# Presentation: 059496

## The STAR Detector at RHIC

**The STAR Heavy Flavor Tracker**
**An Introduction and Brief Review of the Physics Goals at Mid-rapidity**
**Jim Thomas**
**Lawrence Berkeley National Laboratory**
**December 7****th****, 2006**

## The Heavy Flavor Tracker

**The Heavy Flavor Tracker**

**A new detector**
**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**
**Charm Energy Loss to test pQCD in a hot and dense medium at RHIC **
**The proposed Tracking Upgrades include**
**HPD  (1 layer)**
**IST    (2 layers)**
**SSD  (existing layer)**

**+ HPD + IST + SSD**

## Direct Topological Identification of Open Charm

- The STAR Inner Tracking Upgrade 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

## The Light Quark Program at STAR is Compelling

- Its hot

- Its dense

- and it flows
- at the partonic scale

-  and , too!

- Spectra

**V****n**

- Jets & Rcp

- Now we can make these measurements in the charm sector

## Flow: Probing Thermalization of the Medium

**p****y**

## Flow: Constituent Quark Number Scaling

- In the recombination regime, meson and baryon v2 can be obtained from the quark v2 :

## 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

## “Heavy Flavor” is the Final 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.

## Hints of Elliptic Flow with Charm

- Shingo Sakai, QM 2006 for the PHENIX Collaboration

**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 50 M ****Au+Au**** events in the HFT **
**Smoking gun for thermalization at RHIC!**

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

**Notes:**
Figure 7:  Non-photonic electron elliptic flow at RHIC.  Blue and Red symbols are data of non-photonic electron v2 from PHENIX[i] and STAR[ii]. Only statistical errors are shown for both results.  The dashed-dotted-line represents the input D-meson v2 distributions, assuming the scaling of constituent quarks for meson v2. The corresponding decayed electron v2 are shown as the hatched band[iii]. The dashed-line represents the results from Greco[iv] et al.  In this coalescence calculation, similar v2 distributions for c- and u-quarks are assumed.  
Hence, thermalization of light quarks is likely to have been reached through partonic re-scattering. 
Figure 7 shows the first indication of charm particle elliptic flow at RHIC measured in the inclusive electron channel35,36.  [jt1]The predictions assume elliptic flow for the light quarks as determined by fits to experimental data.  Presently, the data support the idea that the heavy charm quarks flow.  As shown in the figure, the uncertainties are rather large especially at low transverse momentum where hydrodynamic behavior should occur. A precise measurement of directly reconstructed open charm hadrons to low momentum is essential to confirm and further quantify elliptic flow of the charm quarks.  
 [jt1]These are cross references and really should be the primary references, not cross.  Put the cross refs in the figure captions.

## 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.

## RAA and RCP for D0s... Critical measurements

**Flow and energy loss are intertwined**
**you can’t have flow without E loss**
**R****AA ****is a measure  of E loss**
**Two models for R****AA**** of D mesons**
**scaled light    quark data**
**hydro**
**The measurements cannot be done with single electron spectra**
**the decayed  curves are nearly indistinguishable**

**We need direct topological identification of Charm**

**Notes:**
Figure 5: Nuclear modification factor RAA of D-mesons assuming the measured spectrum from d+Au collisions scales as the light charged hadrons in central Au+Au collisions (blue line, I)  and from a hydro-dynamically inspired parameterization assuming a collective flow velocity of <r> = [jt1]0.4c for D-mesons (red line, II);  the corresponding electron decay-spectra are shown by the blue dashed (I) and red dashed (II) lines[i].
[i] X. Dong,  “Open Charm Production at RHIC” QM2005, nucl-ex/0509038.
The same conclusion is reached when doing the analysis of RAA for heavy flavor spectra. Recall that RAA is the ratio of the charm yield measured in heavy ion collisions relative to the yield in p-p or d-Au. The nuclear modification factor, RAA, for D-mesons is shown in Figure 5.  Two vastly different models are shown.  The figure shows that there are large differences in  the two D-meson RAA ratio curves but the corresponding decayed-electron RAA curves  are essentially identical for all pT.  These are theoretical curves without error bars which suggests that the decayed electron RAA curve is impossible to measure; additionally, any electron measurement will have large systematic uncertainties at low momentum due to the large background from photonic electron production16,17.  At higher pT, electrons from B-meson decays will become more abundant making the electron measurement for charm physics even less realistic.  In order to avoid these difficulties, and for a precise measurement of charm quark flow, a direct reconstruction of D  π + K is required.  This clearly calls for a  direct measurement of heavy flavor hadrons, i.e. D- and B-mesons.

