run 8). It is a major challenge to inject and collide ions
at the low end of the desired energy range, since that is well below the
design injection energy of RHIC, which corresponds to a center-of-mass
energy of 19.6 GeV [^53]. During this study, the goal of the C-AD
accelerator physicists was to demonstrate the injection and collision of
Au ions at energies of 9.2 and 5 GeV. The primary goal was trigger
testing, but there was also a desire to perform preliminary physics
studies. The peak luminosity achieved at 9.2 GeV was determined to be
3.2 x 10^23^ cm^-2^s^-1^ and the average luminosity was 1.5 x 10^23^
cm^-2^s^-1^. These luminosity values are expected to improve by a factor
of 2-10 for future runs.

  ----------------------------------------------------- -------------------------
  ![](media/image50.png){width="3.3020833333333335in"   Figure 4‑35: **Event
  height="3.3854166666666665in"}                        display of a
                                                        high-multiplicity (upper
                                                        panel) and a
                                                        low-multiplicity (lower
                                                        panel) Au+Au collision at
                                                        9 GeV.**

  ----------------------------------------------------- -------------------------

shows the display of tracks in the STAR TPC for two events. The event
shown in the top panel is for a typical high-multiplicity event and the
one in the bottom panel is for a lower-multiplicity event. The
reconstructed tracks clearly show the position of the vertex in the
event display and will allow a precise extraction of the vertex position
in off-line analysis. 9 shows a preliminary distribution of the vertex
spread in the beam (*z*) direction. The preliminary values of the
transverse size of the beam are about 2 mm, which is only slightly
larger than values found at higher energies. Detailed analysis is in
progress to establish the beam characteristics more precisely and also
to perform a variety of physics analyses. The STAR measures of various
trigger rates will be combined with information from the accelerator
physicists to establish a much more reliable estimate of the rates to be
expected in the future energy scan.

  ---------------------------------------------------- --------------------------
  ![](media/image51.png){width="3.301388888888889in"   Figure 4‑36: **Vertex
  height="2.2583333333333333in"}                       distribution for Au+Au
                                                       collisions at 9 GeV.**

  ---------------------------------------------------- --------------------------

## Beam use request for Run 10

In establishing our request for Run 10, we have considered how to
maximize the physics impact while keeping the duration of this initial
low energy physics run within reasonable bounds. To start the beam
energy scan program at RHIC, we request 14 weeks of Au+Au collisions
dedicated to the energy scan. The goals that we can reasonably expect to
accomplish are limited by the low event rates expected at the lower beam
energies. These event rates can be improved through upgrades such as
electron cooling in RHIC. However, these upgrades will not be available
by Run 10.

We expect to gain new insight into the near-side correlations by
gathering at least 5 million usable events at 12.3 and 17.3 GeV, and 10
million events at 28 and 39 GeV. It should be noted that the exact
values of all beam energies used will depend on matching the RHIC RF
frequency and harmonic number to optimize collisions at both PHENIX and
STAR, and, at approximately 39 GeV, to avoid the transition energy in
RHIC. These approximate values of the energy are selected in order to
provide data near the critical point estimates suggested by Gavai and
Gupta [^54], and in order to gain at least two more energies where we
expect to be able to resolve a near-side peak in the two-particle
correlations. Possible trends suggesting the disappearance of the
near-side peak in *p*~T~ and number correlations can also be verified
with these data sets.

In order to check for a discontinuity in the K/ ratio as suggested by
NA49 data and to improve the precision of the K/ fluctuations, we also
require at least one million events for energies at 6.1, 7.7 and 8.6
GeV. In most cases the SPS energies cannot be exactly matched due to
accelerator limitations. These data sets will allow improvements to the
K/ fluctuation measurements but may not allow for more differential
analyses, which are useful for a more detailed interpretation of the
measurements. Such analyses and the very lowest data points such as 5
GeV and 6 GeV must be revisited once better rates can be provided by
RHIC. Towards this goal, we request about 7 days for an engineering run
at 5 GeV during Run 10. The program as described above allows us to
accomplish a significant fraction of our physics goals in a 14-week
period, while taking into account the limitations of the RHIC facility.
Greater statistics and significant event samples for energies near 5 and
6.GeV will need to be accomplished during future runs after accelerator
upgrades. 0 shows the expected uncertainty after the initial beam energy
scag.

  ----------------------------------------------------- --------------------
  ![](media/image52.png){width="3.8270833333333334in"   Figure 4‑37:
  height="3.2666666666666666in"}                        **Current
                                                        measurements of the
                                                        dynamic width of the
                                                        distribution of kaon
                                                        to pion ratios along
                                                        with the estimated
                                                        errors from a Run 10
                                                        beam energy scan.
                                                        Estimates are made
                                                        for one million
                                                        minimum bias events
                                                        at each energy.
                                                        Larger event samples
                                                        requested will allow
                                                        for more
                                                        differential
                                                        studies.**

  ----------------------------------------------------- --------------------

Table 5: Run plan for a 14 week physics +1 week commissioning beam
energy scan.

  ------------- ----------- ----------------- -------------- --------------
  **s~NN~       **~B~       **Approximate     **Goal         **Duration
  \[GeV\]**     \[MeV\]**   Event Rate        \[Events\]**   \[Days\]**
                            \[Hz\]**                         

  5.0           550         0.5               Testing        7

  6.1           491         1.4               1M             23

  7.7           410         2.7               2M             20

  8.6           385         4                 2M             15

  12.3          300         10                5M             15

  17.3          229         25                10M            12

  27            151         30                10M            7

  39            112         50                10M            6
  ------------- ----------- ----------------- -------------- --------------

Table 6: Run plan for a 12 week physics +1 week commissioning beam
energy scan.

  ------------ ------------ ----------------- -------------- --------------
  **s~NN~      **~B~        **Approximate     **Goal         **Duration
  \[GeV\]**    \[MeV\]**    Event Rate        \[Events\]**   \[Days\]**
                            \[Hz\]**                         

  5.0          550          0.5               Testing        7

  6.1          491          1.4               1M             20

  7.7          410          2.7               2M             20

  8.6          385          4                 1.5M           10

  12.3         300          10                5M             15

  17.3         229          25                5M             6

  27           151          30                10M            7

  39           112          50                10M            6
  ------------ ------------ ----------------- -------------- --------------

# Heavy ion collisions at 200 GeV

## Triggered probes and heavy flavor with heavy ions

With the luminosities available at RHIC, collision rates are too large
for all events to be written to tape with the STAR DAQ. Therefore,
triggers are needed to sample the events for rare probes, the most
discriminating of which use the Barrel Electromagnetic Calorimeter in a
number of ways. A simplified history of sampled luminosity is shown
inError: Reference source not found Table 7, which compares by run and
species the integrated luminosity sampled by the Barrel EMC High Tower
trigger with the highest threshold, requiring all detectors to be read
out. A comparison between species can be made based on the binary
scaling hypothesis, in which the pp-equivalent luminosity in collisions
between species A and B scales as A\*B. This approximation is only
roughly correct: for probes that are suppressed, such as hadrons at high
p~T~, the combination of suppression and centrality cuts can reduce the
rates of triggered probes relative to this expectation by up to an order
of magnitude. The integrated luminosity has been steadily progressing,
so that we now have on the order of 10 pb^-1^ pp-equivalent integrated
luminosity sampled in p+p, d+Au, and Au+Au collisions at 200 GeV.

Table 7: Integrated luminosity in highest threshold Barrel EMC high
tower trigger with full detector readout. pp-equivalent luminosity for
collisions between species A and species B is calculated as
A\*B\*luminosity.

+------+-----------+----------------+---------------+------------------+
| **R  | *         | **Threshold    | **Integrated  | *                |
| un** | *System** | \[GeV\]**      | Luminosity**  | *pp-equivalent** |
|      |           |                |               |                  |
|      |           |                |               | **L \[pb^-1^\]** |
+------+-----------+----------------+---------------+------------------+
| 3    | d+Au      | 4.5            | 1.7 nb^-1^    | 0.7              |
+------+-----------+----------------+---------------+------------------+
|      | p+p       | 4.5            | 0.17 pb^-1^   | 0.17             |
+------+-----------+----------------+---------------+------------------+
| 4    | Au+Au     | 3/sin(theta)   | 50 ub^-1^     | 2                |
+------+-----------+----------------+---------------+------------------+
|      | p+p       | 4.6/sin(theta) | 0.15 pb^-1^   | 0.15             |
+------+-----------+----------------+---------------+------------------+
| 5    | Cu+Cu     | 3.75           | 860 ub^-1^    | 3.4              |
+------+-----------+----------------+---------------+------------------+
|      | p+p       | 4.2            | 2.8 pb^-1^    | 2.8              |
+------+-----------+----------------+---------------+------------------+
| 6    | p+p       | 5.4            | 11 pb^-1^     | 11               |
+------+-----------+----------------+---------------+------------------+
| 7    | Au+Au     | 5.5            | 0.5 nb^-1^    | 20               |
+------+-----------+----------------+---------------+------------------+
| 8    | d+Au      | 8.4            | 36 nb^-1^     | 14               |
+------+-----------+----------------+---------------+------------------+
|      | p+p       | 4.3            | 3.1 pb^-1^    | 3.1              |
+------+-----------+----------------+---------------+------------------+

Before Run 9, the STAR TPC has limited the luminosity that can be
sampled, since it had linear deadtime that would rise with the rate of
triggers that are written to tape. In practice, this led to a loss of
50-70% of the events that could be sampled by the trigger, since
triggered programs competed with large samples of minbias and central
events, and the size of the triggered datasets themselves also need to
be considered. With DAQ1000 for Run 9 and beyond, this limitation has
been largely removed, so that the triggered program can both easily
coexist with the untriggered program and essentially double its
efficiency in sampling the integrated luminosity delivered by the
accelerator.

