# Presentation: 608288

## The STAR Detector at RHIC

**The STAR Heavy Flavor Tracker **
**An Introduction and Brief Review of the Technical Design and Physics Goals at Mid-rapidity**
**Jim Thomas**
**Lawrence Berkeley National Laboratory**
**November 1****st****, 2007**

## The Heavy Flavor Tracker

**The Heavy Flavor Tracker**

**The PXL:  2 layers of Si at mid rapidity**

**The PXL is new technology**
**30 ********m silicon pixels**
**	****to yield 10 ********m space  point resolution**
**Direct Topological reconstruction of Charm**
**Detect charm decays   with small c********, including          D****0**** ******** K ******
**	****New physics**
**Charm collectivity        and flow to test thermalization at RHIC**
**C & B  Energy Loss to  test pQCD in a hot and dense medium at RHIC **
**The proposed Tracking Upgrades include**
**PXL   (2 layers)**
**IST    (1 or 2 layers)**
**SSD  (existing layer)**

**= PXL + IST + SSD**

## Summary of Si Detectors Inside the TPC

**Goal: graded resolution to achieve highly efficient tracking from the outside ******** in **
**TPC – SSD – IST – PXL**
**TPC pointing resolution at the SSD is  ~ 1 mm**
**SSD pointing at the IST   is  ~ 300 ********m **
**IST pointing at the HFT  is  ~ 250 ********m **
**HFT pointing at the VTX   is  ~ 50 ********m**

**Summary of Si Detectors Inside the TPC**

## The Heart of the Beast

**The Heart of the Beast**

- The PXL detector is a thin detector using 50 m thick Si to finesse the limitations imposed by MCS

- See the talk by Howard Wieman for the latest developments (and photos)

## Direct Topological Identification of Open Charm

- The STAR Heavy Flavor Tracker will identify daughters of the decay and do direct topological reconstruction of open charm hadrons.
- No Mixed events, no random background subtraction.

- Goal: Put a high precision detector near the IP to extend the TPC tracks to small radius

## “Heavy Flavor” is the Next Frontier 

**The QGP is the universally accepted hypothesis at RHIC**
**The next step in confirming this hypothesis is the proof of thermalization of the light quarks in RHIC collisions**
**The key element in proving this assertion is to observe the flow of charm ... because charm and beauty are unique in their mass structure**
**If heavy quarks flow**
**frequent interactions among all quarks**
**light quarks (u,d,s) likely to be thermalized**

- Current quark: a bare quark whose mass is due to electroweak symmetry breaking

- Constituent quark: a bare quark that has been dressed by fluctuations in the QCD sea

**Notes:**
An important example of such a vacuum condensate is the Higgs vacuum, which is introduced in the Standard Model of particle physics to generate the masses of quarks, leptons, and the gauge bosons of the weak interaction. The vacuum expectation value of the Higgs field, hφi = 246 GeV, is uniquely determined in the Standard Model; the quark and lepton masses differ from one another only due to the different strength of the coupling of each fermion to the Higgs field. The quark masses receive additional contributions from the quark and gluon condensates in the QCD vacuum, which have been measured
to be: h ̄qqi = (235MeV)3,
         hg2GμGμ i = (840MeV)4. (1)
In fact, the contribution of the QCD vacuum condensates to the masses for the three light quark flavours u, d, s considerably exceed the mass believed to be generated by the Higgs field see Fig. 1).

Electro weak symmetry breaking is responsible for the current quark masses.  Chiral symmetry breaking is responsible for the constituent quark masses.

A constituent quark is a current quark that is dressed by the QCD vacuum.  It includes contributions from fluctuations in the QCD sea.
At RHIC, the heavy quarks do not interact with the QCD vacuum because they are too heavy.  The light quarks are modified and can change inside a QGP.  The higgs scale is too high for electro-weak symmetry breaking to be affected at RHIC energies.  So Charm and Beauty masses are invariant at RHIC.