## 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. 

## e-h Correlations in p+p: Beauty vs. Charm

**Beauty dominates single electron spectra above 5 GeV**
**First measurement of open-bottom at RHIC**
**Significant contribution from bottom**
**Very close to FONLL predictions**
**Xiaoyan Lin’s talk for STAR at QM2006**

## 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 **

## Coalescence of Charm Quarks? 

**Charm fragments into  a variety of hadrons**
**If Charm quarks equilibrate with the surrounding medium then they may coalesce with the light quarks**
**which would imply they travel a large distance in the thermalized medium**
**Coalescence Increases the yield of ****Λ****c  ****and J/******** by 80% and a factor of 10, respectively**
**Systematic errors cancel in D****s****+****/D****+**** due to similar decay channels**

|  | e-p and e+-e 
average | Pythia  | Statistical coalescence |
| --- | --- | --- | --- |
| (c  D + )  | 0.232  | 0.162  | 0.21   |
| (c  D 0 )  | 0.549 | 0.639  | 0.483   |
| (c  Ds+ )  | 0.101  | 0.125  | 0.182   |
| (c  c+ )  | 0.076  | 0.066  | 0.080   |
| (c   J/ )  |  | 0.006  | 0.057   |

**Table 1:  Charm quark fragmentation functions. The D****+**** and D****0**** yields include feed-down from D*****+**** and D*****0**** decays.  **

- Andronic et al.,. *Phys. Lett.* B571, 36 (2003).

- Ds+**/ **D0,  Ds+**/ **D+  and  J/ **/** D0 are sensitive probes of thermal charm production & history

**Notes:**
Table 1:  Charm quark fragmentation functions.  The left column is from Ref. [46,47].  The right column is from Ref. [45].  The D+ and D0 yields include feed-down from D*+ and D*0 decays. 
Measuring the total charm and bottom yields requires measuring the yields of several different hadrons.  Indeed, charm quarks may fragment into a variety of hadrons as shown in the first column of Table 1.  These fragmentation ratios have recently been compiled[i],[ii] using e-p and e+-e- collision data.  The ratios are found to be independent of the collision energy and the collision system (e-p or e+-e-).  They are likely to be the same in p-p collisions at mid-rapidity where the HFT can be used to detect charmed particles at RHIC.  Note that the ratios calculated using PYTHIA[iii], shown in the second column of Table 1, differ significantly from the measured ratios.  To avoid any uncertainties in the charge yield measurement, these ratios will have to be measured in p+p collisions at RHIC energies. 
The effect of statistical coalescence is shown in the last column of Table 1 (see Ref. 45).  The calculation assumes that charm quarks statistically coalesce with the lighter quarks, (i.e. according to the temperature and chemical potential of the light-quark system) so the temperature and chemical potential are set so that the light hadron yields measured at RHIC are reproduced. In addition, the number of charm quarks present in the system is set by pQCD calculations and this is reflected by a charm-chemical potential in the statistical coalescence model.  The results of the model show that statistical coalescence increases the yield of the Ds+ meson by 80% and the J/ yield by a factor of 10 compared to PYTHIA while the yield of D0 and D+ decrease slightly.  Thus, the ratios Ds+/ D0, Ds+/D+ and J/ / D0 are very sensitive probes of thermal charm hadron production.  
Experimentally, we will be able to precisely measure the ratio of Ds+ to D+ yields because most of the systematic errors in the individual spectra cancel out when they are reconstructed in very similar decay channels: D+  K π+ π+ and Ds+  K π+ K+. 