The next step is to increase these sampled luminosities by orders of
magnitude. With stochastic cooling in RHIC II, in one RHIC year STAR can
sample up to 20 nb^-1^ in Au+Au collisions. A strawman for the
progression of luminosities is shown in. Table 8. The following section
will demonstrate a sample of the measurements that can be made within
this scenario.

Table 8: Strawman for increase in integrated sampled luminosity at s~NN~
= 200 GeV over the next five years.

+------+--------+---------------+-------------+-----------------------+
| **R  | **Sy   | **Total       | **pp        | **Physics from run**  |
| un** | stem** | Integrated    | -equivalent |                       |
|      |        | Sampled       | Sampled     |                       |
|      |        | Luminosity**  | L           |                       |
|      |        |               | uminosity** |                       |
+------+--------+---------------+-------------+-----------------------+
| 9-10 | p+p    | 50 pb^-1^     | 50 pb^-1^   | Reference data for:   |
|      |        |               |             |                       |
|      |        |               |             | First attempt at      |
|      |        |               |             | separation of Upsilon |
|      |        |               |             | 1S from 2S+3S         |
|      |        |               |             |                       |
|      |        |               |             | Further study of e-D  |
|      |        |               |             | from B                |
|      |        |               |             |                       |
|      |        |               |             | J/Psi to high p~T~    |
+------+--------+---------------+-------------+-----------------------+
| 9-10 | Au+Au  | 0.5 nb^-1^    | 20 pb^-1^   | Electron spectra for  |
|      |        |               |             | R~AA~ with low        |
|      |        |               |             | material to resolve   |
|      |        |               |             | PHENIX/STAR           |
|      |        |               |             | discrepancy           |
+------+--------+---------------+-------------+-----------------------+
| 11   | p+p    | 100 pb^-1^    | 100 pb^-1^  | Extension of          |
|      |        |               |             | reference data from   |
|      |        |               |             | Runs 9-10             |
+------+--------+---------------+-------------+-----------------------+
| 12   | Au+Au  | 5 nb^-1^      | 200 pb^-1^  | High precision        |
|      |        |               |             | -hadron, Upsilon,     |
|      |        |               |             | J/Psi, e-D with HFT   |
|      |        |               |             | for bottom            |
+------+--------+---------------+-------------+-----------------------+
| 13   | p+p    | 200 pb^-1^    | 200 pb^-1^  | Reference for Run 12  |
+------+--------+---------------+-------------+-----------------------+
| 13+  | Au+Au  | 20 nb^-1^     | 800 pb^-1^  | High precision        |
|      |        |               |             | utilization of RHIC   |
|      |        |               |             | II luminosity         |
+------+--------+---------------+-------------+-----------------------+

**-hadron correlations:** Correlations between a direct photon and a
hadron have been termed a "golden probe" of parton energy loss in the
medium. A direct photon does not interact with the medium, and so does
not lose energy. RHIC sits at a sweet spot in terms of backgrounds, as
the nearly exclusive source of coincident photons and hadrons in the
relevant kinematic regime is QCD Compton scattering between a quark and
a gluon. Photons from pion decay are subdominant, and
Next-to-Leading-Order processes such as parton fragmentation into
photons are expected to be small, on the order of 10%, as a first
measurement from PHENIX indicates. From this clean process, both the
identity and kinematics of the parent parton of the associated hadron
are cleanly tagged. This allows for a precise measurement of the
modification of the fragmentation function of a quark due to its
interactions in medium.

The challenge with photon-hadron correlations is the low production
rate. Figure 5-38 shows results to date, shown at QM2008, for
measurements of direct photon-hadron correlations. While the results are
within errors of theoretical predictions, the size of the uncertainties
do not provide sufficient distinguishing power between different
theoretical scenarios.

  ----------------------------------------------------- ------------------------
  ![](media/image53.png){width="3.6909722222222223in"   **Figure 5‑38:
  height="2.9208333333333334in"}                        Suppression of away-side
                                                        hadrons associated with
                                                        direct photons from Run
                                                        7 Au+Au (0.5 nb^-1^) and
                                                        Run 6 p+p (9 pb^-1^).
                                                        Bars at the origin at
                                                        unity denote uncertainty
                                                        in the p+p reference,
                                                        error bars indicate
                                                        Au+Au statistical
                                                        uncertainties, and grey
                                                        bands denote systematic
                                                        uncertainties.**

  ----------------------------------------------------- ------------------------

+--------------------------------+-------------------------------------+
| ![](media/image54.pn           | ![](media/image5                    |
| g){width="2.759027777777778in" | 5.png){width="3.0680555555555555in" |
| height="1.961111111111111in"}  | height="1.979861111111111in"}       |
|                                |                                     |
| Figure 5‑39: **Prediction for  | Figure 5‑40: **Projection for       |
| sensitivity of suppression of  | statistical uncertainties in        |
| -hadron away-side correlations | -hadron suppression in a scenario   |
| to different scenarios, where  | of luminosity progression in        |
| E~T~ \> 15 GeV. From T. Renk,  | different years. Projection is for  |
| Phys. Rev. C74 (2006)          | E~T~\> 15 GeV, associated particle  |
| 034906.**                      | p~T~ from 4-6 GeV/c.**              |
+--------------------------------+-------------------------------------+

In order to make progress, more luminosity is needed. Figure 5-39 shows
one set of predictions under different theoretical scenarios of the
suppression pattern, for a somewhat higher trigger E~T~, \> 15 GeV, than
STAR has measured to date. This higher trigger E~T~ is needed to ensure
that the associated hadron is both at relatively low fractional momentum
with respect to the trigger photon and at high enough p~T~ to be cleanly
associated with jet phenomena. Figure 5-40 shows a projection of the
evolution of the uncertainties in a representative bin, 4-6 GeV/c
associated particle p~T~ for one scenario of evolving luminosity sampled
by STAR in the coming runs. This bin shows the largest deviation between
theoretical scenarios, though the scenarios with the largest deviations
are currently disfavored. As shown in the figure, incremental progress
can be made over the next five years, as cooling in RHIC ramps up to the
full luminosity of RHIC II. Beyond this, studies of suppression focusing
on the orientation of the trigger with respect to the reaction plane,
and for identified hadrons combining the full Barrel Time of Flight
detector with dE/dx in the TPC, will be possible in certain kinematic
regions. With RHIC II luminosities, precise determination of the
modification of quark fragmentation functions in medium will be possible
with these correlation measurements.

+--------------------------------+-------------------------------------+
| ![](media/image56.pct          | ![](media/image5                    |
| ){width="2.8097222222222222in" | 7.png){width="2.9305555555555554in" |
| height="1.9895833333333333in"} | height="2.6868055555555554in"}      |
|                                |                                     |
| Figure 5‑41: **J/Psi           | Figure 5‑42: **J/Psi R~AA~ in Cu+Cu |
| differential cross section at  | collisions, as comparted to a       |
| mid-rapidity from p+p          | theoretical expectation.**          |
| collisions at √s=200 GeV.**    |                                     |
+--------------------------------+-------------------------------------+

**Quarkonia:** At Quark Matter 2008, STAR showed J/Psi spectra out to
transverse momenta of 14 GeV/c from triggered data from p+p and Cu+Cu
collisions in Runs 5 and 6, as shown inError: Reference source not found
.As by comparing STAR data across species and with PHENIX data, it
appears that J/Psi suppression decreases at high p~T~ and R~AA~
approaches unity, in contradiction to some theoretical expectations, se
Figure 5-42. However, this statement is statistically limited, at
approximatey 2- sigma, so further measurements are sorely nee Error:
Reference source not found Figure 5-4310 shows a projection of
statistical uncertainties after the integrated sampled luminosities
proposed for Runs 9-10, assuming that the increased bandwidth of DAQ1000
allows the thresholds to remain constant as luminosity increases, and
that R~AA~ is unity. Should J/Psi be suppressed at high p~T~,
uncertainties will increase accordingly. While such integrated
luminosities would greatly extend the reach of J/Psi measurements at
RHIC, precision measurements await the higher luminosities of RHIC II,
as sown in Error: Reference source not found Figure 5-44 for
luminosities expected by Run 13. At lower p~T~, the addition of the full
Time of Flight barrel will greatly decrease backgrounds, and should
enable significant measurements, though effective triggering is still
under study to understand the statistical significance that can be
achieved.

