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Charm energy loss at the LHC with ALICE

Charm energy loss at the LHC with ALICE. Andrea Dainese Padova – University and INFN. Outline. Heavy-quark energy loss Estimates of D-meson suppression at LHC ALICE sensitivity via D 0  K p Conclusions. path length L. hard parton. Parton Energy Loss.

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Charm energy loss at the LHC with ALICE

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  1. Charm energy loss at the LHC with ALICE Andrea Dainese Padova – University and INFN Hot Quarks 2004 - July 24, 2004 Andrea Dainese

  2. Outline • Heavy-quark energy loss • Estimates of D-meson suppression at LHC • ALICE sensitivity via D0 Kp • Conclusions Hot Quarks 2004 - July 24, 2004 Andrea Dainese

  3. path length L hard parton Parton Energy Loss • Due to medium-induced gluon radiation • Average energy loss (BDMPS model): Casimir coupling factor: 4/3 for quarks 3 for gluons Medium transport coefficient gluon density and momenta R.Baier, Yu.L.Dokshitzer, A.H.Mueller, S.Peigne' and D.Schiff, (BDMPS), Nucl. Phys. B483 (1997) 291. Hot Quarks 2004 - July 24, 2004 Andrea Dainese

  4. Q Lower Loss for Heavy Quarks? • In vacuum, gluon radiation suppressed at q < mQ/EQ “dead cone” effect1 • Dead cone implies lower energy loss2(Dokshitzer-Kharzeev, 2001) • Recent detailed calculation confirms this qualitative feature3(Armesto-Salgado-Wiedemann, 2003) 1. Yu.L.Dokshitzer, V.A.Khoze and S.I.Troyan, J. Phys. G17 (1991) 1602. 2. Yu.L.Dokshitzer and D.E.Kharzeev, Phys. Lett. B519 (2001) 199 [hep-ph/0106202]. 3. N.Armesto, C.A.Salgado and U.A.Wiedemann, Phys. Rev. D69 (2004) 114003 [hep-ph/0312106]. Hot Quarks 2004 - July 24, 2004 Andrea Dainese

  5. Yu.Dokshitzer DK dead-cone effect • Dokshitzer-Kharzeev: energy distribution w dI/dw of radiated gluons suppressed by angle-dependent factor • suppress high-energy tail of gluon radiation spectrum sizeable reduction of energy loss Hot Quarks 2004 - July 24, 2004 Andrea Dainese

  6. study D meson RAA and compare it to that of h (or p0) Experimental study of energy loss • Compare pt distributions of leading particles in pp and nucleus-nucleus collisions (+ p-nucleus as a control) Nuclear modification factor: Hot Quarks 2004 - July 24, 2004 Andrea Dainese

  7. The dependences ofRD/h • Initial state effects (PDF shadowing) (low pt; <10 GeV/c) • Parton energy loss: • (c) quark vs gluon (Casimir factor) RD/h > 1 (all pt) • mass effects RD/h > 1 (“moderate” pt; <20 GeV/c) • fragmentation: • (much) harder for charm quarks w.r.t. gluons RD/h • slope of pt distribution: • harder for charm RD/h • recombination / in-medium hadronization (low pt; <10 GeV/c) Hot Quarks 2004 - July 24, 2004 Andrea Dainese

  8. use the DK dead-cone factor to correct for charm First estimate • use QW specifically calculated for massive quarks2 New estimate • use QCD theory estimate • use model extrapolation based on RHIC data Energy-loss simulation: ingredients • BDMPS Quenching Weights1: -dependent distrib. • Transport coefficient for central Pb-Pb at LHC • Realistic path length of partons in the dense medium (Glauber-model based) 1. C.A.Salgado and U.A.Wiedemann, Phys. Rev. D68 (2003) 014008 [hep-ph/0302184]. 2. N.Armesto, A.D., C.A.Salgado and U.A.Wiedemann, in preparation. Hot Quarks 2004 - July 24, 2004 Andrea Dainese

  9. QGP @ LHC cold nucl. matter R.Baier, Nucl. Phys. A715 (2003) 209. First estimate (last year) Shadowing included via EKS98 chosen which gives Hot Quarks 2004 - July 24, 2004 Andrea Dainese A.D. Eur. Phys. J. C33 (2004) 495 [nucl-ex/0312005].

