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Future of STAR Measurements in the Heavy Quark Sector

Future of STAR Measurements in the Heavy Quark Sector. James C. Dunlop Brookhaven National Laboratory. Outline. Physics motivation: what remains to be learned in the heavy quark sector? Current detector and upgrade strategy/timeline

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Future of STAR Measurements in the Heavy Quark Sector

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  1. Future of STAR Measurements in the Heavy Quark Sector James C. Dunlop Brookhaven National Laboratory Heavy Quark Workshop, Dec. 2005

  2. Outline • Physics motivation: what remains to be learned in the heavy quark sector? • Current detector and upgrade strategy/timeline • Limitations of current detector and how upgrades remove these STAR: nucl-ex/0511005, nucl-ex/0510073, nucl-ex/0510063 Heavy Quark Workshop, Dec. 2005

  3. A central question: the relative yield of c and b The observed suppression of non-photonic electons (NPEs) is not presently understood! Attempts to reproduce it have completely changed the paradigm for the energy loss of light and heavy quarks Armesto et al, private comm. Resolving this is a crucial next step Heavy Quark Workshop, Dec. 2005 Djordjevic et al, nucl-th/0507019

  4. Further measurements of NPEs alone won’t solve the problem The low end The high end M. Djordjevic et al The relative yield of charm and bottom is highly uncertain The collisional and radiative energy loss for the two is predicted to be different The charm spectra must be measured directly to untangle the two contributions (Bears on the interpretation of the suppression for light quarks as well) Heavy Quark Workshop, Dec. 2005

  5. Kinetic Equilibration: Spectra and Flow at low pT • Charm or “charm resonance” interact with the medium via scattering: • Its phase space shape may be changed at low pT (<3-5 GeV/c) Moore and Teaney, PRC 71(2005) 064904 • Charm could pick up elliptic flow from the medium • Measurements of charm pT spectra and elliptic flow may give us a hint that the partonic matter might be thermalized Hees and Rapp, PRC 71(2005) 034907 Heavy Quark Workshop, Dec. 2005

  6. Chemical Equilibration: Open Charm Yields • No thermal creation of c or b quarks; m(c) = 1.1GeV >> T • c and b quarks interact with lighter quarks  thermal recombination ? • Ds+/D0 very sensitive • J/y: suppression vs recombination ? • Precision necessary for J/y, dilepton baseline Heavy Quark Workshop, Dec. 2005

  7. Temperature and density: Onium RHIC • Yields and Spectra of the onium states (J/, Upsilon, and excited states) to measure the • thermodynamics of deconfinement through varying dissociation temperatures • To deeply probe the plasma through studies of (Debye) screening length l ~ 1 /gT and map in-medium QCD potential • Study vs. Pt • Study vs. centrality • Study in lighter systems • Study vs. a control ( the Upsilon) • Upsilon rate ~ 10-3 J/Y • To date: Only an upper limit from ~30 ub-1, year 4 This physics requires luminosity upgrade 10 Heavy Quark Workshop, Dec. 2005

  8. STAR Detectors Heavy Quark Workshop, Dec. 2005

  9. Upgrades relevant to heavy flavor • Barrel Electromagnetic Calorimeter (EMC): High pt e • ¾ barrel of run 5 has been instrumented to full azimuthal coverage, -1 < h < 1, for next RHIC run: COMPLETE • Barrel Time of Flight (TOF): Particle ID (e, hadrons) • Current prototype patches to be upgraded to full azimuth, -1 < h < 1. • Project is funded and proceeding • Forward Meson Spectrometer (FMS): CGC studies • Full azimuthal EM Calorimetry 2.5 < h < 4.0 • Possibility of charm measurements in this region • Project is proceeding: complete by next d+Au run • Data acquisition upgrade (DAQ1000): Data rate 10x • Upgrade TPC readout an order of magnitude, ~double effective Luminosity • Target for completion: RHIC run in 2008 • Heavy Flavor Tracker (HFT): Displaced vertices • High precision (<10 um) measurements for displaced vertices • Goal: standalone detector in place for RHIC run in 2009 Heavy Quark Workshop, Dec. 2005

  10. STAR (Central) Coverage p Endcap EMC Barrel EMC Tracking (TPC,SVT,SSD,HFT) Tracking (TPC,SVT,SSD) Tracking (degraded) f TOF -p -1 0 1 2 h Heavy Quark Workshop, Dec. 2005

  11. RHIC Upgrade Timeline llllll From recent STAR PAC Talk: STAR will make much, much more effective utilization of AuAu beams in the 2008-2009 timeframe once several key upgrades have come on-line Heavy Quark Workshop, Dec. 2005

  12. Electron ID - TOF p K p K e |1/–1| < 0.03   e • TOF measures particle velocity • TPC measures particle energy loss • The cut |1/-1|<0.03 excludes kaons and protons • TPC dE/dx further separates the electron and pion bands Heavy Quark Workshop, Dec. 2005