+-----------------------------------+-----------------------------------+
| ![]                               | ![](media/image59.                |
| (media/image58.png){width="3.0in" | png){width="2.9131944444444446in" |
| height="2.247916666666667in"}     | height="2.223611111111111in"}     |
|                                   |                                   |
| Figure 5‑43: **Projection for     | Figure 5‑44: **Projection for     |
| statistical uncertainty in J/Psi  | statistical uncertainty in J/Psi  |
| R~AA~ at high p~T~, assuming      | R~AA~ at high p~T~, assuming      |
| R~AA~=1, from integrated sampled  | R~AA~=1, from integrated sampled  |
| luminosity of 0.5 nb^-1^ in Au+Au | luminosity of 5 nb^-1^ in Au+Au   |
| collisions and 50 pb^-1^ in p+p   | and 200 pb^-1^ in p+p collisions, |
| collisions, as proposed in Runs   | as proposed** **to occur by the   |
| 9-10.**                           | end of Run 13.**                  |
+-----------------------------------+-----------------------------------+

Upsilons remove many of the ambiguities that surround J/Psi suppression,
since the b quark production cross-section is not large enough for
recombination to be appreciable, and interactions between the Upsilon
and co-moving hadrons are expected to be weak, leading to a cleaner
signature of suppression at early times. Different states of Upsilon
(1S, 2S, and 3S) are expected to be affected by the medium at different
levels: the 1S state is expected to be unsuppressed, while the 2S and 3S
will be successively more strongly suppressed. By studying the full
pattern of suppression, one can place constraints on the temperature
and/or energy density of the medium. STAR has begun the Upsilon program
with Runs 6 and 7. Shown in Figure 5-45 and **Figure 5-46Error:
Reference source not found** are mass peaks from p+p collisions in Run 6
and Au+Au collisions in Run 7, both peaks of which have significance of
approximatel 4- sigma. A comparable dataset was collected in d+Au
collisions n Rrun 8.

+------------------------------------+---------------------------------+
| ![](media/image6                   | ![](media/image61.pn            |
| 0.png){width="3.029166666666667in" | g){width="2.8319444444444444in" |
| height="2.6222222222222222in"}     | height="2.4902777777777776in"}  |
|                                    |                                 |
| Figure 5‑45: **Upsilon (1S+2S+3S)  | Figure 5‑46: **Upsilon          |
| mass peak from p+p run 6, 9        | (**1S+2S+3S) mass peak from     |
| pb^-1^.**                          | Au+Au R**un 7, 0.3 nb^-1^.**    |
+------------------------------------+---------------------------------+

After Run 7, STAR removed the inner tracking detectors the SVT and SSD,
which greatly reduce the amount of material seen by electrons, and so
reduces the broadening in the mass resolution in the di-electron channel
from Bremsstrahlung. Figure 5-47Error: Reference source not found shows
the expected resolution from simulation in the current configuration.
With this resolution, and 50 pb^-1^ sampled integrated luminosity from
p+p collisions in Run 9, a first measurement separating the 1S from the
2S and 3S states appears to be feasible. This will need to be followed
up with later runs integrating larger sampled luminosity in both p+p and
Au+Au.

  ----------------------------------------------------- ---------------------
  ![](media/image62.png){width="3.5416666666666665in"   Figure 5‑47:
  height="3.1in"}                                       **Expected mass
                                                        resolution in Runs 8
                                                        and beyond, with no
                                                        inner tracking
                                                        detectors**

  ----------------------------------------------------- ---------------------

The ultimate goal from RHIC II luminosity is shown in Error: Reference
source not found, which shows expected uncertainties in Upsilon(1S) that
can be achieved with 20 nb^-1^ sampled integrated luminosity in Au+Au
collisions and 100 pb^-1^ sampled integrated luminosity in p+p
collisions. With this dataset, precision measurements of the
dissociation of quarkonium states will be made possible.

  ------------------------------------------------------ ----------------
  ![](media/image63.png){width="4.35in" height="3.25in"} Figure 5‑48:
                                                         **Projection of
                                                         uncertainties in
                                                         Upsilon(1S)
                                                         R~AA~ for two
                                                         sets of
                                                         integrated
                                                         luminosity.**

  ------------------------------------------------------ ----------------

**Open Heavy Flavor:** Energy loss of heavy quarks tests perturbative
calculations of partonic energy loss in the medium in a way orthogonal
to photon-hadron correlations. Non-photonic electrons, produced
predominantly from decays of beauty and charm, are strongly suppressed,
providing a strong challenge to these calculations. There are, however,
a number of limitations to the measurements that cloud their
interpretation.

First, as shown in Error: Reference source not found, the uncertainties
on the spectrum of non-photonic electrons are quite large, and there is
an apparent discrepancy between STAR and PHENIX (though R~AA~ is
consistent between the experiments). Data from Run 8 will help to
resolve these issues, since the sampled integrated luminosity increased
by a factor of 30 relative to the published results, and without the SVT
and SSD the backgrounds from conversion electrons in STAR are comparable
to those in PHENIX.

The major limitation comes from the uncertainty in the relative
contribution of charm and beauty to the non-photonic electrons. Energy
loss calculations predict significant differences between the
suppression of charm and beauty, but perturbative calculations cannot
accurately predict the relative contribution of charm and beauty to
these non-photonic electrons. Without measuring this fraction
accurately, we are left with an unconstrained problem. First attempts at
measuring this fraction in p+p collisions, using correlations between
the electron and hadrons or fully reconstructed D's, are shown in Error:
Reference source not foundFigure 5-50. The measurements are consistent
with the most advanced calculation, a Fixed-Order Next-to-Leading-Log
calculation, but the data do not strongly constrain the uncertainties in
the calculation.

![](media/image64.png){width="4.623611111111111in"
height="3.5965277777777778in"}

Figure 5‑49: **Ratio of non-photonic electron spectrum in p+p collisions
at 200 GeV to a theoretical FONLL calculation. From a sampled integrated
luminosity of 0.1 pb^-1^.**

+-----------------------------------+-----------------------------------+
| ![](media/image65.                | ![](media/image66                 |
| png){width="2.8222222222222224in" | .png){width="2.832638888888889in" |
| height="2.1666666666666665in"}    | height="2.245833333333333in"}     |
|                                   |                                   |
| Figure 5‑50: **Relative           | Figure 5‑51: **Per-trigger yields |
| contribution of beauty to         | of reconstructed D mesons as a    |
| non-photonic electrons, from      | function of azimuth relative to a |
| correlation measurements, as      | trigger non-photonic electron     |
| compared to the uncertainty band  | with p~T~ between 4 and 8 GeV/c.  |
| from a FONLL calculation. Sampled | From 9 pb^-1^ of p+p collisions   |
| integrated luminosity in Run 6    | in Run 6.**                       |
| was 9 pb^-1^**.                   |                                   |
+-----------------------------------+-----------------------------------+

There are a number of future directions to constrain this situation.
First, with larger untriggered datasets, and with the Heavy Flavor
Tracker, direct D measurements out to high p~T~ will be possible. With
the TPC and TOF, with 200M events of any system D spectra can be
measured with 3-sigma accuracy out to p~T~ of 4-5 GeV. In p+p, with
DAQ1000, this is planned for Run 9. With the Heavy Flavor Tracker, D
spectra and R~AA~ can be measured with high accuracy out to 10 GeV by
Run 13.

In the triggered sector, correlations between electrons and fully
reconstructed D mesons are the most promising, and the most starved for
luminosity. Figure 5-51 shows the per-trigger yield of fully
reconstructed D^0^ mesons associated with a triggered non-photonic
electron within 4-8 GeV/c. From simulations, the near side peak is
almost exclusively from B decays, while the away-side peak is from a
combination of B and D decays. With 50 pb^-1^ sampled integrated
luminosity from p+p collisions in Run 9, a measurement of both the near
and away side peak will be possible at the 20% level. Studies of these
correlations are ongoing in the Cu+Cu and Au+Au datasets: precision
measurements of the suppression of these correlations in medium will be
possible in future years, aided greatly by the Heavy Flavor Tracker.

+-----------------------------------+----------------------------------+
| ![](media/image67.                | ![](media/image68.p              |
| png){width="3.1180555555555554in" | ng){width="2.9916666666666667in" |
| height="2.9916666666666667in"}    | height="2.9916666666666667in"}   |
|                                   |                                  |
| Figure 5‑52: **Signed DCA between | Figure 5‑53: **The same quantity |
| secondary vertex of e and K       | as shown in** **Figure           |
| candidates and the primary vertex | 5-52Error: Reference source not  |
| for all particles, for injected   | found, on**ly for Au+Au data     |
| B+, and the subtraction of        | taken during R**un 7. Total      |
| negative from positive signed DCA | sampled integrated luminosity,   |
| (blue). From simulations with     | after vertex cut, was 50 ub^-1^, |
| PYTHIA B decays injected into     | leading to a signal of high      |
| Hijing. Invariant mass between    | significance, shown in blue.**   |
| the e and K is required to be     |                                  |
| greater than 2 GeV to suppress D  |                                  |
| decays.**                         |                                  |
+-----------------------------------+----------------------------------+

A first step towards direct tagging of electrons from B decays, using
e-K correlations at high invariant mass and secondary vertex
reconstruction, was begun using the SVT and SSD in Run 7 Au+Au. The
analysis follows a similar approach to that taken by CDF. Electrons,
triggered with the Barrel Electromagnetic Calorimeter with p~T~ \> 4
GeV, are paired with hadron candidates to form secondary vertices which
approximate the decay vertex of a heavy (D or B) meson. Since the D mass
is 1.8 GeV, a cut on the invariant mass between the electron and hadron
above 2 GeV strongly suppresses electrons from D decays. The signed
distance of closest approach between the secondary and primary vertex,
signed by cos() between the DCA vector and the hadron momentum vector,
provides further discriminating power and the ability to measure
backgrounds, since those pairs from B decay occur predominantly at
positive values in this quantity. By subtracting the counts at negative
signed DCA from positive signed DCA one obtains a clean signal for
electrons from B decays, as shown in Figure 5-52 from simulation and
Figure 5-53Error: Reference source not found Error: Reference source not
found from data. With the integrated luminosity sampled n Rrun 7,
approximately 0 ub^-1^ after cuts on primary vertex position, a clear
signal of high significance is seen in the data. Future measurements at
much higher luminosity and vertex reconstruction precision will be
possible with the HFT, allowing for precision study of the interaction
of b quarks with the medium.

