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Perspectives for charm measurements at RHIC

Perspectives for charm measurements at RHIC. Ralf Averbeck State University of New York at Stony Brook. Workshop on “Open-charm production in nucleus-nucleus collisions” CERN, December 2-3, 1999. Outline. Motivation RHIC Charm at RHIC Charm in PHENIX Charm in STAR Summary.

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Perspectives for charm measurements at RHIC

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  1. Perspectives for charm measurementsat RHIC Ralf Averbeck State University of New York at Stony Brook Workshop on “Open-charm production in nucleus-nucleus collisions” CERN, December 2-3, 1999

  2. Outline • Motivation • RHIC • Charm at RHIC • Charm in PHENIX • Charm in STAR • Summary R. Averbeck, SUNY SB

  3. Charm in heavy-ion collisions Examples of physics related to charm: • Charmonium suppression (Debye screening) • Charm suppression (gluon shadowing) • Charm enhancement due to: • thermal pair production in QGP phase • pre-equilibrium production • Modification of spectra due to: • energy loss in parton medium • Complex and complicated scenario! Systematic charm measurements in pp, pA, and AA are one key element in the upcoming RHIC program! R. Averbeck, SUNY SB

  4. RHIC overview beams: nuclei and protons (all combinations) beam energy up to ~Z/A x 250 GeV = 200A GeV (Au-Au) = 500 GeV (p-p polarized) design luminosity Au-Au: 2 x 1026 cm-2s-1 p-p : 1 x 1031 cm-2s-1 4 experiments: BRAHMS: 2 magnetic dipole spectrometer experiment PHOBOS: table-top silicon detector experiment PHENIX: large scale multi-purpose experiment STAR: large acceptance tracking experiment Relativistic Heavy-Ion Collider RHIC at BNL 2 independent super conducting rings 3.8 km circumference 6 interaction points 60 bunches per ring R. Averbeck, SUNY SB

  5. RHIC schedule STAR TPC: beam-gas event during comissioning • Past: • comissioning in summer ‘99 • Present: • preparations for first physics run • Near Future (2000): • Jan: cooldown • Feb-Mar: startup • Apr-Aug: first physics run • mostly Au-Au at maximum energy • expected luminosity: up to 10 % of design value • total number of Au-Au collisions: ~108 • 1 RHIC year Au-Au @ design luminosity: ~15 x109 collisions R. Averbeck, SUNY SB

  6. Charm production at RHIC • cc production cross section prediction: • open charm hadro production in pN and pN: • measured in detail at (CERN, FNAL) • NLO perturbative QCD calculations: • tuned to describe “low-energy” data • extrapolated to : (P.L. McGaughey et al.: Int.J.Mod.Phys. A 10(95)2999) (I. Sarcevic, P. Valerio: PRC 51 (95)1433) • uncertainties up to factor 2 in predictions • extrapolation from pp to AA: • assume: point-like hard process; no shadowing: • consistent with FNAL data at (SPS: Ncc~0.2 ; LHC: Ncc~540) • J/Yproduction cross section: ~scc/100 (mass dependence:a=0.92(D.M. Alde et al.: PRL 66(91)133)) R. Averbeck, SUNY SB

  7. Charm measurements at RHIC • Large charm cross section at RHIC: Ncc~ 8 • Unfortunately: dNch/dy~1000 => tremendous hadronic background • How can charm be measured? • Charmonium (J/Y): dilepton decays • Open charm: semileptonic decays • single leptons: • lepton-pair continuum: • em coincidences: • D meson: • PHENIX will measure all channels! • STAR can contribute (“J/Y in STAR”) Leptons are the key!! R. Averbeck, SUNY SB

  8. The PHENIX experiment 11 sub systems: • event characterization: • BBC, MVD, ZDC • tracking: • DC, PC, TEC, MuTR • PID: • RICH, TOF, EMCAL (PbGl, PbSc), TEC, MuID 3 magnets: • central magnet: ~axial • 2 muon magnets: ~radial • large-scale complex detector setup: • e, g, h in 2 central arms: |y|<0.35; 30O<|f|<120O • m in 2 “forward” arms: 1.2<|y|<2.3; full azimuth • high-rate experiment: • event rate: up to 25 kHz; bandwidth: up to 20 MB/s • 1st physics run: EACH Au-Au collision will be recorded • day-one configuration: • global event-characterization detectors • both central arms • no muon arm (south arm: 2001; north arm: 2002) R. Averbeck, SUNY SB

  9. Lepton ID in PHENIX • Charm measurements <=> lepton measurements • Electrons in the central arms: available on Day 1 • excellent momentum measurement; high-momentum limit: Dpt/pt=0.3%pt (DC+PC1+PC2+TEC+PC3) • redundant electron identification: pion misident.<10-4 (RICH+TEC+EMCAL) • but: no Dalitz & conversion rejection (upgrade option) • Muons in the “forward” arms: available from Year 2 • muon tracking: 3 cathode strip chamber stations / arm • muon ID: 5 steel absorber layers interspersed with 6 streamer tube layers / arm • passive hadron absorber: central magnet steel and 1 copper cone / arm • pion misidentification < 10-4 (as for electrons) R. Averbeck, SUNY SB