  13. electrons hadrons d K p p electrons electrons hadrons Electron ID - EMC • TPC: dE/dx for p > 1.5 GeV/c • Only primary tracks • (reduces effective radiation length) • Electrons can be discriminated well from hadrons up to 8 GeV/c • Allows to determine the remaining hadron contamination after EMC • EMC: • Tower E ⇒ p/E • Shower Max Detector (SMD) • Hadrons/Electron shower develop different shape • Use # hits cuts • 85-90% purity of electrons • (pT dependent) • h discrimination power ~ 103-104 Heavy Quark Workshop, Dec. 2005

  14. Triggering Capabilities from the EMC • EMC provides a Level 0 high-pT electron trigger • Runs for every RHIC crossing (10 MHz) • Multiple ET thresholds in prescale ladder • For this plot, 2.5 and 5 GeV • Enhancement proven to be >1000 for pT > 5 GeV/c • Utilizes full RHIC Luminosity (modulo deadtime, currently ~50%) • More sophisticated triggers: • Upsilon • Limited only by luminosity • ~15K Upsilon in 30 nb-1 • J/Psi • Needs TOF for discrimination in Au+Au Heavy Quark Workshop, Dec. 2005

  15. Putting it together: current NPE measurements STAR Preliminary • TOF spectra measured in p+p, d+Au, Au+Au minbias, 0-20%, 20-40%, 40-80% • EMC spectra measured in p+p, d+Au, Au+Au minbias, 0-5%, 10-40%, 40-80% • Non-photonic electron spectra measured by TOF and EMC are consistent with each other QM05 Proceedings: J. Bielcik, nucl-ex/0511005 H. Zhang, nucl-ex/0510063 Current limitation in pT reach: coverage (half EMC, small patch TOF) and integrated luminosity (~50 ub-1 Au+Au on tape) Heavy Quark Workshop, Dec. 2005

  16. PID Capabilities III: Direct Reconstruction PRL 94 (2005) 062301 • Direct reconstruction using Minv • Uncertainty limitation is combinatoric background TOF: cleanly identify daughters HFT: identify displaced vertices NEvents for 3s D0→Kp Signal Number of Au+Au events required for 3s signal. FOM=reduction in Nevents from TOF Heavy Quark Workshop, Dec. 2005

  17. DAQ Limitations (and their removal) • Current limit from TPC front-end electronics is 100 Hz • Limits size of datasets • ~100M events/nominal RHIC run • Affects available luminosity • Deadtime scales linearly with rate • 50 Hz = 50% dead, i.e. 50% drop in luminosity available to rare triggers: usual compromise • Proposal to replace TPC electronics with ALICE chips to increase maximum rate by order of magnitude • Rate of events to disk increased (though timely processing of events on disk is an issue) • Removes deadtime: effective doubling of RHIC luminosity Heavy Quark Workshop, Dec. 2005

  18. Measurements with upgraded capability in RUN VIII Upgrades in place: DAQ1000, Half-barrel TOF • DAQ1000: untriggered AND triggered at same time • 15 weeks: ~900 ub-1 sampled AND few x 100M minbias events • Significant capabilities brought by large-acceptance TOF • Identified particle correlations in the intermediate pT regime • Dileptons: Significant (~10 s) signal in f→e+e- • Initial survey (statistical) measurement of D0→K + p to 4-5 GeV/c Heavy Quark Workshop, Dec. 2005

  19. D Measurements: Brute Force and its Limitations Event mixing technique Select K and  tracks from PID by energy loss in TPC Combine all pairs from same event  Signal+Background • Combine pairs from different events Background • Signal = same event spectra – mixed event spectra • More details about this technique can be found at • PRC 71 (2005) 064902 and PRL 94 (2005) 062301 Residual background Signal is tiny compared to background: loss in statistical power Heavy Quark Workshop, Dec. 2005

  20. Cost/benefit of techniques • Hadronic decay channels:D0Kp, D*D0p, D+/-Kpp • Advantage: complete reconstruction of final state • Disadvantage: not triggerable (need high DAQ rate and cross-section) • Improvement: displaced vertex reco • Semileptonic channels: • c  e+ + anything (B.R.: 9.6%) • D0 e+ + anything (B.R.: 6.87%) • D e + anything (B.R.: 17.2%) • Advantage: triggerable (full Luminosity) • Disadvantages: • Kinematics incomplete • No measurements at low pT: flow? • Mixture of B, D, and photonic decays Heavy Quark Workshop, Dec. 2005

  21. An enabling technology for event-by-event charm measurement in STAR: the Heavy Flavor Tracker • Heavy flavor collectivity • Charm quark kinetic equilibration • Heavy flavor (c,b) energy loss • Vector mesons → e+e- • Significant progress on: • Simulations • Mechanical design • integration and installation • support • alignment • calibration • Sensor prototyping • Readout design • Two layers of Active Pixel • Sensors (APS) • around a new thin • (0.5mm) small radius (14 mm) • beam pipe • 108 pixels, (30 mm)2 • Crucial for low pT: thin • 50 mm thick • 10 mm point resolution Heavy Quark Workshop, Dec. 2005