- A vigorous vector meson program, too ... because the HFT + HPD + IST + SSD is a powerful tool to identify (and eliminate) background for the npe signal

## 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 sector**
**Recombination and R****AA**** in the Charm sector**
**Vector Mesons**
**Charm Angular Correlations**
**non-photonic electrons**
**...**

## The Methodology: Surround the Vertex with Si

**The Methodology: Surround the Vertex with Si**

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

- Add the HPD, IST, and SSD to form the Inner Tracking Upgrade ( ITrUp )

## Summary

**The HFT & IST will explore the Charm sector at RHIC**
**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**

**Backup slides**

## The STAR Inner Tracking Upgrade is Unique at RHIC

**The Inner Tracking Upgrade will cover 2******** in ********azimuth**
**PHENIX Si covers 2********in********but the rest of the detector is 2 arms of ********/2**
**The Inner Tracking Upgrade will cover ± 1 unit of ******
**PHENIX Si covers ± 1 unit but the rest of the detector covers 1/3 unit**
**The HFT uses 30x30 ********m pixels for high resolution tracking**
**PHENIX uses 50x425 ********m pixels (... strips ...)**
**The HFT uses 50 ********m thick Si in each of 2 layers**
**PHENIX uses 350 ********m thick Si (sensor plus readout) in 2 layers and 1250 ********m thick Si in 2 more layers**
**The HFT is 0.25% radiation lengths thick per ladder**
**PHENIX needs cooling ... their first layer is 1.2% thick**
**The HFT will have 10 ********m pointing resolution**
**PHENIX will have 50 ********m pointing resolution**
**Our p****T**** threshold for D****0****s will be ~700 MeV**
**PHENIX will have ~2 GeV ... we get 5 times the spectrum yield**
**The large RHIC collaborations have similar physics goals**
**PHENIX does single electron spectra very well**
**We will do this plus the direct topological reconstruction of open Charm!**

## Where does Charm come from?

**Gluon Fusion and qq-bar annihilation dominate the production of charm at RHIC **
**Initial state**
**Thermal processes are important but not dominant**
**Final state effects**
**Instantaneously equilibrated QGP shown for reference **
**In the real world, thermal distributions are less important due to the large mass of the c quark (not true in the strange quark sector)**

**pre-thermal: scattering between free streaming partons**
**thermal: assumes parton equilibration**
**Assume 3.5 GeV/fm****3**** at instant of equilibration**

- Levai, Mueller, and Wang, PRC 51, 3326 (1995).

**Notes:**
In heavy ion collisions, final state interactions may also enhance charm production relative to the binary scaling of initial parton collisions and may also lead to additional production of charm via “thermal” processes.  An analogous mechanism dominates strangeness production.  The heavy quark channels should be greatly suppressed by the thermal factor due to the heavy quark mass (mc ≈ 1.2 to 1.8 GeV, mb ≈ 4.5 to 5.0 GeV)[i], making heavy quark production rates primarily sensitive to the dynamics of the initial collisions.  This is especially true for the b quark.  Figure 2 shows the calculated contributions to the total charm production at y = 0 at √sNN = 200 GeV, with an energy density of 3.2 GeV/fm3 at the moment of thermal equilibration.  
[i] P. Levai, B. Mueller, and X.N. Wang, Phys. Rev. C51, 3326 (1995) : P. Levai and R. Vogt, Phys. Rev. C56, 2707 (1997).).
Figure 2: Contributions to charm production at RHIC energies include the initial gluon fusion and q annihilation phase (solid line), a pre-thermal phase characterized by scattering between free-streaming partons, and a thermal phase that assumes parton equlibration.  The distributions were calculated with an energy density of 3.2 GeV/fm3 at the moment of thermal equilibration.  As a reference, the faint dotted line is the thermal production of charm due to an initial fully equilibrated QGP. The figure is from Ref. [15].

## How many c c-bar pairs per collision?

**Many ingredients are required to understand the formation of charmed hadrons at RHIC including the parton distribution functions for the projectile and target and the cross section for gluon fusion and qq-bar annihilation.**
**The cross-sections can be calculated in NLO perturbative QCD**
**The pdf’s come from e-p data**
**Ramona Vogt updates these estimates every few years **
** ****R. Vogt, hep-ph/0203115, hep-ph/0203151 **
**The nucleon-nucleon cross sections are extrapolated to Au-Au by assuming ~1000 binary scatterings in a central collision**

|    Theory:  NN (c )  =  289 - 445 μb
   Exp:       NN (c )  =  900 - 1400 μb

   20 - 30 c  pairs per central Au+Au collision at √sNN = 200 GeV |
| --- |
|    Theory:   NN (b)  = 1.64 - 2.16 μb 
   Exp:        NN (b)  =  ??

