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The STAR Detector at RHIC

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 HFT: 2 layers of Si at mid rapidity. Mid-rapidity Pointing Devices: HPD + IST + SSD.

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The STAR Detector at RHIC

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  1. The STAR Detector at RHIC

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

  3. The HFT: 2 layers of Si at mid rapidity Mid-rapidity Pointing Devices: HPD + IST + SSD The Heavy Flavor Tracker + HPD + IST + SSD • A new detector • 30 mm silicon pixels to yield 10 mm space point resolution • Direct Topological reconstruction of Charm • Detect charm decays with small ct, including D0 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)

  4. 50-150 m Direct Topological Identification of Open Charm Goal: Put a high precision detector near the IP to extend the TPC tracks to small radius 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.

  5. Lattice results The Light Quark Program at STAR is Compelling Its hot Spectra Its dense Jets & Rcp and it flows at the partonic scale Vn Now we can make these measurements in the charm sector  and , too!

  6. Semiperipheral collisions y Coordinate space: initial asymmetry Momentum space: final asymmetry px x Signals early equilibration (teq 0.6 fm/c) Flow: Probing Thermalization of the Medium py

  7. Flow: Constituent Quark Number Scaling In the recombination regime, meson and baryon v2 can be obtained from the quark v2 :

  8. V2 / nq Scaling as a Function of (mT – m0) STAR Preliminary work by Yan Lu • The light quark sector scales beautifully with v2/nq .vs. (mT – m0)/nq • Note that pT < 1 GeV always did scale ! • The strange quark sector also scales with <v2> 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

  9. “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

  10. 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: g 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! Shingo Sakai, QM 2006 for the PHENIX Collaboration Better if we can do direct topological identification of Charm

  11. 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 RAA measurements • The decayed spectra are shown in black. The sensitivity to the original spectra are reduced. • We heard this message many times at QM H. Liu et al., Phys. Lett. B 639 (2006) 441. S. Batsouli et al., Phys. Lett. B 557 (2003) 26. We need direct topological identification of Charm

  12. RAA and RCP for D0s… Critical measurements • Flow and energy loss are intertwined • you can’t have flow without E loss • RAA is a measure of E loss • Two models for RAA 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

  13. Heavy Flavor Energy Loss … RAA 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?

  14. 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

  15. Baryons vs. mesons • Coalescence and fragmentation conspire at intermediate pT 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 Λc will be a fascinating test … and we might be able to do it with the HFT via Λc / D

  16. 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 Λc and J/ by 80% and a factor of 10, respectively • Systematic errors cancel in Ds+/D+ due to similar decay channels Andronic et al.,. Phys. Lett. B571, 36 (2003). Table 1: Charm quark fragmentation functions. The D+ and D0 yields include feed-down from D*+ and D*0 decays. Ds+/ D0, Ds+/ D+ and J/ / D0 are sensitive probes of thermal charm production & history

  17. 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

  18. 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 RAA in the Charm sector • Vector Mesons • Charm Angular Correlations • non-photonic electrons • …

  19. 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 )

  20. 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 • V2 • Energy Loss • Charm Spectra, RAA & Rcp • Vector mesons • Angular Correlations • The technology is available on an appropriate schedule

  21. Backup slides

  22. The STAR Inner Tracking Upgrade is Unique at RHIC • The Inner Tracking Upgrade will cover 2p in f azimuth • PHENIX Si covers 2p in f but the rest of the detector is 2 arms of p/2 • The Inner Tracking Upgrade will cover ± 1 unit of h • PHENIX Si covers ± 1 unit but the rest of the detector covers 1/3 unit • The HFT uses 30x30 mm pixels for high resolution tracking • PHENIX uses 50x425 mm pixels (… strips …) • The HFT uses 50 mm thick Si in each of 2 layers • PHENIX uses 350 mm thick Si (sensor plus readout) in 2 layers and 1250 mm 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 mm pointing resolution • PHENIX will have 50 mm pointing resolution • Our pT threshold for D0s 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!

  23. 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) Levai, Mueller, and Wang, PRC 51, 3326 (1995). • pre-thermal: scattering between free streaming partons • thermal: assumes parton equilibration • Assume 3.5 GeV/fm3 at instant of equilibration

  24. 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

  25. 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.

  26. 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 pT spectra from  conversions reconstructed by requiring TPC tracking or TPC hits and 2 HFT hits. The rejection factor is about 16:1

  27. 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

  28. 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

  29. An example: The w and f 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 p0 Dalitz decays by measuring both electrons, of a pair, in the TPC 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.

  30. 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

  31. Install and run MimoSTAR IV Detector (Full) Install MimoSTAR III Ladder Install MimoSTAR IV Prototype Detector Install UltraSTAR Detector (Full) X X X X X 06 07 08 09 10 Install and test Prototype detector. Reduced diameter BP is required. HFT R&D and Installation Timeline Install MimoSTAR II Telescope Proposed HFT Timeline – the HPD and IST come about 1 year later X marks the installation dates. Running comes after installation.

  32. 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

  33. Charm Yields • Open charm yields have been measured at RHIC by STAR and PHENIX • Single electron spectra and direct toplogical reconstruction of open charm • D0 K± + p ± • 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 mc= 1.2 GeV/c2, µF = 2mc, µR = 2mc

  34. High pT Suppression for Charm and Beauty • 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” 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. Heavy Quark energy loss should be lower than for light quarks.

  35. 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  = /4and /2, respectively.

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