1 / 22

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.

suki
Télécharger la présentation

Perspectives for charm measurements at RHIC

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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

More Related