**Heavy Flavor Tracker:** Precision measurements of charm and beauty
require the Heavy Flavor Tracker (HFT). The HFT is still a project in
its early days, but is an important upgrade to STAR. In connection with
the CD-0 review at BNL on February 25 and 26 the necessary beam
conditions were outlined to fulfill the science milestones for the
project. The actual year when the project will be ready to take data is
of course uncertain, depending on funding and construction success. The
technical driven plan presented at the CD-0 review have completion in
time for the Run 12. It is, however, noted that the current planned
start of construction funding in FY10 will imply a delayed schedule.
Table 9 below is based on this technical schedule.

## Triggered probes and heavy flavor beam use request

Table 8, in the previous section, provides a strawman plan for the
year-by-year increase in luminosity towards the ultimate goal of RHIC II
for triggered probes. In Runs 9-11, the main goals are to increase the
statistical power of the reference in p+p collisions, and to make use of
moderate amounts of integrated luminosity in Au+Au, in the time window
between the removal of the SVT/SSD and the installation of the HFT, to
make measurements in the electron sector that benefit from a low amount
of material. Starting with Run 12, we expect to begin the precision
program of RHIC II in earnest, with an order-of-magnitude increase in
sampled integrated luminosity in Au+Au collisions and the installation
of the Heavy Flavor Tracker. Specific needs for the Heavy Flavor tracker
are described bellowed and surmised in Table 9.

  ----------------- ------- ---------------------- ------------------------
                                                   

                                                   

                                                   

                                                   

                                                   

                                                   

                                                   

                                                   

                                                   

                                                   

                                                   

                                                   

                                                   
  ----------------- ------- ---------------------- ------------------------

**Run 11:** It is an important intermediate goal for the HFT upgrade to
have the mechanical structures for the Pixel, and beam pipe in place for
an engineering run. The Pixel would be instrumented with a few (2-3)
ladders with sensors of the Phase-1 type. The primary purpose is to
ensure the sensors and readout systems work in the RHIC environment, and
gain experience with quality of data, alignment in preparation for the
complete installation. The configuration will be chosen such that a
partial measurement of directly identified D^0^ spectrum is possible in
a limited pt range.

**Run 12:** In the first run after completion of the HFT we anticipate
taking 500M minimum bias events of Au+Au collisions at 200 GeV. This
will allow us to measure v~2~ of D^0^ in the p~t~ range from 600 MeV/c
up to a p~t~ of larger than 5 GeV/c. The absolute error on the
measurement will be 0.009 for the p~t~ bin from 600 to 1000 MeV/c and
0.007 for the p~t~ bin from 3.75 to 4.25 GeV/c. This will address the
question if the light quarks are thermalized.

From the same 500M events we also will be able to measure the p~t~
dependence of R~cp~ of D^0^ mesons in the p~t~ range from 1 GeV/c to 10
GeV/c. For the central events we take the 10% most central events and
for the peripheral events we chose the centrality bin of 60 to 80%.
R~cp~ will be determined to 5% in the p~t~ bin 1.75 to 2.25 GeV/c and to
16% in the p~t~ bin 7.5 to 8.5 GeV/c. The errors are for the case that
the open charm suppression is the same as for light quark mesons. This
measurement will further our understanding of the energy loss mechanism
in the hot and dense medium created at RHIC. All errors are statistical
only.

**Run 13:** In the second year we anticipate a long p+p run at 200 GeV.
In a run of 3 months we expect to be able to record 2 billion minimum
bias events, with which we will measure the p~t~ dependence of D^0^
production and achieve a precision of 17% for the p~t~ bin between 7 and
8 GeV/c. The total D^0^ production cross section will be measured with a
precision of better than 15%. This is important input for pQCD
calculations and the combination of p+p and Au+Au data will enable us to
determine R~AA~.

**Run 14:** In the third year we anticipate a high statistics Au+Au run.
With 1,000M minimum bias events we will be able to perform impact
parameter selected v~2~ and R~AA~ measurements. In the centrality bin
with the largest v~2~ signal, 40 -- 60%, the statistical error in the
D^0^ production will be less than 2.5% in each p~t~ bin, with p~t~ = 0.5
GeV/c, in the p~t~ range from 1 to 4 GeV/c. Finite reaction plane
resolution will dominate the v~2~ errors, the maximum error being about
10%. The systematic measurement of the centrality dependence will allow
us to address the issue of light quark thermalization and determine the
collective velocity of charm quarks. In the third year we will have
accumulated a ~c~ spectrum with about 14,000 identified ~c~ in the p~t~
range of 2 to 10 GeV/c.

Table 9: Heavy Flavor Tracker Timeline and Beam Use Request

+----------------+------+---------------------+-----------------------+
| **Inst         | **   | **Physics**         | **Request**           |
| allation/Run** | FY** |                     |                       |
+----------------+------+---------------------+-----------------------+
| Run 11         | FY11 | Commissioning       | HFT engineering run   |
|                |      |                     | (A+A at 200 GeV)      |
+----------------+------+---------------------+-----------------------+
| Run 12         | FY12 | v~2~(pt) for D^0^;  | 200 GeV Au+Au, 500M   |
|                |      | R~CP~               | minbias               |
+----------------+------+---------------------+-----------------------+
| Run 13         | FY13 | D^0^ reference      | 200 GeV pp, 2,000M    |
|                |      | spectrum to 10      | minbias               |
|                |      | GeV/c               |                       |
+----------------+------+---------------------+-----------------------+
| Run 14         | FY14 | C                   | 200 GeV Au+Au, 1,000M |
|                |      | entrality-dependent | minbias               |
|                |      | v~2~(pt)            |                       |
|                |      |                     |                       |
|                |      | ~c~ spectrum        |                       |
+----------------+------+---------------------+-----------------------+

## Non-triggered datasets at 200 GeV: Recent heavy ion physics results 

**Recent relativistic heavy ion physics results:** A key strength of
STAR is the ability to identify a wide variety of the hadron species
produced in the collision. Stable charged particles are identified
through dE/dx measurements and weakly decaying strange and multi-strange
hadrons from their decay vertices. Various resonance peaks can be
revealed by combining particles in pairs either without identification,
or by using one or both of the identification techniques mentioned
above. These capabilities have allowed us to address a variety of
physics topics by measuring the yields and transverse momentum spectra
of a wide range of particles. The other major advantage of the STAR
detector is the large azimuthal acceptance, which has allowed many
correlation and other event-by-event analyses to be made. RHIC has
provided A+A collisions at several energies and with two species (Au and
Cu). This has enabled detailed systematic studies as a function of
system size, geometry and energy to be performed.

**Results on Spectra and Yields:** The recent progress in this area has
been on two fronts. One is the use of the Run 5 Cu+Cu data from s~NN~ =
62 and 200 GeV collisions to explore the dependence of particle yields
and spectra on the system size and, for systems with a similar number of
participant nucleons (N~part~), the geometrical shape. The strange
particle yields as a function of N~part~ have long been of interest to
further understand the origins of strangeness enhancement. Cu+Cu data
covered a region in N~part~ where the uncertainty on N~part~, determined
from a Glauber model, for semi-peripheral Au+Au collisions is rather
large. It is over this region (N~part~ 100) that some of the largest
changes in thermal fit parameters occur. Although the analysis is being
finalized and the interpretation is not settled we can say that the
enhancement for Cu+Cu and Au+Au at the same N~part~ is not identical, in
that the enhancement in Cu+Cu is larger than in Au+Au collisions.[^55]

This could be related to the energy density achieved if the collision is
more compact or, more speculatively, could be due to a core-halo
scenario where the ratio of core and halo contributions differs
depending the shape (surface to volume effect). In a core-halo model,
the independent collisions in the halo should be like p+p collisions.
One of the weaknesses in our current data comes from the large errors on
the p+p collision yields, in particular for Omega, where the fractional
error is 80%[^56]. A measurement using 600 M minimum bias p+p events
would provide as accurate a mesurement for the Omega as the current Xi
measurement made with 15 M events. It is expected that the TOF system
will relax this target somewhat. This measurement would complete the
program with strange particle yields at s~NN~ = 200 GeV.

