# Presentation: 944527

**The STAR Heavy Flavor Tracker**
**Jim Thomas**
**Lawrence Berkeley Laboratory**
**11 / 07 / 2006**

## “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.

## 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 :

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

## 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

- The STAR Inner Tracking Upgrades will identify the daughters in the decay and do a direct topological reconstruction of the 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 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 **
**R&D with HFT + SSD**
**A proposal has been submitted and a TDR     is in preparation**

**The Heavy Flavor Tracker**

## R&D is Driven by the Fabrication Schedule

**R&D is Driven by the Fabrication Schedule**

- Driven by the availability of CMOS Active Pixel Sensors

| Fab-1999 | Fab-2001 | Fab-2003 | Fab-2004 | Fab-2005 | Fab-2006 | Fab-2007 | Fab-2009 |
| --- | --- | --- | --- | --- | --- | --- | --- |
| Mimosa-1 | Mimosa-4 | Mimosa-8 | MimoSTAR-1 | MimoSTAR-2 | MimoSTAR-3 | MimoSTAR-4 | UltraSTAR |

**Build a full detector with each**

- Alexandre Shabetai & Xianming Sun

## Selected Parameters and Specifications

| Min I efficiency | 98% |
| --- | --- |
| Accidental rate | < 100 /cm2 |
| Position resolution | < 10 m |
| Number of pixels | 135,168,000 |
| Pixel dimension | 30 m  30 m  |
| Detector chip active area | 19.2 mm  19.2 mm  |
| Detector chip pixel array | 640  640 |
| Number of ladders | 33 |
| Ladder active area | 192 mm  19.2 mm |
| Number of barrels | 2 |
| Inner barrel (9 ladders) | r = 2.5 cm |
| Outer barrel (24 ladders) | r = 7.0 cm |

## Surround the Vertex with Si

**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 STAR Inner Tracking Upgrade ( ITUp )

## The Heavy Flavor Tracker

## Andrew Rose, Sevil Salur, et al.

**Goal: graded resolution from the outside – in **
**TPC – IST – HPD – HFT **
**TPC pointing resolution at the SSD is  ~ 1 mm**
**SSD pointing at the IST   is  ~ 300 ********m **
**IST pointing at the HPD  is  ~ 150 ********m **
**HPD pointing at the HFT  is  ~ 100 ********m**
**HFT pointing at the VTX   is  ~ 50 ********m**

## Keep the SSD, it is a beautiful detector!	

**The SSD is thin**
**1% - double sided Si**
**The SSD lies at an ideal radius**
**23 cm - midway between IP and IFC**
**The SSD has excellent resolution **
**(rumor says better than design)**
**The SSD is too large to be replaced**
**The money is better spent, elsewhere**

## Hand Calculations of HFT + TPC Performance

**Hand Calculations of HFT + TPC Performance**

- Yan Lu & JT

## Hand Calculations

**Hand Calculations**

## 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!**

## Summary

**The STAR Inner Tracking Upgrade will explore the Charm sector**
**We will do direct-topological-reconstruction of open 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**