  10. Single electrons in central arms “single electron” spectrum : (dN/dy(p0)~400) • History: e/p~10-4 @ ISR: open charm • PHENIX: 1st year observable • dominant contributions: • pt<1-2 GeV/c: po(h) Dalitz • pt>1.5 GeV/c : charm • pt>4 GeV/c: bottom • large e yield from charm: • ~3M / year @ design Lumin. • ~23k: 1st year (pt>1.5 GeV/c) • no combinatoric BG • high-pt: • e/p~1/1000 and pion re-jection better than 1000 • S/B~1 • BG: Dalitz, Conversion • BG spectra can be measured • sensitive to: • cross sections • energy loss of quarks electron/pion ratio : Y. Akiba: UCRL-ID-121571 (95)131 R. Averbeck, SUNY SB

  11. Single muons in forward arms Simulated pt spectrum of single muons detected in the PHENIX south muon arm: • available in year 2: one of the muon arms • similar to electrons in central arms, but: • different acceptance • different background • different systematics • complementary probes • dominant contributions: • pt >1.5 GeV/c: charm • pt>4 GeV/c: bottom • expected yield / year: >106m with pt>2 GeV/c • background: • p and K decay in flight before hadron absorber • extended vertex region in PHENIX (RMS~20 cm) => asymmetric vertex distribution for p/K decaying into detected muons (M. Brooks, J. Moss) R. Averbeck, SUNY SB

  12. Lepton-pair spectroscopy: e+e- • Statistics: ~ 5x108 collisions (1/3 RHIC year or 50 1st runs) • conservative estimate for charm production: Ncc=2.4 • charm dominates in the mass region 4-6 GeV • cc~10xDY above M~4 GeV • bb contributes significantly above M~8 GeV (not shown) • first data in year 1, sufficient statistics needs longer: • >4GeV: 5-10x103 pairs / year (muon arms: 4-5 times more) • Lepton-pair mass spectrum measured in PHENIX: • low-mass vector mesons (note the excellent resolution) • Charmonia (and Bottonia in the di-muon channel) • correlated and uncorrelated open charm and bottom • Drell-Yan • thermal radiation • Dalitz, conversions (di-electron channel) • combinatorial background (to be subtracted by event mixing, like-sign pair subtraction) • Example: simulated electron-pair mass spectrum in the central arms (Y. Akiba) R. Averbeck, SUNY SB

  13. Lepton-pair spectroscopy: m+m- PHENIX north muon arm: mass spectrum before and after like-sign pair subtraction. From PHENIX CDR: • Year 2 measurements • Vector mesons in the PHENIX muon arms (central AuAu, 1 year): • >300k r • >140k f • 700k J/Y (DM=105MeV) • 10k Y’ • >2k U (DM=180MeV) • S/B>>1 after like-sign pair subtraction • continuum in range 4<M<9 GeV/c2: domina-ted by open charm Lepton-pair spectroscopy in PHENIX provides: • excellent quarkonium measurement • sensitivity to open-charm continuum with different acceptance and systematics R. Averbeck, SUNY SB

  14. em-coincidence measurement From PHENIX CDR: • Correlated unlike-sign electron-muon pairs: • ISR: em coincidences in pp at attributed to charm (A.Chilingarov et al. PLB83(79)136) • PHENIX: unique em capabi-lity at RHIC, but: • year 2 measurement • PHENIX CDR simulation: • no energy loss taken into account • conservative charm cross section (scc=150 mb) • bottom ignored (significant above 6 GeV/c2) • main background: Dalitz electron + pion decay • background reduction: like-sign pair subtraction • cuts: pe>1 GeV/c, pm>2 GeV/c, fe-m>90o, qm>25o • pair accept.: M>1.5 GeV/c2, 0.2<y<1.5, all pt • expected yield: >100k pairs / RHIC year • cut strategies and bottom-charm separation are being revisited (H. Sato) R. Averbeck, SUNY SB

  15. Direct open-charm measurement: D->Kp Kp inv. mass distribution in pp events with open-charm pair (PYTHIA: Y. Akiba) • None of the semi leptonic channel is unambiguous • Direct measurement as supplement for indirect charm measurements: • K and p ID: TOF, RICH in central arms • lepton tag in central or muon arms to increase S/B • only feasible for: pp, pA, very light (peripheral) AA • expected yield in pp: 100 D0 with e tag per pb-1 (1 day) R. Averbeck, SUNY SB