The second area of progress is the extension of identified particle
spectra to higher p~T~. STAR reported on the nuclear modification
factors (R~AA~) of protons and pions, and showed that their partonic
sources had similar energy loss[^57]. This was contrary to initial
expectations, as gluons are thought to favor baryon production and, due
to the fundamental coupling strengths in QCD, gluons are expected to
interact more strongly with the medium than quarks. We have begun to
explore this finding with further analysis, the most recent being the
extension to higher p~T~ of the rho measurement, presented at Quark
Matter 2008. Clearly it would be of benefit to measure R~AA~ for further
hadron species. The Run 5 Cu+Cu and Run 4 Au+Au data sets are large
enough to allow the strange particle spectra to be measured to above 7
GeV in p~T~, and using the Run 7 data on tape can extend the latter
dataset. There is, however, no suitable reference data. A large minimum
bias p+p sample is required for this reference. Our estimates suggest
that at least 300 M events would be required.

The biggest single improvement for future Au+Au running comes from the
installation of the STAR TOF system. This will extend particle
identification above the present limits from dE/dx at low p~T~, where
proton to pion and pion to kaon separation ceases to be possible at
around p~T~ \~ 1.2 and 0.7 GeV, respectively. The TOF also provides
clean electron and positron identification. The resonance program would
benefit in a major way from minimum bias Au+Au collsions with the TOF.
K\* resonance production and v~2~ measurements will be greatly improved.
The measurement of phi by its decay to e^+^e^-^ becomes possible, which
provides reconstruction from particles decaying earlier in the collision
process, the decay products of which will not be affected by the later
hadron gas stage of the collision. It is estimated that a 10% error on
the peak in the data integrated over all p~T~ can be made with 7 M
minimum bias events, so a few hundred million events should be
sufficient to obtain p~T~ spectra. The continuum di-lepton spectra below
the mass peak can also be studied. Finally, we intend to search for U(1)
symmetry restoration by looking at the eta meson [^58]

  ---------------------------------------------------- ------------------------------------------------------
  ![](media/image69.png){width="3.498611111111111in"   ![](media/image70.jpeg){width="2.3743055555555554in"
  height="2.217361111111111in"}                        height="1.6395833333333334in"}

  Figure 5‑54: Preliminary v~2~/(n~qpart~) versus      Figure 5‑55: Preliminary measurements of the relative
  p~T~/n~q~ and (m~T~-m)/n~q~ for various centralities v~2~ fluctuations compared to various models as a
  in Au-Au collisions at 200 GeV.                      function of impact parameter.
  ---------------------------------------------------- ------------------------------------------------------

Progress has also been made in our understanding of v~2~ fluctuations.
Most theories indicate that the magnitude of the momentum-space
anisotropy (v~2~) is driven by the spatial anisotropy of the initial
state. However these calculations also show that this spatial anisotropy
can vary significantly between events with the same impact parameter.
Such eccentricity fluctuations should result in fluctuations in v~2~
that is preserved through the evolution of the system. ) shows STAR's
measured relative v~2~ fluctuations as a function of impact parameter.
The brown shaded region indicates the upper limit of the measured
relative fluctuation, while the curves indicate various theoretical
calculations. STAR's data are consistently beneath those derived from a
Glauber calculation using the standard eccentricity calculation.
However, the data is consistent with the magnitude of the fluctuations
calculated using the participant eccentricity and slightly above those
predicted from a calculation invoking Color-Glass-Condensate initial
conditions. In all cases the data rule out large fluctuations and
correlations from sources other than event-by-event differences in
geometry of the initial conditions.

**Non-triggered Correlations Results:** Also reported at QM2008 were
STAR's new results on non-triggered two-particle correlations. In this
analysis no transverse momentum requirement is placed on either of the
two particles, so that the correlations are formed from all unique pairs
of particles in an event. The resulting correlations will be sensitive
to any process that induces relative momentum correlations between final
state particle pairs in the acceptance. Such processes include, but are
not limited to, elliptic flow, quantum statistics, resonances and,
minijets. These minimum-bias jets with no a-priori p~T~ cut (though
dominated by \~1 GeV/c hadrons) are the focus of this analysis. Each
dynamical process generates a characteristic correlation structure on
relative azimuth and pseudorapidity. It has been shown by STAR that the
correlation data can be readily decomposed into a few analytic
structures that can be related to the above sources.

Minijet correlations, measured as the same-side (relative φ \< π/2)
two-dimensional Gaussian correlation structure, are clearly observed in
p+p collisions. One important application of this analysis is to study
what happens to these correlation patterns in heavy-ion systems as the
collision centrality increases. The null hypothesis is that, barring an
increase in amplitude in proportion to the number of binary collisions,
the minijet correlation will remain unchanged. While we observe an
approximate binary scaling in Au+Au collisions from most peripheral to
mid-central collisions, at about 55% and 40% centralities for the 200
and 62 GeV data, respectively, a sharp transition point is observed, as
shown in I. For more central collisions the minijet structure changes
dramatically. The peak amplitude grows rapidly and the relative η width
broadens by a factor of 2-3, while the φ width (not shown) narrows
significantly. This correlation structure becomes so large that it is
estimated to include approximately one third of all produced particles
in central collisions. STAR also observes that for both energies the
same-side peak amplitude, η width, and correlation volume scale with
transverse particle density, estimated as the number of final-state
particles per unit rapidity per unit of initial collision overlap area,
as shown inXI. This non-triggered correlation analysis technique enables
unprecedented access to the dynamics of low momentum particles and
provides a sensitive probe of the physics affecting the majority of
particles produced in heavy ion collisions. For example, these
correlation measurements can provide new information about the degree of
thermalization in RHIC collisions

  -----------------------------------------------------------------------

  -----------------------------------------------------------------------

**Hadron-Triggered Correlation Results**: The ongoing heavy ion physics
program with the STAR detector at RHIC has enabled remarkable advances
in the study of hot strongly interacting matter [^59]. An extended reach
in particle momentum with ongoing analysis of Run 4 Au+Au data as well
as analysis of the base-line p+p data sets from Runs 2 through 6 allowed
for further development of "hard probes" to study the behavior of the
created medium that was difficult to access at lower collision energies.
Data samples from lighter collided ions complement these developments by
providing a better handle on system size variations. Use of penetrating
probes is becoming increasingly sophisticated and is currently at the
brink of new advances, both with hadron-triggered correlations in large
non-triggered datasets, and, as has been discussed in an earlier
section, with datasets triggered by the Barrel Electromagnetic
Calorimeter. With the above-mentioned data sets STAR continues the
exploration of fundamental properties of QCD matter, providing further
constrains on theoretical models. Full capabilities for particle
identification coupled with large acceptance of STAR have enabled
advanced analyses of correlations and fluctuations, necessary for
quantitative tests of QCD predictions on energy loss, jet fragmentation
and hadronization in QGP medium. These capabilities have afforded a
number of first "proof-of-principle" measurements, documenting STAR's
ability to take advantage of planned upgrades and increased luminosity
available at RHIC II. Here we summarize briefly recent advances of
various correlations analyses and provide an outlook for the near term
future and planning.

**Identified Di-hadron Correlations:** The suppression of hadron yields
and back-to-back correlations has firmly established that jets are
quenched by very strong interactions with the matter produced in central
Au+Au collisions. The results are consistent with the effects of parton
energy loss in traversing the dense medium, predicted before the data
were available [^60]^,^[^61]. Those pQCD calculations appear to
efficiently describe both the centrality- and p~T~-dependence of the
observed suppression if energy densities of about 2 orders of magnitude
higher than normal nuclear density are assumed. Further explorations of
the hadron suppression with identified particle species provided new
challenges for the pQCD models. At intermediate p~T~, as illustrated in
Figure 5-56 and Figure 5-57 with proton and antiproton R~CP~, all
measured baryons show less suppression, with respect to binary scaling,
than measured mesons (see, for example, charged pions R~CP~ in the same
figure). As seen in the figure, the baryon/meson effects disappear at
high p~T~, where baryons and mesons show a common degree of suppression.
The theoretical calculations[^62]^,^[^63] expect significant (factor of
3) differences in energy loss for quarks and gluons, which should appear
in the nuclear modification factor for baryons and mesons due to
differences in contributions from quark vs. gluon fragmentation.

Di-hadron correlations provide additional means to probe the medium.
They permit the experimental observation of the energy lost by partons,
which appears in the form of an excess of softer emerging hadrons.
Measurement of particle spectra and ratios for associated hadrons in the
jet cones could address this issue of color-charge dependence of energy
loss explicitly. The current STAR program includes di-hadron
correlations with a variety of identified particles. Such measurements
test for the presence of meson/baryon effects at intermediate p~T~, and
explore the interplay between various hadronization mechanisms.
Hyperon-hadron correlations, shown in Figure 5-57, show significant and
similar, correlations for all species of hyperons up to and including
the Omega, in apparent contradiction to predictions from a coalescence
picture.

  ----------------------------------------------------- ------------------------------------------
  ![](media/image72.png){width="2.5381944444444446in"   ![](media/image73.png){width="2.84375in"
  height="2.0381944444444446in"}                        height="2.188888888888889in"}

  Figure 5‑56: R~CP~ for ++− and p+p in 200 GeV Au+Au   Figure 5‑57: Hyperon-hadron azimuthal
  collisions[^64]. The solid lines show jet quenching   correlations in central Au+Au collisions
  predictions for pions[^65].                           at 200 GeV.
  ----------------------------------------------------- ------------------------------------------

Higher statistics Au+Au data samples are required to address the
constituent quark dependences of correlation structures, as well as to
decompose the observed correlation peaks into spectral measurements.