## Flow: Probing Thermalization of the Medium

**p****y**

##  -mesons Flow: Partonic Flow

******-mesons are very special:**
-  they do not re-interact in hadronic environment
-  they show strong collective flow
-  they are formed via coalescence with thermal s-quarks
*	**	*
*	**	**	**STAR Preliminary: QM06, S. Blyth*
* **	**	**	**Hwa and Yang, nucl-th/0602024; *Chen et al., PRC73 (2006) 044903

## Scaling as a Function of  (mT – m0)

**The light quark sector scales beautifully with v****2****/n****q****  .vs.  (m****T**** – m****0****)/n****q**
**Note that p****T**** < 1 GeV always did scale !**
**The strange quark sector also scales   with <v****2****> and the scaling holds at all centralities**
**Even the ******** meson**
**See S. Blythe QM2006**

**Does it work in the Charm Sector? **
**A strong test of the theory**

- Yuting Bai, QM 2006 for the STAR Collaboration

- STAR Preliminary work by Yan Lu

**p****T**** distributions of electrons from semi-leptonic decay of heavy flavor mesons (left D-mesons, right B-mesons) as a function of parent p****T****.  The inserted plots represent the projections to the corresponding heavy flavor distributions. The widths of the electron p****T**** windows are indicated by dashed boxes.**

## Single Electron Spectra ... are not sufficient

**Hydro and Pythia are extreme models on opposite ends of the   model spectrum **
**Charm in red, Beauty in Blue ... Hydro is the solid line, Pythia is dashed**
**Single electron spectra make it difficult to distinguish hard and soft physics below 3 GeV**
**Also true for R****AA**** measurements**
**The decayed spectra are shown in black.  The sensitivity to the original spectra are reduced. **
**We heard this message many times at QM**

- S. Batsouli et al., Phys. Lett. B 557 (2003) 26.

**We need direct topological identification of Charm**

- H. Liu et al., Phys. Lett. B 639 (2006) 441.

**Notes:**
Figure 4: Solid- and dashed-curves represent the charm- (red) and beauty-hadron (blue) spectra from Blast-Wave and Pythia model calculations, respectively. The corresponding heavy flavor decayed electron spectra are shown as black curves.  The data are the measured single electron distributions measured in 10% central Au+Au collisions at 130 GeV by the PHENIX collaboration.  The figure is adapted from Ref. [20] 
When direct measurements of heavy flavor hadrons are not possible, non-photonic electrons from heavy flavor decays can be used to study charm production.  However,  there are serious limitations in such situations. As shown by Batsouli, Kelly, Gyulassy and Nagle in Ref. [[i]], the decayed electron distributions are insensitive to the intrinsic shape of the D-meson transverse momentum distribution. Due to the decay kinematics and the light mass of the electrons and positrons, the dynamical information in the primary spectrum is washed out.  This is shown in Figure 4.  There is a clear difference between the zero mean free path hydrodynamic flow prediction (solid) and the infinite mean free path pQCD Pythia calculations (dashed-line) for D-mesons; but the resulting electron spectra are nearly indistinguishable. In order to extract useful information about heavy flavor production in heavy ion collisions, we have to measure the charm and beauty-hadrons by direct topological reconstruction. Single electron spectra are not sufficient.  
[i] Batsouli et al., Phys. Lett. B557, 26 (2003).  nucl-th/0212068.

## Heavy Flavor Energy Loss ... RAA for Charm

**Heavy Flavor Energy Loss ... R****AA**** for Charm**

**Heavy Flavor energy loss is an unsolved problem**
**Gluon density            ~ 1000 expected from light quark data**
**Better agreement with the addition of inelastic E loss**
**Good agreement only if they ignore Beauty ...**
**Beauty dominates single electron spectra above 5 GeV**
**We can separate the Charm and Beauty by the direct topological identification of Charm**