   0.04 - 0.06 b pairs per central Au+Au collision at √sNN = 200 GeV |

## 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.**

## Backgrounds can be reduced with the HFT

**Direct topological reconstruction of charm avoids the single electron background problem**
**But the HFT can also reduce the conversion electron backgrounds by judicious cuts in the TPC & HFT **
**The HFT enables better single electron measurements**

**Figure:  Electron p****T ****spectra from ******** conversions reconstructed by requiring TPC tracking or TPC hits and 2 HFT hits.  The rejection factor is about 16:1**

**Notes:**
Figure 13:  Electron pT spectra from  conversions reconstructed by requiring TPC tracking, a TPC hit and 2 SVT hits, or a TPC hit and 2 SVT hits and 2 HFT hits.  The rejection factor with an additional SVT hit (2 hits) is 3 and with an additional HFT hit is 50.[HM1]
These electrons are, to a large extent, from photons converting into electron-positron pairs   e+ e- in the detector material.  The HFT detector will reduce the background of electrons and positrons from these  conversions.  By requiring hits in the HFT, electrons from photon conversion outside the HFT, i.e. the SSD, and TPC inner field cage, are rejected. Figure 13 shows the resulting yield from pure Monte Carlo simulations (no tracking involved) of conversion electrons when requiring hits in the TPC only (open circles), hits in the TPC and hits in the TPC+SSD+HFT [HM1](closed circles). The requirement of hits in the HFT reduces the detected yield of conversion electrons by a factor of 50.  Full simulations with appropriate pile-up events show that the rate of random ghost tracks is 5%. This means that we can achieve a maximum  rejection by a factor of 20. To estimate the signal-to-background ratio in the vector meson measurements, we take a reasonably conservative approach and assume that the HFT can reject  conversions by a factor of 10.  Another source of electron background comes from the semi-leptonic decays of heavy quark hadrons.  The dominant source at intermediate mass[i],[ii] is from the semi-leptonic decay of open charm.  Due to the large charm yield at RHIC energies, the number of e+e- pairs from charm-anti-charm decays is comparable to that from  conversion and  and  Dalitz decays after the HFT rejection factor, described above, is applied.  Due to the lifetime of ~ 100 m of heavy-flavor hadrons, this charm semi-leptonic decay background can be greatly reduced by measuring the displaced decay vertex with high resolution as can be provided by the HFT detector.  Detailed simulations show that HFT is capable of rejecting 75% of e+e- pairs from the D0 decay while preserving 50% of the primordial e+e- pairs. This increases the signal-to-background ratio by a factor of 2[HM2]. 

## Working with the rest of STAR   ...   (       )n

**	****Combining the power of the STAR TOF barrel to identify electrons with**
**	****the ability of the TPC+HFT to identify and eliminate conversion electrons **
**	****means we can execute a vigorous single electron and di-electron program of measurements**

**Notes:**
Figure 12:  Left top: TPC dE/dx vs p in d+Au collisions.  Left bottom: TPC dE/dx vs p after TOFr PID selection of | 1- | < 0.03.   Clean electron identification is achieved.  Right: dE/dx from the TPC after TOFr PID selection for 1.0 < p < 1.5 GeV/c.[HM1]

At STAR, electron identification is made possible by a combination of two measurements: a measurement of the energy lost by charged particles due to ionization in the TPC gas (dE/dx), and a velocity measurement with the time-of-flight system.  The relativistic rise of the electron dE/dx separates the electron dE/dx from those of hadrons except at the crossovers with pions at a momentum of  ~ 0.2 GeV/c, kaons at ~ 0.6 GeV/c, protons at ~ 1.1 GeV/c and deuterons at  ~ 1.5 GeV/c.  A time-of-flight measurement, using a requirement that |1-| < 0.03, eliminates slow hadrons and cleans up the crossovers.  This results in clean electron identification, as shown in Figure 12. The top plot on the left shows the conventional dE/dx measurement in the TPC gas as a function of particle momentum. The bottom plot on the left shows the same dE/dx measurement but now requiring |1-| < 0.03 from the TOF detector.  The upper electron band is clearly separated from the lower pion band. Heavier particles (e.g. kaons and protons) are completely removed by the TOF requirement.  The right hand plot of Figure 12 shows a dE/dx projection in the transverse momentum range pT = 1.0-1.5 GeV/c.  Even at this rather large momentum, the distribution of electrons is clearly separated from the distribution of charged hadrons 