**Mechanisms of Jet-Medium Interactions:** STAR is actively working on
understanding the mechanisms of jet interactions with a hot and dense
QCD medium via studies of jet-shape modifications in heavy ion
collisions. As seen in the 2, modifications of the correlation shapes
are quite significant in the intermediate and soft sectors. For some
particular kinematic selections of correlated particles the overall
broadening of the away-side distribution is accompanied by the
developing of a dip \~ . This shape of the away side distributions
stirred many theoretical discussion and
investigations^\[^[^66]^,^[^67]^,^[^68]^,\]^. In particular, shock wave
induced conical flow ("sonic boom"), Čerenkov gluon radiation, and a
"deflected jet" induced by interactions between the propagating partons
and the flowing medium were proposed to explain the oserved
modifications.

Preliminary STAR results on the three-particle azimuthal correlation
analysis [^69]^,^[^70] are shown in 3. The correlation surface reveals
more details about the jet fragmentation and propagation through the
medium. The elongation of away-side peak along the diagonal in the 1,2
space is already observed in pp and d+Au data and is interpreted as
possible effects of k~T~ broadening. This elongation becomes more
pronounced in Au+Au collisions potentially indicating the presence of
some additional mechanisms, such as for example deflected jets,
contributing to shape of the observed correlation. Observation of
off-diagonal peaks evident in more central Au+Au data is consistent with
conical flow or Čerenkov radiation scenarios. More advanced studies, for
example differential in p~T~, are needed to provide the discriminating
power between the two scenarios. Such studies require larger data
samples.

  ----------------------------------------------------- ---------------------------
  ![](media/image74.png){width="3.0416666666666665in"   Figure 5‑58: Comparison of
  height="2.063888888888889in"}                         the background-subtracted
                                                        distributions for d+Au and
                                                        central Au+Au data for the
                                                        associated particles in the
                                                        (1.3, 1.8) p~T~ range.

  ----------------------------------------------------- ---------------------------

+-----------------------------------------------------------------------+
| *p+p* d+Au Central Au+Au                                              |
|                                                                       |
| ![](media/image75.png){width="1.8381944444444445in"                   |
| height="1.55                                                          |
| 27777777777778in"}![](media/image76.png){width="1.8381944444444445in" |
| height="1.55                                                          |
| 27777777777778in"}![](media/image77.png){width="1.8381944444444445in" |
| height="1.5527777777777778in"}                                        |
+-----------------------------------------------------------------------+
| Figure 5‑59: **Background subtracted 3-particle azimuthal             |
| correlations for p+p (left), d+Au (center), and ZDC triggered 12%     |
| central Au+Au (right). Trigger particles are selected in the          |
| transverse momentum range of 3.0\<p~T~\<4.0 GeV/c, and the            |
| correlation function is build with hadrons with transverse momentum   |
| between 1.0\<p~T~\<2.0 GeV/c.**                                       |
+-----------------------------------------------------------------------+

Another three-particle correlation technique has been recently explored
by STAR to differentiate between various jet-medium interaction
scenarios proposed to accommodate the data. We capitalize on earlier
direct di-jet signatures, in which at high enough associated and trigger
p~T~, associated hadrons at the away-side in azimuth from the triggered
hadron show a clear back-to-back jet-like peak. Assuming that the
combination of trigger and hard away-side associated hadron determine a
jet-axis, we correlate a third, softer, hadron with the axis. The
comparisons of the projections of such tri-hadron (2+1) correlation for
central Au+Au and d+Au data at RHIC top energy reveal no modification of
shapes or suppression of yields for di-jets in the same kinematic range
where a typical di-hadron analysis shows dramatic changes. For the first
time for soft associated hadrons, the away-side correlation structure is
also peaked in both and , further confirming the di-jet nature of these
events. Statistics available limit the ability to vary the away-side
trigger threshold, necessary to further explore the geometric,
fragmentation, and energy-loss fluctuation biases inherent in the
theoretical explanation of these correlations.

**Jets and Ridges in correlations at RHIC:** The correlation structure
of charged hadrons in Au+Au collisions are dramatically modified not
only in , but also in the direction compared to a simpler *p+p*
reference. 4 shows the 2-dimensional correlation in -- space between a
high p~T~ trigger hadron and a lower p~T~ associated hadron in *p+p*
(left) and central Au+Au collisions (right). The near-side peak,
symmetric in -- in *p+p* collisions, is substantially broadened in the
direction in Au+Au collisions.

Within a two-component model, the structure appears to be composed of a
"jet" with properties similar to those seen in *p+p* collisions and a
"ridge" with properties similar to those of the bulk. The origin of the
ridge is still uncertain, but it has been speculated that the "ridge"
may be due to redistribution of energy lost to the medium through some
mechanism, such as expansion or unstable color modes, that breaks the
symmetry in azimuth and pseudorapidity.

  -------------------------------------- ---------------------------------------
  ![](media/image46.png){width="2.5in"   ![](media/image45.png){width="2.75in"
  height="1.9909722222222221in"}         height="1.9152777777777779in"}

  Figure 5‑60: Di-hadron correlations in 
  p+p (left) and central Au+Au           
  collisions (right). Trigger particles  
  are selected in the transverse         
  momentum range of 3\<p~T~\<4 GeV/c,    
  while the correlation function is      
  built with associated hadrons with     
  transverse momentum greater than 2     
  GeV/c.                                 
  -------------------------------------- ---------------------------------------

Of particular interest for multi-particle production models are
di-hadron correlation studies with multi-strange triggers. Due to the
quark mass difference it is expected that multi-strange hadron
production will be dominated by thermal quark recombination in the
momentum range accessible at RHIC. Di-hadron azimuthal correlations with
multi-strange hadrons such as , and φ will provide experimental
constrains on the validity or the extent of recombination influence in
hadronization at RHIC. Initially, parton recombination models predicted
that no jet-like azimuthal correlations would exist for multi-strange
baryons at the intermediate (2-4 GeV/*c*) p~T~ region. STAR measurements
of multi-strange hadron correlations in d+Au and Au+Au collisions,
showing jet-like structures for all measured species with no variation
in correlation strength based on strangeness content, has already
rejected those models. The theoretical explanations that followed those
results have suggested that observed azimuthal correlation signal is not
a direct result of jet fragmentation, but rather expanding medium
response to the propagating parton or \"phantom jet\". Both scenarios
preserve the correlation in azimuthal space, but in pseudorapidity space
the latter scenario predicts a ridge-like structure. To advance our
understanding of the nature of these correlations, extension of the
correlation analysis into - space for and baryons is necessary, which is
not possible with the limited statistics of our current datasample.

The details of hadro-chemistry of jet fragments can provide additional
constraints for the models. Preliminary STAR results show significantly
lower baryon/meson ratios in the jet-cone around a 4 GeV/c trigger
particle, as compared to non-triggered distributions. At the same time,
the ratios in the "ridge" part of the correlation structure suggest
enhanced production of the baryons that might be related to the
fragmentation of the gluons radiated of the hard-scattered quark
resulted in trigger.

  ----------------------------------------------------- -----------------------------------
  ![](media/image78.png){width="2.9034722222222222in"   
  height="2.191666666666667in"}                         

  Figure 5‑61: 3-particle correlations in - for 200 GeV 
  minimum bias d+Au (left) and central Au+Au collisions 
  (right). The trigger particle is selected in          
  transverse momentum range between 3--10 GeV/c and     
  associated particle p~T~ range is between 1--3 GeV/c. 
  ----------------------------------------------------- -----------------------------------

Further understanding of the source of the "ridge" can be provided by
adding an additional particle, and moving to a 3-particle correlation
analysis in - space. Preliminary results, with unidentified hadrons, are
presented in Figure 5-61 for minimum bias d+Au collisions (left) and
central Au+Au collisions (right) at 200 GeV. A clear jet peak is
observed in these correlations at (,)\~0 for both d+Au and Au+Au
collisions. In addition, a uniform overall excess of associated
particles is observed in Au+Au collisions. This observation is contrary
to most model predictions, which would lead to an excess of yield along
the diagonals or strips across the ~~=0 and ~2~=0. These early results
from a statistics-starved analysis, consistent with jet fragmentation
accompanied by an overall excess of associated yields with no further
correlation among the associated particles, already provide more
stringent constraints on the interpretation of ridge formation, and set
high expectations for future higher statistics data samples.

Correlations Relative to the Reaction Plane: In addition to the above
experimental observations in the near side from high p~T~ triggered
di-hadron correlations, STAR has begun a study of the near side "jet"
and "ridge" structures with respect to the reaction plane. The new
analysis can study path-length effects in greater detail, by exploiting
non-central collisions where the overlap region between the two
colliding nuclei is anisotropic: the size in the reaction plane
direction is shorter than that perpendicular. By selecting the trigger
particle direction with respect to the event plane, φ~S~=\|φ~T~-ψ~EP~\|,
one selects different path-lengths of the medium the away-side parton
traverses, providing more differential information than the inclusive
jet-correlation measurements. STAR has previously performed an
exploratory measurement of jet-correlations at high p~T~ with trigger
particles in-plane (\|φ~T~-ψ~EP~\| \< /4) and out-of-plane (\|φT-ψ~EP~\|
\> /4) in non-central Au+Au collisions[^71], in which away-side
particles correlated with out-of-plane trigger particles were found to
be more suppressed than those correlated to in-plane trigger particles.
We have extended these measurements to finer bins in \|φ~T~-ψ~EP~\| and
to lower associated and trigger p~T~ ranges. Minimum bias data from Run
4 provided approximately 20 million events for this analysis. The
results discussed below are for the trigger particle p~T~ range of
3\<p~T~^trig^\<4 GeV/c and centrality bin of 20-60%. Data from central
ZDC triggered sample and the d+Au run have been used for comparison
purposes.