- Theory from Wicks et al. nucl-th/0512076v2

**Where is the contribution from Beauty?**

**Notes:**
Figure 9: Non-photonic electron RAA in central Au+Au collisions from STAR (open-symbols, top 5%) and PHENIX (filled-symbols,  top 10%) from Refs. [58,59,60].  Theoretical predictions for charmed-hadron and charm and beauty-hadron decayed electrons are shown as blue- and yellow-bands, respectively. In these calculations, the initial gluon density was assumed to be unrealistically large: ~3500,  see Ref [63] .
Figure 9 shows RAA for non-photonic electrons[i],[ii],[iii] from STAR and PHENIX.  The data extend up to pT ~ 10 GeV/c.  The figure shows that the suppression factor for single electron spectra is in the range of 0.2-0.3 which is almost exactly the same as the suppression factor observed for charged hadrons and pions[iv],[v] (i.e. light quarks).  Using an unrealistically large initial gluon density of dNg/dy = 3500, Djordjevic, Gyulassy, Vogt and Wicks have done a pQCD calculation[vi], including gluon radiative energy-loss, which can barely reproduce the electron RAA (see the blue-line in Figure 9).  The model ignores the contributions from beauty-hadrons, and when beauty is added to the model, the resulting prediction for RAA cannot describe the data at all (see the yellow band in Figure 9 ); the model with beauty over-predicts the data by a factor of 2-3.   Note that for light-quark hadrons, like pions, a gluon density of ~1000 has been used to  reproduce the observed RAA[jt1].  These results raise a serious challenge to our understanding of both the mechanism for heavy-quark production and the mechanism for energy-loss in a hot and dense medium. In order to resolve these important issues, we have to do direct topological reconstruction of open charm.  It is experimentally and theoretically too difficult to make definite conclusions from the single-electron measurements due to the complications from the mixing of the electrons from D-meson decay and electrons from bottom hadron decay as well as the smearing of the decay kinematics. 

## Baryons vs. mesons

**Coalescence and fragmentation conspire at intermediate p****T ****to give constituent quark number scaling and Baryon-Meson differences.**
**Coalescence and fragmentation of charm quarks is different than for light quarks  ... so it is a strong test of the theory**

**Coalescence of light quarks implies deconfinement and thermalization prior to hadronization**
**How do baryons and mesons behave in the Charm sector?**
**The ****Λ****c ****will be a fascinating test ... and we might be able to do  it **** ****with the HFT via ****Λ****c ****/ **** ****D **

## The Properties of the Open Charm Hadrons

| Particle | Decay Channel | c (m) | Mass (GeV/c2) |
| --- | --- | --- | --- |
| D0 | K +         (3.8%) | 123 | 1.8645 |
| D+ | K + +   (9.5%) | 312 | 1.8694 |
|  | K+ K +   (5.2%)
+ + -     (1.2%) | 150 | 1.9683 |
|  | p K +     (5.0%) | 59.9 | 2.2865 |

## A Rich Physics Program	

**There is a rich physics program when all of the STAR physics detectors are working together**
**Flow in the Charm sector**
**dE/dx in the Charm and Beauty sector**
**Recombination and R****AA**** in the Charm and Beauty sector**
**Vector Mesons**
**Charm Angular Correlations**
**now accessible due to STAR’s large acceptance**
**non-photonic electrons**
**...**

**Simulation results**

## Finding the D0  ...  (whew, it works)

**The open circles show the primary vertex resolution in central Au+Au collisions.  (absolute value of 3D distance)**
**The solid circles show the D****0**** secondary decay-vertex resolution.  The mean decay distance, c******** = 123 ********m for the D****0****, is shown to guide the eye. **
**Each D****0**** decay length was scaled by the appropriate ******** factor to provide a universal peak for the purposes of the illustration.**

## D0 Reconstruction Efficiency

**D****0**** Reconstruction Efficiency**

**The yield of reconstructed D****0****s divided by the simulated D****0**** yield**
**The red squares show the maximum possible efficiency for reconstructing a D****0**** based upon the single track efficiencies for the daughter particles.  **
**The black circles show the efficiency for actually finding these D****0****s after applying the topological cuts.  **

## Significance of the D0 measurement

**The expected significance of the D****0**** measurement with 100 M central events as a function of p****T****.  For p****T**** above 5 GeV/c, the upper limit on the signal significance is shown because we do not have sufficient statistics to properly estimate the background.  **
**UPC electrons and pileup of minimum-bias events during the integration time of the PIXEL detectors are not included in this simulation**

## c   Reconstruction with the HFT

******c  ****reconstruction is difficult due to 3 body final state and the short decay length, however**
**a ********c**** spectrum can be measured in 50 M central Au-Au events**
**an R****AA**** measurement with 20% error bars can be made out to 4 GeV with 500 M events**

