1. EM Probes in STAR A Look into the Future Thomas Ullrich, STAR/BNL International Workshop on Electromagnetic Probes of Hot and Dense Matter ECT, Trento June 10, 2005

2. 2 Current STAR Layout and EM Capabilities Detectors used for EM Probe Detection:  TPC: tracking p T > 0.2 GeV/c, PID via dE/dx for p T < 0.7-1 GeV/c (-1.3 <  < 1.3)  BEMC & EMC: e/  PID best for p > 1.5 GeV/c, trigger (0 <  < 2)  ToF: electron PID (   PMD:  detection, p > 20 MeV/c (2.3 <  < 3.7)  FPD: e,  PID for p > 10 GeV, x F > 0.4, small p T (3.4 <  < 4)

3. 3 Current EM Capabilities EMC+BEMC:  not optimized for low p T EM probes  large coverage and efficiency for l high-p T electrons (p > 1.5 GeV/c)  open charm, , Z (  s = 500 GeV) high-E photons  high-p T  0,  -jet, jet-jet ToF Patch:  good electron PID for p T < 3 GeV/c in conjunction with TPC l successfully used for non-photonic single electrons (open charm)  acceptance of present “prototype” too small for e + e  physics PMD:  photon detection down to 20 MeV/c l DCC studies  multiplicity and rapidity distributions in forward region FPD:  only for low-p T, high-p, x F > 0.4 physics (only p+p, d+Au. or peripheral Au+Au)  0, open charm, J/  (  ), at high x F

4. 4 Electron PID with MRPC TOF/TPC and EMC EMC 1.use TPC for p and dE/dx 2.use Tower E  p/E 3.use SMD shape to reject hadrons 4.e/h discrimination power ~ 10 5 5.works for p T > 1.5 GeV/c ToF 1.use TPC and ToF PID 2.works for p T < 3 GeV/c

5. 5  and  0 Studies Using the TPC Only STAR reconstructs  0,  from conversions in material inside the TPC  Material budget crucial l Sweet spot: ~6% radiation length from vertex to TPC  eff(  0 ) ~ eff(e  ) 4 PRC 70 (2004) 044902  130 GeV Au+Au  Inclusive  from 0 to 2.5 GeV/c   E/E = 2%  Fraction of    contribution to inclusive yield decreases in most central events  Large systematic uncertainties ~40%   normalization l complex interplay of corelated und un-correlated uncertainties

6. 6 Same Idea: Photonic Single Electron Background Subtraction Works well for photonic background rejection in single electron studies: 1.Combine candidate electron with opposite sign tracks anywhere in TPC 2.Reject tagged track when m < m cut ~ 0.1 – 0.15 MeV/c 2 Rejection Efficiency:  conversion and  0 Dalitz decay reconstruction efficiency ~60% Invariant Mass Square Rejected Signal Opening Angle  conversion and  0 Dalitz decay reconstruction efficiency : ~60% at p T >1.0 GeV/c

7. 7 Studies on EM Probes in STAR PMD: 62 GeV Au+Au Centrality dependence of dN  /dy (nucl-ex/0502008) FPD: Forward  0 production in 200 GeV p+p (PRL 92 (2004) 171801) Excellent (e,  )-h separation Other studies:  -HBT using TPC and EMC/TPC (  a la WA98)

8. 8 The Next Step: Upgrades Barrel Electromagnetic Calorimeter (EMC)  Current ¾ barrel will be instrumented to full azimuthal coverage, -1 <  < 1, for next RHIC run Barrel Time of Flight (TOF)  Current prototype patches to be upgraded to full azimuth, -1 <  < 1.  Project is in President’s budget. Forward Meson Spectrometer (FMS)  Full azimuthal EM Calorimetry 2.5 <  < 4.0  Possibility of charm measurements in this region  Proposal submitted to NSF Data acquisition upgrade (DAQ1000)  Upgrade TPC readout an order of magnitude, ~double effective Luminosity Heavy Flavor Tracker (HFT)  High precision (<10 um) measurements for displaced vertices

9. 9 Time-of-Flight: MRPC   /K separation up to 1.6 GeV/c  p/K separation up to 3 GeV/c  Thus cover wider range of (p,K  p T  Full ToF: -1 <  < 1, 2  Relevant for EM Probes: ToF and HFT Heavy Flavor Tracker (HFT) Two layers  1.6 cm radius  4.8 cm radius 24 ladders  2 cm by 20 cm  MIMOSA Active Pixel Sensor (CMOS) Precise (<10  m), thin and low power  50  m thick chip - air cooling l 0.36% radiation length l Power budget 100 mW/cm 2

10. 10 Time-of-Flight: MRPC  ToF + EMC l complement one another  ToF + TPC l Electron PID p < 3 GeV/c l Exactly where needed for J/Y l Low mass dileptons spectra l vector mesons allows us to trigger on J/   ToF used as fine granular  veto  ToF PID of K, , p allows D meson measurements up to higher p T Relevant for EM Probes: ToF and HFT Heavy Flavor Tracker (HFT)  SVT + HFT l Clean D meson sample (v 2 !) l Test statistical models Pythia p-p 200 GeV Au-Au Statistical recombination* D + /D 0 0.330.455 D s + /D 0 0.200.393  c + /D 0 0.140.173 J/  /D 0 0.00030.0004 (No suppr.) l Disentangle b,c contributions to non-photonic singel electron spectra  bb through B  J/  + X (?) A. Andronic et al., PLB 571,36 (2003).