  ----------------------------------------------------- -----------------------------------------------------
  ![](media/image80.png){width="2.7777777777777777in"   ![](media/image81.png){width="2.7777777777777777in"
  height="1.9965277777777777in"}                        height="1.9965277777777777in"}

  Figure 5‑62: **The near-side di-hadron correlation    
  amplitudes (\|\|\<0.35) for jet part (red) and ridge  
  part (blue) as a function of trigger particle         
  azimuthal angle from the event plane, φ~S~            
  =\|φ~T~-ψ~EP~\|. The systematic uncertainties on the  
  ridge amplitude are significant and indicated by the  
  curves**, **while those on the jet data are small.    
  The results are for 3\<p~T~^trig^\<4 GeV/*c* and      
  1.0\< p~T~^assoc^\<1.5 GeV/*c* in 20-60% (left panel) 
  and top 5% (right panel) Au+Au collisions.**          
  ----------------------------------------------------- -----------------------------------------------------

The away-side correlation broadens (and becomes double-peaked) with
increasing trigger particle azimuthal angle from the reaction plane. In
top 5% collisions, the away-side correlation is broader at small trigger
particle angle, but at trigger particle angle of 90^o^ from the reaction
plane, little difference is found between the two centralities, perhaps
consistent with the variations in the away-side medium path-length. The
near-side jet component varies little with trigger particle angle from
the reaction plane and generally agrees with that in d+Au. However, the
ridge component decreases significantly with the trigger particle angle
in collisions from 20-60% in centrality. The change is weaker in the
most central 5% collisions, consistent with the more spherical geometry.
The near-side modifications are quantified in Figure 5-62, where the
amplitudes of correlations are shown as function of φ~S~ for the "ridge"
part (blue points) in the 20-60% centrality (left panel) and the top 5%
centrality (right panel), respectively. The "jet" part amplitudes are
shown in the same figure in red, for the 20-60% centrality (left panel)
and the top 5% centrality (right panel), respectively.

The preliminary results of path-length effects studies suggest that the
near-side jet in the reaction plane direction interacts with the medium,
losing energy and generating a sizeable ridge over a long range. The
near-side jet perpendicular to the reaction plane suffers minimal
interaction with the medium, with no significant ridge and with the
correlated multiplicity only slightly higher than that in d+Au. One
should note that although the results are dominated by systematics, the
systematic uncertainties are correlated, thus statistical precision is
important, and a higher statistics sample is needed to assess the
reliability of systematic uncertainty estimates, and to expand these
studies as function of trigger p~T~.

## Future EM probes in STAR

Electromagnetic probes[^72] uniquely provide direct access to the
in-medium modifications of hadronic states (vector mesons ρ(770),
ω(782), φ(1020)) via dilepton invariant-mass spectra, which can
illuminate the nature of hadron mass generation and thus the origin of
∼98% of the visible mass in the universe (as well as related changes in
the structure of the QCD vacuum including the restoration of chiral
symmetry). In addition, at masses above ∼1 GeV there exists the
possibility of detecting novel nonperturbative (resonant) correlations
in a sQGP. Electromagnetic probes are also unique in inferring the
temperature of the system during its hottest phases via direct thermal
radiation of photons and dileptons radiation; additional HBT
interferometry of thermal photons offers the cleanest measurement of
early system sizes.

At STAR electron identification is made possible by a combination of a
measurement of the energy loss by charged particles due to ionization
(dE/dx) of the Time Projection Chamber (TPC) gas and a velocity
measurement with the TOF system[^73]. The relativistic rise of the
electron dE/dx separates electrons from hadrons, except where electrons
cross with pions at momenta of ∼0.2GeV/c, with kaons at ∼0.6GeV/c, with
protons at ∼1.1GeV/c, and with deuterons at ∼1.5GeV/c. A time-of-flight
measurement, with the requirement that \|1 − β\| \<0.03, eliminates slow
hadrons and cleans up the crossing regions. This results in clean
electron identification[^74]^7^. In addition to direct measurements of
open-charm hadrons (via Kπ decays), the STAR HFT [^75] will serve as a
powerful device to discriminate primordial electrons from background
electrons, both from photonic sources such as conversions and from
correlated decay of charm and beauty. Rejection of π^0^ and η Dalitz
decays by a factor of 3 (single track) can be achieved by measuring both
electrons of the pair, which is possible due to the large acceptance of
the STAR TPC. The large reduction in electron background will enable us
to observe the electromagnetic signal from low-mass vector mesons and
the radiation of intermediate-mass dileptons with a few hundred thousand
central Au+Au events in STAR. With the upgrades we expect to detect 6K φ
and 22K ω decays in 200 Million recorded central Au+Au collisionsError:
Reference source not found^,Error:\ Reference\ source\ not\ found^.
These measurements will be of comparable accuracy to thoss presented by
NA60 at QM05[^76] in central In+In collisions, which consisted of
signas: ∼ 6K for the ω and ∼ 10K for the φ.

In the absence of HFT, and with 50% of the Barrel TOF, a possible
configuration for Run 9, we expect that the electron background level
will be similar to that of the PHENIX detector as presented in Ref.[^77]
From yields measured by STAR in the hadronic decay channel (K^+^K^-^),
we estimate that a signal with 25 significance and signal-to-background
ratio between 1:25 to 1:10 will be obtained in 300M minimum bias events.
This is an example of the high quality measurements that can be made
with electron pairs in such a configuration.

We also propose to place a 0.1 radiation length lead photon
converterError: Reference source not found - concentric with the beam
pipe with a radius of approximately 43 cm - into STAR for portions of
two upcoming runs, ideally Runs 10 and 12. The driving physics
motivation is to make measurements of direct photon HBT in full energy
Au-Au collisions, which could provide unique and extremely interesting
information about all stages of the evolving collision system. The
inclusion of a converter in STAR would give an increased efficiency for
reconstructing photon pairs at low relative momentum by using pairs in
which one photon converts and is reconstructed via its daughter tracks
in the TPC and the other is reconstructed in the barrel EMC. We envision
a short (approximately 1 week, 50M central Au+Au collisions) time of
running with the converter during Run 10. This would serve chiefly as a
proof of principle for the photon HBT measurement although other physics
topics would benefit from the increased photon efficiency. When this 1
week of running is complete, the converter would be withdrawn to a
location that sits largely outside of the STAR acceptance. If the
results of this trial run prove to be satisfactory, we would propose a
significantly longer run with the converter during Run 12. The goal of
this run would be to collect on the order of 10^9^ full energy central
Au-Au events with the converter in place and using this data, to expand
the HBT measurement into the more interesting kinematic range and
increase the detail of the measurement.

## U+U collisions at RHIC

The Uranium nucleus is an oblate spheroid so central U+U collisions can
be very asymmetric. The conversion of that spatial asymmetry to momentum
space can test the validity of the zero mean-free-path limit which some
believe has been achieved in central Au+Au collisions at RHIC. The new
EBIS ion source, which will be commissioned in 2009, will allow RHIC to
provide U+U collisions as early as 2011.

Grazing (non-central) collisions of spherical nuclei produce elliptic
shaped overlap areas. The expansion of and radiation from the produced
spatially asymmetric fireball yield asymmetries in momentum space. The
momentum space anisotropy may probe the dynamics of the expanding
system. For nearly central Au+Au collisions at 200 GeV, the momentum
space anisotropy seems to reach the value predicted for a system with
zero mean-free-path where a hydrodynamic description of the expansion
may be valid. Figure 5-26 shows heavy-ion data for the ratio of the
momentum anisotropy to the intial spatial anisotropy (v~2~/ε). The data
at the right of the plot are for central 200 GeV Au+Au collisions where
the transverse charged particle density (1/S dN/dy) is largest.

A central U+U collision is expected to reach densities 30% larger than
central Au+Au. Due to the oblate shape of the Uranium nucleus, even very
central U+U collisions will have a large initial spatial deformation.
For these reasons, U+U collisions will allow RHIC to test for the
presence of a hydrodynamic limit. The ratio v~2~/ε may continue to rise,
as the current trend indicates or it may begin to saturate. A leveling
off of the trend would be a strong indicator of the attainment of a zero
mean-free-path, hydrodynamic limit. These collisions will provide a
stringent test of the models shown in the figure and lead to a
significant extension to higher densities of the measurement of v~2~/ε
into previously unexplored territory.

![](media/image82.pct){width="4.454861111111111in"
height="3.6347222222222224in"}

Figure 5-26: Heavy ion data on eccentricity scaled v~2~ vs transverse
particle density. A central U+U collision is expected to reach densities
30% larger than central Au+Au while the eccentricity is large.