  10. New estimate (1): Parton Quenching Model • PQM: BDMPS quenching weights + realistic coll. geometry • Parton-by-parton calculation of distrib. • Centrality evolution included through Glauber model • Theoretical uncertainty band considered (finite parton-energy limit) • needed1,2 to match RAA at RHIC (200 GeV) 1. PQM: A.D., C.Loizides and G.Paic, hep-ph/0406201. 2. K.J.Eskola, H.Honkanen, C.A.Salgado and U.A.Wiedemann, hep-ph/0406319. Hot Quarks 2004 - July 24, 2004 Andrea Dainese

  11. New estimate (2): PQM from RHIC to LHC … • Assumption: (initial volume-density of gluons) • Extrapolation to LHC according to saturation model1 gives: • Most partons are absorbed • Only those from the surface can escape the medium … energy loss saturated 1. K.J.Eskola, K.Kajantie, P.V.Ruuskanen and K.Tuominen, Nucl. Phys. B570 (2000) 379 [hep-ph/9909456]. Hot Quarks 2004 - July 24, 2004 Andrea Dainese

  12. New estimate (3):mass effect much smaller red “band”: massless blue “band”: massive (1.2 GeV) Hot Quarks 2004 - July 24, 2004 Andrea Dainese

  13. Comparison, step by step • [For lower bound of the uncertainty band] Hot Quarks 2004 - July 24, 2004 Andrea Dainese

  14. The ALICE Detector |h| < 0.9 TPC + silicon tracker Hot Quarks 2004 - July 24, 2004 Andrea Dainese

  15. Exclusive charm in ALICE: D0 K-p+ • Exclusive reconstruction direct measurement of the pt distribution ideal tool to study RAA • Large combinatorial background (dNch/dy=6000 in central Pb-Pb!) • Main selection: displaced-vertex selection • pair of opposite-charge tracks with large impact parameters • good pointing of reconstructed D0 momentum to the primary vertex Invariant mass analysis to “count” D0 Hot Quarks 2004 - July 24, 2004 Andrea Dainese

  16. Results Stat. and syst. errors on D pt distr. estimated for pp and Pb-Pb (K,) Invariant Massdistribution (pt –integrated) in Pb-Pb (~1 monthrun) Statistical significance: 1 < pt < 14 GeV/c N.Carrer, A.D. and R.Turrisi, J. Phys. G29 (2003) 575. A.D. PhD thesis (2003), nucl-ex/0311004. Hot Quarks 2004 - July 24, 2004 Andrea Dainese

  17. Measuring D quenching with ALICE Hot Quarks 2004 - July 24, 2004 Andrea Dainese

  18. Conclusions • Direct D reconstruction in ALICE: powerful tool to address experimentally the rich m > 0 domain • gluon radiation suppressed at small angles • smaller energy loss / suppression ??? • Theoretical picture on charm energy loss is evolving … • DK dead-cone approximation over-estimates effect of mass • within current th. uncertainties, D-meson suppression may not be affected by “dead cone” • further improvement in treatment of finite parton energies will indicate kinematic range where mass effects are more significant Hot Quarks 2004 - July 24, 2004 Andrea Dainese

  19. BACK-UP SLIDES Hot Quarks 2004 - July 24, 2004 Andrea Dainese

  20. Go for deep deconfinement at LHC Next step in the “quest for QGP” … • LHC: factor 30 jump in w.r.t. RHIC much larger initial temperature study of hotter, bigger,longer-living ‘drops’ of QGP •  closer to ‘ideal’ QGP • easier comp. with theory (lattice) ‘Deep de-confinement’ Hot Quarks 2004 - July 24, 2004 Andrea Dainese

  21. c Pb Q Pb c Hard Processes in AA at the LHC • Main novelty of the LHC: large hard cross section • Hard processes are extremely useful tools • large virtuality Qhappen at t = 0 small “formation time”Dt ~ 1/Q (for charm: Dt < 1/2mc ~ 0.1 fm/c << tQGP ~ 5–10 fm/c) • Initial yields and pt distributions in AA can be predicted using pp measurements + pQCD + collision geometry+“known” nuclear effects • Interactions with the medium can induce deviations from such predictions medium formed in the collision time Hot Quarks 2004 - July 24, 2004 Andrea Dainese

  22. charm q Q q gPb(x)/gp(x) “they are so close that they fuse” xa • Shadowing: • reduces initial hard yield at low pt • scales trivially from pA to AA xa+xb xb Initial-state effects: Shadowing • Bjorken-x: fraction of the momentum of the proton ( ) carried by the parton entering the hard scattering • At the LHC • Pb ion @ LHC ~ 105-106 partons (mainly gluons) Hot Quarks 2004 - July 24, 2004 Andrea Dainese