11. 11 Low Mass Dileptons: What STAR Can Do Upgraded detectors: Full TOF+TPC SVT+HFT (  -Vertex detector) Electrons PID Reject electrons not from primary vertex (  conversion + Dalitz) NIM Article in preparation: Studies on Particle Identification with TPC and ToF  γ conversion and π0, η Dalitz decay background  How can μVertex detector deal with γ conversion subtraction?

12. 12 STAR not hadron blind  a low level of hadron contamination crucial Study in 62.4 GeV Au+Au  Hadron contamination increases for p T > 1.5 GeV/c (eff = const.) l need to accept slightly lower efficiency at intermediate p T l This is the p T range where EMC because effective  Hadron Rejection Power ~ 10 -5 for p T < 1 GeV/c l Def: (hadron contamination)  (e/h) / (electron efficiency) Low-p T Electron PID with ToF Evaluated through dE/dx fits

13. 13 Low-Mass Dileptons: Background Rejection Dalitz decay background/event: ~5∙10 -6 /25MeV (ω) ~5∙10 -7 /25MeV (Φ) Total background/event : ~10 -4 /25MeV (ω) ~2∙10 -5 /25MeV (Φ) 1 M PYTHIA Events Require TPC+SVT+μVertex (HFT): ~98% electrons from gamma conversion rejected Dalitz decays become dominant sources!!! Background inv. mass spectrum Conversion Electrons only

14. 14 What one wants … R. Rapp, hep-ph/0010101

15. 15 Vector Mesons Rate Estimate Assume: From PDG: TOF match+PID eff ≈ 80%TPC+SVT+μVertex eff ≈ 60% (?) Au+Au#events for ω with 3σ signal #events for Φ with 3σ signal TOF+TPC7M2M TOF+TPC+SVT+ μVertex (HFT) 800K (350K) 150K (50K) Preliminary estimates:

16. 16 b Quark Measurements with HFT 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 Using displaced vertex tag is the most promising method p T ~ 15 GeV/c:  (Au+Au) ~ 20  b/Gev  10 nb -1  yields 200k bb pairs Non-photonic electrons in d+Au Tagging in Au+Au (w/ HFT)

17. 17 DAQ Upgrades (1000 Hz) Current limit from TPC front-end electronics is 100 Hz  Limits size of datasets l ~100M events/nominal RHIC run  Affects available luminosity l Deadtime scales linearly with rate l 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 l though timely processing of events on disk is an issue  Removes deadtime: effective doubling of RHIC luminosity

18. 18 Summary  STAR has proven capabilities for EM probes and heavy flavor measurements at RHIC l PMD: Photon multiplicity FPD: forward  and electron detection - high x F physics l Electron identification using three detector systems (TPC, TOF, EMC) from 1 to >10 GeV/c l Direct reconstruction of charmed mesons  Shortcoming in PID, vertexing, and acceptance STAR has a clear path for improving its capabilities in the near future  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 for heavy flavor physics and photonic electron rejection Low Mass Vector Mesons and Thermal Dileptons Will Become Part of STAR’s Program

19. 19 Argonne National Laboratory Institute of High Energy Physics - Beijing University of Bern University of Birmingham Brookhaven National Laboratory California Institute of Technology University of California, Berkeley University of California - Davis University of California - Los Angeles Carnegie Mellon University Creighton University Nuclear Physics Inst., Academy of Sciences Laboratory of High Energy Physics - Dubna Particle Physics Laboratory - Dubna University of Frankfurt Institute of Physics. Bhubaneswar Indian Institute of Technology. Mumbai Indiana University Cyclotron Facility Institut de Recherches Subatomiques de Strasbourg University of Jammu Kent State University Institute of Modern Physics. Lanzhou Lawrence Berkeley National Laboratory Massachusetts Institute of Technology Max-Planck-Institut fuer Physics Michigan State University Moscow Engineering Physics Institute City College of New York NIKHEF Ohio State University Panjab University Pennsylvania State University Institute of High Energy Physics - Protvino Purdue University Pusan University University of Rajasthan Rice University Instituto de Fisica da Universidade de Sao Paulo University of Science and Technology of China - USTC Shanghai Institue of Applied Physics - SINAP SUBATECH Texas A&M University University of Texas - Austin Tsinghua University Valparaiso University Variable Energy Cyclotron Centre. Kolkata Warsaw University of Technology University of Washington Wayne State University Institute of Particle Physics Yale University University of Zagreb 545 Collaborators from 51 Institutions in 12 countries STAR Collaboration