## Non-triggered probes at 200 GeV: Au+Au beam use request for Runs 9 and 10

The collection of \~300M minbias events combined with the improvement in
particle identification via the ToF will allow for a significant
improvement in the measured directed flow (v~1~) of protons. If 600M
minbias events can be recorded then a detailed study of both directed,
elliptic and v~4~ can be made as a function of p~T~ and particle species
including the and K^0^~s.~ With such a large minbias sample STAR will
also be able to perform large correlation studies. Detailed measurements
of all the Fourier harmonics (v~n~) are important for furthering our
understanding of the bulk dynamics of the matter created in RHIC
collisions. For instance the observation of a "wiggle" in the v~1~
measurement around mid-rapidity could indicate a change in the
compressibility of the matter created. The higher order harmonics can be
used as a more sensitive test of the initial conditions. Both the v~1~
and v~4~ results have been performed by STAR with limited statistics but
finer granular rapidity measurements using PID are needed to further
constrain models of both the initial conditions and the dynamical
evolution of the system.

Systematic studies of the centrality dependence of jet-like correlation
structures as function of trigger and associate hadron p~T~ in di-hadron
and di-triggered (2+1) correlations should shed additional light on
questions of the initial gluon density, parton thermalization, and QGP
formation and subsequent hadronization. An increase of the minimum bias
dataset by more than an order of magnitude will provide higher-p~T~
access for triggered di-hadron correlation studies with respect to
reaction plane, and measurements more differential in centrality. While
current trigger selections are limited to p~T~ of 3-4 GeV/*c*, a dataset
of this size will shift the upper limit to 6 GeV/*c*. The current
reaction-plane dependent analysis concentrates on a wide centrality bin
from 20-60%, which can be binned more finely with such a larger dataset.
Detailed exploration of qualitative features feasible at higher p~T~
should greatly enhance our leverage in disentangling some of the
competing physics effects.

With a few hundred million minimum bias events and 50-100% of the TOF,
STAR's two particle relative correlation analysis at low transverse
momentum will be extended to explore identified (, K, p) particle
correlations, focusing on the flavor and baryon-to-meson content of the
same-side correlations below and above the transition centrality point.
In Run 10 it is desirable to have a dedicated Au+Au run at full energy
with full barrel TOF and complete DAQ 1000. We would like as many events
as practically possible, focusing (assuming prior fulfillment of our
statistic requirements for minimum bias data) on utilizing both minimum
bias, central and specialized rare probes triggers, including high tower
triggers at 6 GeV and/or software trigger selecting high-p~T~ events of
interest. The progress in physics analyses mentioned below requires a
minimum factor of 10 increases in the statistics, which corresponds to
approximately 200 M central-triggered events.

Many of the existing theoretical descriptions of final-state
interactions at RHIC assume vacuum fragmentation of partons, which may
be valid in the high-energy limit, when the fragmentation time exceeds
the medium traversal time of the leading parton. It is therefore
important to understand the features of fragmentation functions that may
distinguish parton energy loss in a QGP from other possible sources of
jet softening and broadening. It is necessary to employ the extended
particle identification capabilities of STAR in the hard sector studies,
addressing energy loss mechanisms and modification of jet fragmentation
functions with both identified and unidentified angular correlations
in - space. Such measurements will enable the study of the transition
between jet fragmentation dominant and multi-parton mechanisms of hadron
formation. A new extended sample of central Au+Au data would allow
quantitative studies of spectral distribution for associated hadrons
with identified (baryon/meson) triggers, as well as spectral
measurements of identified species in jets and ridges. 3-particle
correlation technique developed with the Run 4 data available would
capitalize on the high statistics sample, providing systematic studies
of correlation structures as function of trigger and associate hadron
momenta.

Other physics topics that would become feasible with the requested
sample include jet-induced Lambda polarization, resonance production in
jets. Additionally, rare particle searches and resonance studies will
become possible even in central Au+Au collisions, benefiting greatly
from TOF availability.

As well as the pp2pp program discussed above, STAR plans to continue its
ultra-peripheral collisions (UPC) program in Au+Au events. The Au+Au UPC
analysis of STAR focuses on heavy vector mesons, in particular the
J/production, and four-prong particle decays. The four-prong data will
be used to improve the previous reported measurement of the parameters
of the '(\~1450) and/or '(1700) resonance, and a search for heavier
resonances decaying to 4 pions. With 50 M minbias UPC triggered Au+Au
events, the collection of which is made possible via DAQ1000, we expect
to reconstruct \~1500 ' candidates. A search for the four-pronged KK
and, statistics permitting, KKKK final states will also be performed.
With such high statistics a 6-prong final state study will also be
feasible as will the first attempt to observe the two-photon production
of the f~2~(1270). Meanwhile the reconstruction of the J/ will enable
the study of the gluon distribution functions around x=0.01 and
Q^2^=(1.5 GeV)^2^. Finally multiple vector meson production in a single
ion-ion collision is within reach with such a data set. Approximately 1
out of 1,000 ^0^ should be accompanied by a second ^0^. Quantum
correlations in the production and in the decay can then be studied, for
example with ^00^ with in the same quantum state, stimulated decay
becomes a possibility.

## Non-triggered probes: p+p beam use request for Runs 9 and 10

For the unbiased references for STAR spectral analyses and correlation
studies it is necessary to collect at least 300 M minimum bias 200 GeV
p+p events as part of proton physics run. With full implementation of
DAQ1000 this can be achieved in a reasonable length of time. Reasonably
low pile-up levels are optimal for those studies, and therefore a
dedicated run is required for at least a significant portion of this
dataset, which can be taken concurrently with the special, low
luminosity conditions during the pp2pp run. The event estimates are
driven by the high-p~T~ program requirements and the Omega baryon
measurement. These data would afford reliable measurements of the
omega-baryon yield, and extend p~T~ spectra reach for identified
protons, pions and strange baryons into the fragmentation region from
approximately 6-10 GeV/c. This limit corresponds to the maximum p~T~
reach achieved in Au+Au and Cu+Cu collisions, and would greatly aid in
systematic studies of hadron suppression via nuclear modification
factors. STAR's p+p data are also world-leading in the area of
identified particle fragmentation functions. Any 500 GeV running would
be useful to test these models tuned with the 200 GeV data.

STAR\'s femtoscopic analysis is also ongoing with data from p+p
collisions. The analysis focuses on two main areas. The first topic is
the continued investigation into the observation that the extracted
radii from Au+Au events seem to scale linearly to those from p+p when
pion pairs with the same mean m~T~ are selected. This is surprising
since radial flow, which is expected to affect the HBT radii, in Au+Au
events is very different to that in p+p. There are also unexplained long
range correlations observed that need further study. Second a new
analysis of HBT with respect to the jet axis has started. The goal of
this analysis is to see if the presence of a jet affects the
space-momentum correlations of the underlying event. The femtoscopic
measurements as a function of m~T~ require several million minbias
events. While there seem to be sufficient data at 200 GeV there is much
interest in performing these measurements at 500 GeV, to see if the
observed scaling is preserved at a different collision energy. The HBT
analysis with respect to the jet, or thrust, axis needs several million
high tower triggered data. This data set is requested for both 200 and
500 GeV collisions.

## 

## 

## The Uranium nucleus is an oblate spheroid, so central U+U collisions can be very asymmetric. The conversion of that spatial asymmetry to momentum space can test the validity of the zero mean-free-path limit which some believe has been achieved in central Au+Au collisions at RHIC. The new EBIS ion source, which will be commissioned in 2009, will allow RHIC to provide U+U collisions as early as 2011.

## Grazing (non-central) collisions of spherical nuclei produce elliptic shaped overlap areas. The expansion of and radiation from the produced spatially asymmetric fireball yield asymmetries in momentum space. The momentum space anisotropy is a sensitive probe of the dynamics of the expanding system. For nearly central Au+Au collisions at 200 GeV, the momentum space anisotropy seems to reach the value predicted for a system with zero mean-free-path where a hydrodynamic description of the expansion may be valid. Figure 5-26 shows heavy-ion data for the ratio of the momentum anisotropy to the intial spatial anisotropy (v~2~/ε). The data at the right of the plot are for central 200 GeV Au+Au collisions where the transverse charged particle density (1/S dN/dy) is largest. 

## 

## ![](media/image82.pct){width="4.454861111111111in" height="3.6347222222222224in"}

## Figure 5-26: Heavy ion data on eccentricity scaled v~2~ vs transverse particle density. A central U+U collision is expected to reach densities 30% larger than central Au+Au while the eccentricity is large.

## 

## **A central U+U collision is expected to reach densities 30% larger than central Au+Au while the eccentricity is large. Central U+U collisions at 200 GeV will probe larger values of the charged particle transverse density and allow RHIC to test for the presence of a hydrodynamic limit. The ratio** **v~2~/ε** **may continue to rise, as the current trend indicates or it may begin to saturate. A leveling off of the trend would be a strong indicator of the attainment of a zero mean-free-path, hydrodynamic limit. Due to the oblate shape of the Uranium nucleus, the geometry of U+U collisions will be more complicated than that of Au+Au collisions.** **A short run, combined with a possible upgrade of detector capability in the forward direction, will allow for the selection of different collision geometries and a significant extension** **to higher densities** **of the measurement of** **v~2~/ε** **into previously unexplored territory.** 

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