  23. medium IN vs. OUT probe IN (known from pp, pA + pQCD) probe OUT Hard partons probe the medium • Partons travel ~ 5 fm in the high colour-density medium • Energy loss by gluon bremsstrahlung • modifies momentum distributions • jet shapes • … • depends on medium properties PROBE Hot Quarks 2004 - July 24, 2004 Andrea Dainese

  24. vacuum medium kt w BDMPS model Hot Quarks 2004 - July 24, 2004 Andrea Dainese

  25. Background multiplicity in Pb-Pb • What is the background to hadronic D decays? combinatorial background given by pairs of uncorrelated tracks with large impact parameter in central Pb-Pb at LHC Simulations performed using huge combinatorial background! need excellent detector response and good selection strategy Hot Quarks 2004 - July 24, 2004 Andrea Dainese

  26. rf: 50 mm z: 425 mm PIXEL CELL  9.8 M Two layers: r = 4 cm r = 7 cm ALICE Barrel |h|<0.9: B = 0.4 T TOF TPC ITS with: - Si pixels - Si drifts - Si strips Hot Quarks 2004 - July 24, 2004 Andrea Dainese

  27. Tracking Tracking efficiency ~70% with dNch/dy=6000 pions kaons pt resolution = 1% at 1 GeV/c D0 invariant mass resolution: Hot Quarks 2004 - July 24, 2004 Andrea Dainese

  28. rf: 50 mm < 60 mm for pt > 1 GeV/c z: 425 mm PIXEL CELL Two layers: r = 4 cm r = 7 cm Impact parameter resolution • Mainly provided by the 2 layers of Si pixels  9.8 M Hot Quarks 2004 - July 24, 2004 Andrea Dainese

  29. TOF PID TOF Pb-Pb, dNch/dy=6000 Optimization for hadronic charm decays was studied: minimize probability to tag K as p Hot Quarks 2004 - July 24, 2004 Andrea Dainese

  30. D0 K-p+:Signal and background • Signal: • charm cross section from NLO pQCD (MNR program), average of results given by MRS98 and CTEQ5M PDFs (with EKS98 in Pb-Pb) • signal generated using PYTHIA, tuned to reproduce pt distr. given by NLO pQCD • contribution from bBD0 (~5%) also included • Background: • Pb-Pb: HIJING (dNch/dy=6000 ! we expect ~2500 !);pp: PYTHIA; MNR Program: M.L.Mangano, P.Nason and G.Ridolfi, Nucl. Phys. B373 (1992) 295. Hot Quarks 2004 - July 24, 2004 Andrea Dainese

  31. D0 K-p+:Selection of D0 candidates increase S/B by factor ~103! Hot Quarks 2004 - July 24, 2004 Andrea Dainese

  32. pp, 14 TeV 109 events Pb-Pb, 5.5 TeV 107 events 1–14 GeV/c 0–14 GeV/c D0 K-p+:Results Note: with dNch/dy = 3000, S/B larger by  4 and significance larger by  2 Hot Quarks 2004 - July 24, 2004 Andrea Dainese

  33. D0 K-p+:Results 0.5 < pt < 1 GeV/c 2 < pt < 2.5 GeV/c 12 < pt < 14 GeV/c Hot Quarks 2004 - July 24, 2004 Andrea Dainese

  34. What if multiplicity in Pb-Pb is lower? • We used dNch/dy = 6000, which is a pessimistic estimate • Recent analyses of RHIC results seem to suggest as a more realistic value dNch/dy = 3000 (or less) • Charm production cross section: • estimate from NLO pQCD (only primary production, no collective effects) • average of theoretical uncertainties (choice of: mc, mF, mR, PDF) • BKG proportional to (dNch/dy)2 • We can scale the results to the case of dNch/dy = 3000: S/B = 44 % SGNC = 74 (this only from scaling, obviously better with retuning of cuts) Hot Quarks 2004 - July 24, 2004 Andrea Dainese

  35. pp Pb-Pb Estimate of the errors • Statistical error on the selected signal = 1/Significance • Main systematic errors considered: • correction for feed-down from beauty (B.R. B  D0 is 65%!): error of ~8% assuming present uncertainty (~80%) on @ LHC • Monte Carlo corrections: ~10% • B.R. D0 Kp: 2.4% • extrapolation from N(D0)/event to ds(D0)/dy: • pp: error on (~5%, will be measured by TOTEM) • Pb-Pb: error on centrality selection (~8%) + error on TAB (~10%) Hot Quarks 2004 - July 24, 2004 Andrea Dainese