  22. D Measurements with the HFT: Run 9 D0→ K p using HFT, 50M events • From only 50M events, additional rejection power of HFT leads to extremely small uncertainties in both spectra and v2 • Charm quark flow can be fully addressed with this upgrade Also: Measure Ds f + p Heavy Quark Workshop, Dec. 2005

  23. Dominant source at low pT Photonic Background For each tagged e+(e-), we select the partner e-(e+) from TPC global tracks to make invariant mass. γ conversion π0Dalitz decay η Dalitz decay Kaon decay vector meson decays EMC TOF • Combinatorial background reconstructed by track rotating technique. • Invariant mass < 0.15 GeV/c2 for photonic background: removal efficiency ~50-60% Heavy Quark Workshop, Dec. 2005

  24. Future capabilities in photonic background • Current methods • Photonic electrons dominate below ~1 GeV/c • Clear nonphotonic signals above g e+e- Future capabilities: Large suppression by HFT Enabling technology for low-mass dilepton measurements Heavy Quark Workshop, Dec. 2005

  25. Strongly coupled probes: back to the Fragility of RAA Central RAA Data Increasing density K.J. Eskola, H. Honkanken, C.A. Salgado, U.A. Wiedemann, Nucl. Phys. A747 (2005) 511 Surface bias leads effectively to saturation of RAA with increasing density Challenge: Increase sensitivity to the density of the medium Method: decrease coupling of probe to the medium But: Non-photonic electron RAA ALSO in the saturation regime Not coincidental that electron RAA ~ light quark RAA, if both at ~lower bound Provocative question: Do we learn anything from charm RAA beyond the geometrical properties of Glauber overlap? A. Dainese, C. Loizides, G. Paic, Eur. Phys. J. C38(2005) 461 Heavy Quark Workshop, Dec. 2005

  26. How to find a weakly coupled probe: B? • Last hope for appreciable difference in RAA is in the B sector • Requirements for this to happen, given current measurements • B energy loss significantly smaller than charm AND • contribution to NPE not dominant at measured energies • Requirement is to isolate potentially small signal Heavy Quark Workshop, Dec. 2005

  27. b quark measurements The low end The high end • B mesons accessible using semileptonic decay electrons • Issue: nonphotonic electrons will be measured, but what is the real fraction of these from B? Highly model dependent • Subtraction of direct D measurements one possibility • Alternative: Using displaced vertex tag pT ~ 15 GeV/c: s (Au+Au) ~ 20mb/Gev 30 nb-1yields 600K b-bar pairs Tagging in Au+Au (w/ HFT) Heavy Quark Workshop, Dec. 2005

  28. Summary • STAR has proven capabilities for heavy flavor measurements at RHIC • Electron identification using three detector systems (TPC, TOF, EMC) from 1 to >10 GeV/c • Photonic background rejection using topological methods • Triggering capabilities to utilize full luminosity for rare probes • Direct reconstruction of charmed mesons • STAR has a clear path for improving its capabilities • Completion and extension of calorimetric coverage • Extension of TOF coverage to full azimuth for electrons and combinatoric background rejection in direct reconstruction • Upgrade of Data Acquisition to increase effective luminosity and untriggered data samples • Installation of the heavy flavor tracker for displaced vertices Heavy Quark Workshop, Dec. 2005

  29. Backup: Comparison QM05 and proceedings • Cuts on electron identification tightened post-QM: results consistent within errors • Updated results are in proceedings: nucl-ex/0511005, nucl-ex/0510073, nucl-ex/0510063 • Please ask me for data points: happy to provide them (as for all STAR datapoints that are in proceedings) STAR Preliminary Heavy Quark Workshop, Dec. 2005

  30. Heavy Quark Energy Loss • total energy loss comparable but smaller than in the massless case Armesto, Salgado, Wiedemann, PRD69 (2004) 114003 B.W. Zhang, E. Wang, X.N. Wang, PRL93 (2004) 072301 Djordjevic, Gyulassy, NPA733 (2004) 265 Flavor dependence of coupling: Less radiation, and so less suppression, for massive objects • vacuum radiation suppressed in the dead-cone q < m/E Dokshitzer, Kharzeev, PLB 519 (2001) 199 • medium-induced radiation fills the dead-cone Armesto, Salgado, Wiedemann, PRD69 (2004) 114003 massive dI/dk2 massless dead cone k2 Heavy Quark Workshop, Dec. 2005

  31. The STAR Detector Magnet Coils Central Trigger Barrel (CTB) ZCal Time Projection Chamber (TPC) Year 2000 Barrel EM Cal (BEMC) Silicon Vertex Tracker (SVT)Silicon Strip Detector (SSD) FTPCEndcap EM CalFPD TOFp, TOFrYear 2001+ Heavy Quark Workshop, Dec. 2005

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