## Measuring Vector Mesons with Dileptons

**Dileptons are a valuable probe of the early stages in a HI  collision**
**However, the signals are relatively rare and the backgrounds are high (Signal in Black)**
**Red Curve shows the sum or all backgrounds into the 2004 detector (including the SVT)**
**Grey curve is e****+****e****-**** spectrum after rejection by the HFT**
**Blue dashed curve is Dalitz decays after rejection by the TPC**

- Pulling the Signal out of the Background

**Notes:**
Figure 14: Di-electron invariant mass spectrum.  The solid black curve is a prediction from R. Ralf Ref. [78] using the full STAR acceptance. The red curve at the top is the total di-electron invariant mass spectrum seen with the 2004 configuration of STAR but assuming electron PID from Full TOF coverage. The pink dashed line is the e+e- pair invariant mass spectrum from charm semi-leptonic decays. The gray curve is the charm e+e- pair invariant mass distribution after a cut on the HFT DCA < 80um.  The dot dashed line is the Dalitz decays from 0 and  after rejection from TPC. 
Figure 14 shows the di-electron invariant mass distribution for various signals plus the background. The signals for vector mesons and thermal radiation, are shown as a black curve78 and have been calculated using the full acceptance of the STAR detector. The red curve, at the top, is the total di-electron invariant mass yield using the STAR configuration in 2004.  This is obtained from single inclusive electron spectrum measurements in 200 Au+Au collisions with the assumption of electron PID from Full TOF coverage. The pink dashed line is the e+e- pair invariant mass spectrum for charm semi-leptonic decays derived from the non-photonic single electron spectra measured in Au+Au collisions. The gray curve is the e+e- pair invariant mass distribution for charm after a cut on the HFT DCA < 80m. The dot dashed line show the Dalitz decays from 0 and  after rejection from the TPC. The standard method of dealing with the residual background is by a mixed-event method. This method has been used in CERES and NA52/NA60 at the SPS, and will be used in both PHENIX and STAR. The red line is all sources of background (conversions, Dalitz, resonances) assuming the 2004 configuratoin (with the SVT) and asuming a full barell of TOF.  The conversion background is 80% of this.  It can be removed with the TPC and HFT cuts (see previous plot in the propsal.)  Then you have two sources left.  Dalitz decays (which go down due to TPC cuts and a mixed events subtraction) and also the combinatorics of finidng the ccbar pair even if you know all electrons come from charm decays.

## An example: The  and 

**	****The large reduction in photonic background will enable us to observed short lived vector meson decays**
**The conversion background is reduced by requiring a hit in the HFT **
**Charm semi-leptonic decay background filtered by DCA**
**Reject ******** and ********0**** Dalitz decays by measuring both electrons, of a pair, in the TPC**

|  Detectors  |  |   | Comments  |
| --- | --- | --- | --- |
| TPC+TOF+SSD+HFT |  22 K |  6 K |   Pairs per RHIC year |

**Table 3:  The number of vector meson pairs which can be recorded by STAR  in one RHIC year.  We assume 200 M central Au***** + *****Au events will be recorded per RHIC year when DAQ 1000 is operational.**

**Notes:**
Table 2:   Number of central Au + Au events required to observe a 3- signal for  and  in their leptonic decay channels under different detector configurations.
The large reduction in photonic-electron background will enable us to observe electromagnetic decays of short-lived vector meson, e.g. ,   e+e, and intermediate mass dileptons with a few hundred thousand central Au+Au events in STAR.  The rejection of π0 and  Dalitz decays by a factor of 3 (single track) can be achieved by measuring both electrons, of a pair, which is possible because of the large acceptance of the STAR TPC.  Table 2 shows our estimate of central Au+Au collisions needed in order to observe a signal with a statistical significance of 3. Compared to measurements incorporating only the TPC and TOF, the HFT will enhance the sensitivity to electro-magnetic vector meson decay by a factor 20-40. 