## Charm Meson Flow

**The green line describes the expected magnitude of the v****2 ****parameter if only the light quarks in the D****0**** exhibit flow.  **
**The red line illustrates how the flow parameter can increase if the charmed quarks also flow.  **
**The blue error bars shown at the top of the figure show the anticipated errors of measurement in 100 M Au+Au minimum-bias events.**

##  Rates Estimation - v2

**dN/dp****T**** distributions for D-mesons.**
-       Scaled by <Nbin > = 290, corresponds to the minimum bias Au + Au collisions at RHIC.
**(b) Assumed v****2**** distributions for D-mesons.**
-                  ---- PLB 595, 202 (2004)
-        Error bars shown are from 15% systematic errors** **
**(c) D****0**** meson v****2**** rates from minimum bias Au + Au collisions at 200 GeV.**
-       The small and large error bars are for 15% and 30% systematic errors, respectively.  For the v2 analysis, 12 bins in  are used.

| pT(GeV/c) | ∆pT(GeV/c) | # events
qC = qu,d,s | # events
qC = 0 |
| --- | --- | --- | --- |
| 0.6 | 0.2 | 260x106 | 525x106 |
| 1.0 | 0.5 | 70x106 | 140x106 |
| 2.0 | 0.5 | 53x106 | 125x106 |
| 3.0 | 1.0 | 390x106 | 270x106 |
| 5.0 | 1.0 | 520x106 | 880x106 |

- 500 M Events is one RHIC Month with DAQ1000

## Hints of Elliptic Flow with Charm

**D  ********  e +X**
**	****Single electron spectra from PHENIX show hints of elliptic flow**
**Is it charm or beauty?**
**The HFT will cut out  large photonic backgrounds: ***** ***
****** ******** ****e****+****e****-**
**	****and reduce other large stat. and systematic uncertainties**
**STAR can make this measurement with 100M ****Au+Au**** events in the HFT**
**Smoking gun for thermalization at RHIC!**

**Better if we can do direct topological identification of Charm**

## PHENIX PRL 98, 172301 (2007)

**R****AA**** of heavy-flavor electrons in 0%–10% central collisions compared with ********0**** data and model calculations **
**V****2**** of heavy-flavor electrons in minimum bias collisions compared with ********0**** data and the same models. **
**Conclusion is that heavy flavor flow corresponds to ********/s at the conjectured QM lower bound**

- However, Hydro doesn’t know about Quark Coalescence

- 0

- 0

## Charm Meson Flow – coalescence view

**The green line describes the expected magnitude of the v****2 ****parameter if only the light quarks in the D****0**** exhibit flow.  **
**The red line illustrates how the flow parameter can increase if the charmed quarks also flow.  **

## Fridays PRL 99, 172301 (2007) ... new insights 

**Romatschke****2 ****perform relativistic viscous hydrodynamics calculations**
**Data on the integrated elliptic flow coefficient v****2**** are consistent with a ratio of viscosity over entropy density up to ********/s ********0.16**
**But data on minimum bias v****2**** seem to favor a much smaller viscosity over entropy ratio, below the bound from the anti–de Sitter conformal field theory conjecture**

## A possible interpretation

**Assume the PHENIX data are correct, then they may have seen *****charmed meson *****flow**
**A couple of assumptions ...**
**The PHENIX data do not show charmed quarks flowing ... rather they see the light quarks flowing ( 1⁄2 the full value for the flow of the light hadrons)**
**This would be consistent with the ********/s ratio fit by Teaney et al.**
**It would also be consistent with the even lower value of the ********/s ratio found by ****Romatschke & Romatschke for the light hadrons**
**Conclusion: charmed quarks don’t flow and the conjectured QM lower bound has been violated**

- Future data sets and further discussion will be very interesting!

## Direct Topological Identification of Open Charm

**Direct Topological Identification of Open Charm**

**	****The STAR HFT will identify the daughters in the decay and do a direct topological reconstruction of the open charm hadron.  **
**	****No mixed events, no random background subtraction.**

## Summary

**The STAR HFT will explore the Charm and Beauty sectors**
**We will do **_**direct**_** topological reconstruction of Charm**
**Our measurements will be unique at RHIC**
**The key measurements include**
**V****2**
**Energy Loss **
**Charm Spectra, R****AA  ****& R****cp**
**Vector mesons**
**Angular Correlations**
**The technology is available on an appropriate schedule**