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The Charm and Beauty of Heavy Quarks in Heavy Ion Collisions (at RHIC)

The Charm and Beauty of Heavy Quarks in Heavy Ion Collisions (at RHIC). Thomas Ullrich, BNL/Yale Ohio State University Seminar Feb 27, 2004. D mesons. , Y ’, c. Heavy Flavor Production in Hadronic Collisions. Reactions that produce heavy-flavor involve a hard scale  pQCD

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The Charm and Beauty of Heavy Quarks in Heavy Ion Collisions (at RHIC)

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  1. The Charm and Beauty of Heavy Quarks in Heavy Ion Collisions (at RHIC) Thomas Ullrich, BNL/Yale Ohio State University Seminar Feb 27, 2004

  2. D mesons , Y’, c Heavy Flavor Production in Hadronic Collisions • Reactions that produce heavy-flavor • involve a hard scale  pQCD • At LO, charm and beauty production proceeds through • quark-antiquark annihilation • or gluon fusion • Factorize calculations: • pQCD to calculatecc production • cc propagation and hadronization LO, NLO, NNLO calculations magnitude, shape of spectra Charm mass total cross section kTpT spectra Parton Distribution Functions (PDF) rapidity dependence , √s dependence

  3. charm beauty gg / total Heavy Flavor Production in p+p Collisions At high energies, gluon fusiondominates the production cross-section  Heavy Flavor production directly probes gluon distributions of colliding particles Pythia 6.208 General: Heavy-flavor and quarkonia production is theoretically not fully understood … even in p+p collisions

  4. What do we know about charm production in p+p? • Data mostly from fixed target experiments at SPS and Fermilab, but also CDF • energy range : 200–800 AGeV (s = 19 – 38 GeV), s=1.8 TeV • p+A: linear nuclear A-dependence assumed spA = spp Aa , a = 1 charm pp pp beauty pp pp

  5. Charm production in p+p at RHIC energies (s=200 GeV)? • Different PDF sets or quark masses lead to different energy dependences • All the curves are normalized at low energies • the ‘predictions’ for higher energies have a certain spread : •  changing PDF sets : range 400–800 mb at RHIC energies changing c quark mass by 15% : range 300–700 mb RHIC CTEQ6M compiled by H. Woeri and C. Lourenco (SQM2003)

  6. Heavy Flavor Production in A+A Collisons • So far • Production little studied (CERN/SPS low s) • Charmonium suppression (melting of J/Y due to color screening) used as signature of QGP • But: • Low x: heavy quark pair is produced over long distances that can exceed the size of the nuclei (even though the matrix element of hard processes is dominated by short distances)  sensitive to medium  good probe for high density matter (QGP) • RHIC and LHC: • gluon density in nucleii ? • energy loss due to interaction with medium ? • shadowing (higher twist effect  gluon density – steeply rises with E) • cannot study J/Y suppression without understanding open charm

  7. Hidden and Open Charm – Both Needed SPS s = 17 GeV RHIC s = 200 GeV At RHIC open charm production provides reference and may be the only mean to understand charmonium suppression (same gluon conditions in the initial stage)

  8. Heavy Flavor Production in A+A Collisons • So far • Production little studied (CERN/SPS low s) • Charmonium suppression (melting of J/Y due to color screening) used as signature of QGP • But: • Low x: heavy quark pair is produced over long distances that can exceed the size of the nuclei (even though the matrix element of hard processes is dominated by short distances)  sensitive to medium  good probe for high density matter (QGP) • RHIC and LHC: • gluon density in nucleii ? • energy loss due to interaction with medium ? • shadowing (higher twist effect  gluon density – steeply rises with E) • cannot study J/Y suppression without understanding open charm • Heavy Flavor Production is a new frontier in Heavy-Ion Collisions

  9. Thermalization of “Heavy” Quarks • Strangeness: • 1. Lower energy threshold • Key concept is: • TQGP > >TC ~ ms = 150 MeV/c2 • 2. Larger production cross-section • 3. Pauli blocking (finite chemical potential) • Charm: • No thermal production • TQGP<< mc 1.3 GeV/c2 • Charm quarks interact with evolving QGP (light quarks, g) makes thermalization theoretically possible • Should show in • production rate (J/Ψ recombination) • pT-spectra (thermal, flow) • elliptic flow (v2)

  10. Thermalization I • Kinetic Formation Model • Thews, Rafelski hep-ph/0305316 • Charm produced in initial nucleon-nucleon collision • Competition between formation and breakup in region of deconfinement (QGP) • formation c +c + g  J/Y + g • breakup via gluons • formation  N2cc • 30% effect in rate at RHIC • negligible at SPS