  36. inner bars: statistical outer bars: systematic ds(D0)/dy for |y| < 1 and pt > 0 statistical error = 3 % systematic error = 14 % from b = 8 % MC correction = 10% B.R. = 2.4 % sinel = 5 % D0 K-p+:d2s(D0)/dptdy and ds(D0)/dy ds(D0)/dy for |y| < 1 and pt > 1 GeV/c (65% of s(pt > 0)) statistical error = 7 % systematic error = 19 % from b = 9 % MC correction = 10% B.R. = 2.4 % from AA to NN = 13 % Hot Quarks 2004 - July 24, 2004 Andrea Dainese

  37. Sensitivity to NLO pQCD parameters pp, 14 TeV MNR Program: M.L.Mangano, P.Nason and G.Ridolfi, Nucl. Phys. B373 (1992) 295. Hot Quarks 2004 - July 24, 2004 Andrea Dainese

  38. Interpolation pp 14  5.5 TeV Necessary to compare Pb-Pb and pp by RAA In pQCD calculations the ratio of the differential cross sections at 14 and 5.5 TeV is independent of the input parameters within 10% up to 20 GeV/c pQCD can be safely used to extrapolate pp @ 14 TeV to 5.5 TeV Hot Quarks 2004 - July 24, 2004 Andrea Dainese

  39. ‘High’ pt (6–15 GeV/c) here energy loss can be studied (it’s the only expected effect) Low pt (< 6–7 GeV/c) Nuclear shadowing Sensitivity on RAA for D0 mesons Hot Quarks 2004 - July 24, 2004 Andrea Dainese

  40. Effect of shadowing Hot Quarks 2004 - July 24, 2004 Andrea Dainese

  41. Transport coefficient choice • Require for LHC suppression of hadrons as observed at RHIC: RAA ~ 0.2-0.3 for 4<pt<10 GeV/c • pt distributions of hadrons at LHC: • partons (pt>5 GeV/c) generated with PYTHIA pp, 5.5 TeV (average parton composition: 78% g + 22% q) • energy loss: pt’ = pt – DE • (independent) fragmentation with KKP LO F.F. • RAA = (pt distr. w/ quenching) / (pt distr. w/o quenching) Hot Quarks 2004 - July 24, 2004 Andrea Dainese

  42. RAA ~ 0.4–0.5 increasing at high pt RAA ~ 0.7–0.8 decreasing at high pt RAA with Quenching A.D. Eur. Phys. J. C33 (2004) 495 [arXiv:nucl-ex/0312005]. Hot Quarks 2004 - July 24, 2004 Andrea Dainese

  43. D/h ratio: RD/h = RAAD / RAAh RD/h ~ 2–3 in hot QGP sensitive to medium density D/hadrons ratio (1) • Ratio expected to be enhanced because: • D comes from (c) quark, while p, K, p come mainly (~80% in PYTHIA) from gluons, which lose 2 more energy w.r.t. quarks • dead cone for heavy quarks • Experimentally use double ratio: RAAD/RAAh • almost all systematic errors of both Pb-Pb and pp cancel out! Hot Quarks 2004 - July 24, 2004 Andrea Dainese

  44. pthadron = zptparton (ptparton)’ = ptparton – DE (pthadron)’ = pthadron – zDE Energy loss observed in RAA is not DE but zDE zcD 0.8; zgluonhadron 0.4 (for pt > 5 GeV/c) DEc = DEgluon/2.25 (w/o dead cone) zcD DEc  0.9 zgluonhadron DEgluon Without dead cone, RAADRAAh D/hadrons ratio (2) • RD/h is enhanced only by the dead-cone effect • Enhancement due to different quark/gluon loss not seen • It is compensated by the harder fragmentation of charm Hot Quarks 2004 - July 24, 2004 Andrea Dainese

  45. PQM: RAA all centralities Hot Quarks 2004 - July 24, 2004 Andrea Dainese

  46. PQM: IAA Hot Quarks 2004 - July 24, 2004 Andrea Dainese

  47. PQM: v2 Hot Quarks 2004 - July 24, 2004 Andrea Dainese

  48. PQM: RAA @ 62.4 GeV Hot Quarks 2004 - July 24, 2004 Andrea Dainese

  49. PQM: ch. hadrons RAA at LHC Hot Quarks 2004 - July 24, 2004 Andrea Dainese

  50. PQM: <DE / E> vs E Hot Quarks 2004 - July 24, 2004 Andrea Dainese

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