  16. The STAR experiment Sub systems: • event characterization: • CTB, VPD, ZDC • tracking: • TPC, SVT, FTPC’s • PID: • TPC, SVT, TOF, EMC, RICH Magnet: • central solenoid • large acceptance tracking experiment (|h|<4) • designed for: • charged-particle spectroscopy, emphasis on hadrons • event-by-event physics • study of hard-scattering processes • not designed for: • measurement of rare observables (low rate experiment) • measurement of lepton pairs • day-one configuration: • TPC, FTPC, global detectors, RICH, parts of EMC and SVT R. Averbeck, SUNY SB

  17. Can STAR measure charm? • Current emphasis: J/Y -> e+e- measurement in Au+Au (T.LeCompte SN287, SN368; P.Jacobs, T.Ullrich SN391) • Advantages of STAR: • large acceptance: N~acceptance2 • electron ID in TPC, SVT, EMC • Disadvantages of STAR: • not hadron blind in scenario with dNch/dy~1000 • low rate (TPC: ~100 Hz, DAQ: ~1 Hz) • Solution: dielectron invariant-mass trigger • full event reconstruction on processor farm with hierarchical structure (scalable to 100Hz!) • pattern recognition in real time: • TPC tracking + PID (dE/dx & EMC) • track finding eff.: ~90% (|h|<1, pt>0.4 GeV/c) • momentum resolution ~ 1% • J/Y trigger: invariant mass of electron candidates • Essential questions: • J/Y rate and acceptance in STAR? • Is the particle identification good enough for a reasonable background suppression? R. Averbeck, SUNY SB

  18. J/Y rate and acceptance in STAR • Input: • dspp/dy=0.62mb, A dependence ~A2a with a=0.92 • J/Y cross section for 10% most central: 0.4 smin.bias • RHIC design luminosity: 2x1026 cm-2 s-1 = 0.2 mb-1Hz • BR(J/Y->e+e-) = 6 % • (J/Y)/Dy rate (suppression/absorption ignored): = (0.62x1972x0.92x0.4)mb x 0.2 mb-1Hz x 0.06 = 0.05 Hz • J/Y geometrical acc. (TPC, SVT, EMC): eacc = 0.83 • phase-space cut for background suppression pt>1.5 (2.0) GeV/c : eacc = 0.23 (0.06) • reconstruction efficiency: eacc = (0.9x0.7)2 = 0.4 • annual J/Y yield in central Au+Au collisions: 107sx0.05 Hzx0.83x0.23x0.4 = 40k (10k) J/Y / year • acceptance simulation: R. Averbeck, SUNY SB

  19. PID in STAR (e/h separation) • Particle identification below p = 2.5 GeV/c: • dE/dx in TPC (45 samples) and dE/dx in SVT (3 samples) B. Lasiuk: SN312 • hadron rejection factor ~ 10 • Particle identification above p = 2.5 GeV/c: • E/p~1 for e (EMC), shower profile (SMD): (T. Cormier) hadron rejection factor > 100 R. Averbeck, SUNY SB

  20. J/Y measurement in STAR • S = 40k (10k) J/Y / year (PHENIX: 30k with S>>B) • BG is entirely dominated by misidentified hadrons (Dalitz, conversion, other electrons: ~irrelevant) • HIJING simulation (Au+Au, b = 0 fm): • form hadron pairs with 3.0<m<3.2 GeV/c2 • pt>1.5 (2.0) GeV/c => 2.36x109 (8.0x107)h+h- pairs / year • hadron suppression: realistic: 100-200 but: extremely sensitive to the pt cut • assumption: hadron suppression factor = 150 => B = 105000 (3500) / year ( S/B = 1/3 (3/1) ) • Hadron suppression factor ~40 on the trigger level: • required for tolerable dead time • and can be achieved (trigger studies) J/Y measurement in STAR seems feasible ! R. Averbeck, SUNY SB

  21. Charm measurements in STAR • extreme sensitivity to hadronic background • efficient electron ID requires full detector: • EMC (30-40 % end Y2000, 70 % Y2001) • SVT (end Y2000) • J/Y requires software trigger • direct measurement of D decays (Kp): • S/B too low • option: displaced vertex cut (SVT upgrade: microTPC; CCD) • semi leptonic open-charm decays: • electron ID possible (J/Y study) • no invariant-mass trigger • additional cut on displaced vertex • c->K+X: • lack of K identification for pt>1 GeV/c (small-acceptance RICH: up to pt=3GeV/c) R. Averbeck, SUNY SB

  22. Summary • Large charm-production cross sections are predicted for RHIC • Measurements are: • DIFFICULT (huge background) • PROMISING (large physics potential) => Looks like a lot of fun! • Consistent picture will only emerge from measurements of: • open charm AND charmonium • ALL channels (leptons, lepton pairs, hadrons) • in pp, pA, and AA • PHENIX: puts strong emphasis on charm measurements from Day 1 on • STAR: can contribute to J/Y measurements upon availability of invariant-mass trigger • Both experiments: upgrade options to improve on charm physics capabilities R. Averbeck, SUNY SB

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