## Si Pixel Developments in Strasbourg

**Mimosa – 1**
**4k array of 20 ********m pixels with thick epi layer**
**Mimosa – 4**
**Introduce Forward Biased Diode**
**Mimosa – 5  **
**1M array of pixels, 17 ********m pixels using AMS 0.6 process**
**4 msec readout scan rate**
**Mimosa – 8**
**Fast parallel column readout with internal data sparsification**
**200 ********sec readout scan rate**

**MimoSTAR – 1   128x128 pixels  using TSMC 0.25 **
**MimoSTAR – 2   128x128 pixels  using AMS 0.35**
**Duct tape these to the STAR Beam Pipe for 07 run **
**MimoSTAR – 3****	****320x640 pixels  using AMS 0.35**
**MimoSTAR – 4   640x640 pixels  production run**

**Ultra – 1**
**Ultra – 2**

## HFT R&D and Installation Timeline 

- Install MimoSTAR II
- Telescope

- X marks the installation dates.  Running comes *after*  installation.

- Proposed HFT Timeline – the HPD and IST come about 1 year later

## BNL Mid-Term Plan

**The BNL Mid-Term Plan creates a new opportunity**
**The BNL FWP and LBL FWP include the following schedule**
**HFT Funding Profile**
**	****	****06****	****07****	****08****	**** 09 ****	**** 10 ****	**
**	**** ****300K         1M     800K+300K    2.5M       2.5M****	****  **
**	**** ****R&D         R&D   R&D+Const   Const     Const**
**IST+FST+FGT Funding Profile**
**	****	****06****	****07****	****08 ****	**** 09  ****	**** 10 ****	**** 11**
**	**** ****200K         200K      500K         1.5M        4.0M****	****3.0M  **
**	**** ****R&D         R&D        R&D        Const       Const****	****Const**
**The financially driven schedule makes Mimosa-8 technology available in time to complete the project**
**We propose to extend the scope of the project**
**Do extensive R&D and risk analysis with MimoSTAR-4 chips**
**The final detector will be based on MimoSTAR-XXX chips**

## Charm Yields 

**Open charm yields have been measured at RHIC by STAR and PHENIX**
**Single electron spectra and direct toplogical reconstruction of open charm**
** ****D****0**** ******** K****± ****+ ********±**** **
** ****D  ********  e ****±**** + X**
**Yields appear to exceed Pythia and NLO pQCD expectations**

**The blue dashed line depicts a PYTHIA calculation.  **
**The red dot-dashed line depicts a NLO pQCD calculation with m****c**** = 1.2 GeV/c****2****,   μ****F**** = 2m****c****,    μ****R**** = 2m****c**

**Notes:**
Figure 3:  Total c production cross-sections per nucleon-nucleon collision vs. collision energy.  The dashed line depicts a PYTHIA calculation[i].  The dot-dashed line depicts a NLO pQCD calculation with MRST HO, mc = 1.2 GeV/c2, μF = 2mc, μR = 2mc[ii].  The figure is adapted from Ref. [17, 19].
[i]T. Sjostrand et al., Computer Physics Commun.135, 238 (2001).  PYTHIA 6.152 was used with the parameter settings: MSEL=1, CTEQ5M1. Bottom contribution was estimated to be 20-30% in pT=2-3 GeV/c, 40-50% in 3-4 GeV/c to the total non-photonic electrons, and negligible at lower pT.
[ii] R. Vogt, Private Communication. 
STAR and PHENIX have made measurements of charm production in Au+Au, d+Au, and p+p collisions at RHIC.  The PHENIX data for Au+Au collisions suggests that the cross-section for open charm production is consistent with the expectations of pQCD.  These results were derived from non-photonic single electron spectra[i] as shown in Figure 3. The STAR results show that the cross-section for open charm production in d+Au  and Au+Au is consistent with binary collision scaling but the total yield may be larger than NLO pQCD models. Both the pT integrated yield of dN/dy and the value of <pT> of the measured D-meson spectrum are larger than the pQCD model predictions. The STAR results are obtained from a direct reconstruction of the open charm hadrons[ii] as well as through single electron spectra. [i] K. Adcox et al., Phys. Rev. Lett. 88, 192303 (2002).
[ii] J. Adams et al. (STAR Collaboration), Phys. Rev. Lett. 94, 062301 (2005);  nucl-ex/0407006.