  11. Thermalization II • Quark Coalescence Model • Greco, Ko, Rapp nucl-th/0312100 • Approach successful in light-quark sector • “anomalous” p/p ~ 1 ratio • “constituent-quark” scaling of v2 • Applied to charm production: D mesons e from D decays If complete thermalization at RHIC: J/Y: dN/dy increased by factor 3 pT slope increased by factor 2 D-mesons: less pronounced effect in De+X but v2 2  larger preserved in single-e spectra

  12. Suppression of inclusive hadron yield at high pT STAR, nucl-ex/0305015 pQCD + Shadowing + Cronin energy loss pQCD + Shadowing + Cronin + Energy Loss • central Au+Au collisions: factor ~4-5 suppression • pT>5 GeV/c: suppression ~ independent of pT • pQCD describes data only when energy loss is included

  13. Energy Loss of Heavy Quarks Vacuum radiation suppressed in the dead cone for θ < m/E Dokshitzer, Kharzeev, PLB 519 (2001) 199 Medium induced radiation fills dead-cone: total E-loss comparable but smaller than for m=0 Arnesto, Salgado, Wiedemann, hep-ph/0312106  enhancement of D/ ratio at moderate high pT (5-10 GeV/c)

  14. Open Charm at SPS (pA) Mm+m- Only measurement from NA50 via correlated m-pairs fromDD decays in the IMR: DD/Drell-Yan = 4.2  0.9 In pA at 450 AGeV with lots of PYTHIA: scc = 36.2  9.1 mb (|xF| > 0)

  15. Open Charm at SPS (AA) peripheral central Unexpected excess in the IMR in Pb+Pb @ 160 AGeV • excess grows with centrality • mass distribution compatible withDD • pT, y, cosθ* compatible withDD • but checks not conclusive • Still controversial: charm, thermal, • background subtraction method ?

  16. What do we know about charm at RHIC in A+A? PHENIX Au+Au@130GeV Phys. Rev. Lett. 88, 192303 (2002)

  17. TPC (tracking  pT ) ToF (PID < 3 GeV/c) EMC (PID > 1 GeV/c) Heavy Flavor and the STAR Experiment STAR measuring charm in leptonic channels: b, c  e + X STAR measuring charm in hadronic channels: D0  K(B.R. 3.8%) and K p r(B.R. 6.2%) D K p(B.R. 9.1%) D*±D0π (B.R. 68%) Lc p K p (B.R. 5%) all with TPC only

  18. D0 in STAR: Analysis Methods K p • Event-Mixing Technique • Identify charged Kaon and Pion tracks • through energy loss in TPC • Produce oppositely charged K- pair • invariant mass spectrum in same event • Obtain background spectrum through • mixed event • Subtract background and get D0 spectrum • d+Au: 15.7 M events K- Pair Invariant Mass

  19. D0 in d+Au Collisions D0+D0 to increase statistics 0 < pT < 3 GeV/c, |y| < 1.0 Gaussian function + linear Residual background • Mass and Width consistent • with PDG values considering • detector effects • mass=1.867±0.006 GeV/c2; • mass(PDG)=1.8645±0.005 GeV/c2 • mass(MC)=1.865 GeV/c2 • width=13.7±6.8 MeV • width(MC)=14.5 MeV

  20. D* Mesons in d+Au Collisions D*±D0π (B.R. 68%) Decay Kinematics (Pythia) “Golden channel” for open charm study Standard method: M(D*±) – M(D0)=145.421 MeV Width~1 MeV Difficulty: the low efficiency of the soft pion reconstruction STAR full field: tracks curl up for pT < 100 MeV/c

  21. D* Mesons in d+Au Collisions D0  K-p+p0 (B.R. 13.1%) D*± D0 from D* decays D0 2.4<pt<3.5 GeV/c 2.4<pt<3.5 GeV/c Masses and Widths OK: m(D*)-m(D0) =0.1467±0.00016 GeV/c2 m(D*)-m(D0)(PDG)=0.1454 GeV/c2 m(D*)-m(D0)(MC)=0.1451 GeV/c2 width=0.43±0.14 MeV width(MC)=0.67 MeV

  22. D± 7.4<pt<9.3 GeV/c D Mesons in d+Au Collisions • D± mass=1.864±0.0052 GeV/c2 • D± mass(PDG)=1.869 GeV/c2 • D± mass(MC)=1.868±0.002 GeV/c2 • width = 13.83±3.7 MeV • width (MC)=14.9±1.6 MeV • D±Kππ (B.R. 9.1%) • 3-body decay  more background • high-pT reach

  23. D*/D0 Ratio • CDF: hep-ex/0307080 • HERA: www-h1.de/h1/www/ publications/conf_list.html • e+e-: hep-ph/0312054 • Statistical model: Andronic et al. nucl-th/0209035 Good agreement with other experiments: D+/D0 D*/D0 = 0.40  0.09 (stat)  0.13 (sys)