## High pT Suppression for Charm and Beauty

**Figure 8: The ratio of suppression factors in hot matter for charm (H) and light (L) quarks. The solid line represents results from calculations with unrestricted gluon radiation, while the dashed line is based on calculations with a cut on gluon energies ******** > 0.5 GeV.****  The size of the static medium traversed by the fast quark is assumed to be 5 fm.  From Dokshitzer and Kharzeev.**

**A major contributor to light and  heavy quark energy loss is Gluon Bremsstrahlung**
**The radiation is determined by multiple scattering in the medium and is suppressed when the energy of the radiation is so high that the gluon formation time exceeds the finite size of the medium**
**The radiation comes out in a cone and is suppressed when**
**	**** ******** ****< M/E**
**	****“****The dead cone effect”**

- Heavy Quark energy loss should be lower than for light quarks.

**Notes:**
Figure 8: The ratio of suppression factors in hot matter for charm (H) and light (L) quarks. The solid line represents results from calculations with unrestricted gluon radiation, while the dashed line is based on calculations with a cut on gluon energies  > 0.5 GeV.  The size of the static medium traversed by the fast quark is assumed to be 5 fm.  The figure is from Ref. [51].

 Heavy quarks are also predicted to lose energy as they traverse the medium, however, their energy loss is predicted to be significantly less because of a suppression of gluon radiation at angles  < MQ /E ; where  MQ is the heavy quark mass and E is the heavy quark energy[i].  This kinematic effect is known as the “dead cone” effect. The suppression of small angle radiation has the advantage that the heavy quark fragmentation function and the spectrum of light particles produced in association with the heavy quarks can be calculated perturbatively.
Figure 8 shows the result for the ratio of charm (H) to light (L) quark suppression from QCD calculations assuming a size of about 5 fm for the static medium traversed by the fast quark.  For transverse momentum pT > 7.5 GeV/c this ratio is predicted to be about 2.0 due to the smaller energy loss of the heavy quark.  In the case where heavy quarks suffer the same amount of energy loss as light quarks, this ratio would be 1.0. This ratio is exponentially sensitive to the density of color charges in the medium, and so the measurement of nuclear modification factors of open charm mesons at large pT is a promising tool for the study and further characterization of QCD matter at RHIC. 
[i] Y. L. Dokshitzer and D. E. Kharzeev, Phys. Lett. B519, 199 (2001).

## Charm Angular Correlations

**Correlations between charmed hadrons are a way to separate charm and beauty physics at RHIC**
**Heavy quark production requires a large momentum transfer**
**back to back topology for quarks**
**The amount of broadening of this correlation is a measure of energy loss in the medium**

**Figure 10: D-meson correlation functions for 200 GeV p+p collisions.  Default parameters in the Pythia model were used in these calculations.  A clear back-to-back correlation in the angular distribution of charmed mesons is observed (shown by the open circles).  The Solid-line and the diamonds represent the results with angular smearing for ************ = ********/4 ****and ********/2, respectively.**

**Notes:**
Figure 10: D-meson correlation functions for 200 GeV p+p collisions.  Default parameters in the Pythia model were used in these calculations.  A clear back-to-back correlation in the angular distribution of charmed mesons is observed (shown by the open circles).  The Solid-line and the diamonds represent the results with angular smearing for  = /4 and /2, respectively.
Correlations between charmed hadrons are another way to separate charm and beauty physics at RHIC.  Let the correlation between D mesons be defined as the normalized pair distribution  C() = N(p1,p2)/N0 where p1 and p2 are the momenta of the charmed hadrons, and N0 is the total number of pairs. 
Similar to jet production, heavy-quark production requires a large momentum transfer and so we expect a distinct back-to-back topology  for the quarks (and mesons) as shown by the open-circles in Figure 10.  In this calculation, the Pythia (v6.2) event generator was used with the default sets of parameters. As one can see in the figure, there is a clear back-to-back correlation for the D-mesons.  We propose to utilize this distinct correlation to study the charm-quark energy loss in high-energy nuclear collisions.