  24. D-Meson Spectra in d+Au Assuming σ(D*) = σ(D±) and scale σ(D*) and σ(D±) to match D0 by D*/D0=0.40

  25. Charm Quark Hadronization at RHIC NLO pQCD predictions by R. Vogt, Int. J. Mod. Phys. E12 (2003) 211 Phenix: PRL 88, 192303(2002) D. Kharzeev,hep-ph/0310358 bare c-quark spectrum, normalized to measured dn/dy • Open charm spectra is hard: NLO c-quark spectrum = D spectrum • observed in fixed target exp. at lower energies • solved by intrinsic kT model to counter-balance effect of c-quark hadronization • doesn’t work at RHIC because spectrum is too broad • NLO underestimates s (factor of 3 at pT~10GeV/c for MRST) • Harder fragmentation function ? • Hadronize through quark recombination mechanism (Hwa at QM2004) ?

  26. Tools to Identify Electrons in STAR • Electromagnetic Calorimeter • p+p and d+Au runs at 200 GeV/A • 0 < h< 1.0 (for p+p, d+Au) • Full azimuthal coverage • 60 modules • tower: (Dh, Df)tower ~ (0.05, 0.05) • shower max: (Dh, Df) ~ (0.007, 0.007) • dE/E ~ 16%/√E • Time-of-Flight: MRPC • p/K separation up to 1.6 GeV/c • p/K separation up to 3 GeV/c • Thus cover wider range of(p,K,p) pT • only 10 modules installed (p/30, –0.5<eta<0.)

  27. Electron PID with MRPC TOF/TPC and EMC • EMC • use TPC for p and dE/dx • use Tower E  p/E • use SMD shape to reject hadrons • e/h discrimination power ~ 105 • works for pT > 1.5 GeV/c • ToF • use TPC and ToF PID • works for pT < 3 GeV/c

  28. Inclusive Electron Spectra in p+p and d+Au

  29. Background Subtraction Opening Angle γ conversion π0, η Dalitz decays Kaon decays ρωΦ vector meson decays heavy quark semi-leptonic decays others (Drell-Yan) Background Signal Invariant Mass Square • Select an primary electron/positron (tag it) • Select another opposite sign track anywhere in TPC Signal Rejected “Measure” background

  30. Background Subtracted Electron Spectra in p+p & d+Au Semantics: “background subtracted” in STAR = “non-photonic” in PHENIX

  31. Nuclear Effects (Cronin) ? • Within the errors consistent with binary scaling ... • NOTE: RdAu for a given pT comes from heavy quarks from a wide pT range

  32. Charm Cross-Section and PYTHIA • The more we learn about heavy flavor in PYTHIA the less • we believe to learn something from it. • Vary parameters (K, kT, processes, PDF, mC, etc.) within reasonable limits •  s changes up to factor 2 •  ds/dpT at high pT up to a factor of 10 • Measured D combined with • measured electron • spectra • better scc

  33. Consistency between electron data sets in p+p at RHIC Single electrons in p+p at s = 200 GeV • STAR data slightly higher than PHENIX • STAR: Generated electrons from measured D-meson spectra consistent with non-photonic electron spectra

  34. Energy Loss of Heavy Flavor at RHIC ? • Increasing deviation towards higher pT • Suppression of charm at high-pT likely • magnitude still uncertain but looks big (to me) – remember: p,h factor 4-5 • need STAR Au+Au/STAR p+p (soon …) Central Au+Au (PHENIX) vs. p+p (STAR) Central Au+Au (PHENIX) vs. fit to p+p (PHENIX)

  35. Summary • STAR measured D0,D* and D± in d+Au at √s=200 GeV with a pT coverage of 0<pT<11 GeV/c at mid rapidity • scc = 1.18 ± 0.21 ± 0.39 mb from directly-measured open charm • D*/D0= 0.40±0.09(stat)±0.13(sys) consistent with other experiments • D pT-spectrum is hard and coincides with the bare c-quark distribution from NLO calculations • STAR measured single electrons in the range 1 < pT < 8 GeV in p+p and d+Au at √s=200 GeV • D and single electron spectrum are consistent (pT < 3 GeV/c) • scc = 1.44 ± 0.20 ± 0.44 mb from D0 and single electron spectra • Electrons with pT > 3.5 GeV/c are mostly from beauty decays • First RHIC measurement sensitive to beauty cross section!!! • Comparison with PHENIX Au+Au data indicates charm suppression in Au+Au • Lots of STAR Au+Au data taken as we speak ( D, single electrons …)

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