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Hard (Heavy) QCD probes in heavy-ion collisions at the LHC: ALICE performance

5.5 TeV. 208 Pb. 208 Pb. LHC. Hard (Heavy) QCD probes in heavy-ion collisions at the LHC: ALICE performance. Andrea Dainese, University of Padova for the ALICE Collaboration. Go for deep deconfinement at LHC. Next step in the “quest for QGP” …

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Hard (Heavy) QCD probes in heavy-ion collisions at the LHC: ALICE performance

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  1. 5.5 TeV • 208Pb • 208Pb • LHC Hard (Heavy) QCD probes in heavy-ion collisions at the LHC:ALICE performance Andrea Dainese,University of Padova for the ALICE Collaboration XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, 2004 Andrea Dainese

  2. 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’ XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, 2004 Andrea Dainese

  3. 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 • Deviations from such predictions are due to the medium medium formed in the collision time XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, 2004 Andrea Dainese

  4. QGP @ LHC cold matter Parton Energy Loss • Due to medium-induced gluon emission • Average energy loss (BDMPS model): path length L QCD process: emitted gluon itself radiates  EL2 hard parton 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. C.A.Salgado and U.A.Wiedemann, Phys. Rev. D68 (2003) 014008 [arXiv:hep-ph/0302184]. XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, 2004 Andrea Dainese

  5. D mesons quenching reduced Ratio D/hadrons (or D/p0) enhanced and sensitive to medium properties Lower Loss for Heavy Quarks? • Heavy quarks with momenta < 20–30 GeV/cv << c • In vaccum, gluons radiation suppressed at Q < mQ/EQ “dead cone” effect • Dead cone implies lower energy loss(Dokshitzer-Kharzeev, 2001) • Recent detailed calculation confirms this qualitative feature (Armesto-Salgado-Wiedemann, 2003) see talk by N.Armesto Yu.L.Dokshitzer, V.A.Khoze and S.I.Troyan, J. Phys. G17 (1991) 1602. Yu.L.Dokshitzer and D.E.Kharzeev, Phys. Lett. B519 (2001) 199 [arXiv:hep-ph/0106202]. N.Armesto, C.A.Salgado and U.A.Wiedemann [arXiv:hep-ph/0312106]. XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, 2004 Andrea Dainese

  6. 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: Important step forward at the LHC: Compare quenching of massless and massive probes • Study jets (or ‘jets’): • ‘jets’ - (charged) particle correlations (RHIC tells us they can tell a lot!) • jets - with calorimetry (CMS/ATLAS speciality see talk by C.Roland) see talk by D. d’Enterria XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, 2004 Andrea Dainese

  7. The ALICE Detector |h| < 0.9 TPC + silicon tracker g,e, p, K, p identification 2.5 < h < 4 muons XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, 2004 Andrea Dainese

  8. 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 tracks with large impact parameters • good pointing of reconstructed D0 momentum to the primary vertex Invariant mass analysis to “count” D0 XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, 2004 Andrea Dainese

  9. Results. Example: Pb-Pb pt-integrated (K,) Invariant Massdistribution (pt –integrated) (corresponding to 107 central Pb-Pb events ~1 monthrun) Statistical Significance of the Signal: after background subtraction analysis for Pb-Pb and pp done in bins of pt and main errors estimated Details on selection strategy in: N.Carrer, A.D. and R.Turrisi, J. Phys. G29 (2003) 575. A.D. PhD thesis (2003) [arXiv:nucl-ex/0311004] XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, 2004 Andrea Dainese

  10. Low pt (< 6–7 GeV/c) Nuclear shadowing ‘High’ pt (6–15 GeV/c) here energy loss can be studied (it’s the only expected effect) Sensitivity on RAA for D0 mesons XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, 2004 Andrea Dainese

  11. Energy-loss simulation • Energy loss simulated using BDMPS quenching weights by Salgado and Wiedemann • Plus: • realistic path lengths of partons in the medium • dead-cone correction for heavy quarks • transport coefficient for central Pb-Pb collisions at LHC estimated requiring hadrons RAA ~ 0.2–0.3 (~ RHIC) C.A.Salgado and U.A.Wiedemann, Phys. Rev. D68 (2003) 014008 [arXiv:hep-ph/0302184]. XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, 2004 Andrea Dainese

  12. QGP @ LHC cold matter Average relative energy loss XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, 2004 Andrea Dainese

  13. 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. C, in press [arXiv:nucl-ex/0312005]. XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, 2004 Andrea Dainese

  14. 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! XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, 2004 Andrea Dainese

  15. 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 XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, 2004 Andrea Dainese

  16. S/(S+B) S per 107 Pb-Pb events pt > 2 GeV/c, d0 > 180 mm: 50,000 electrons with S/(S+B) = 90 % Open Beauty in electron channel • Inclusive B e + X: • electron ID + cut on its pt & on its impact parameter d0 XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, 2004 Andrea Dainese

  17. Summary • LHC: study properties for deconfined QCD matter via hard probes and their quenching • ALICE: good potential in the hard probes sector • Outstanding example: ALICE can exclusively reconstruct D0 mesons in Pb-Pb collisions with dNch/dy = 6000! • measure charm production in 0 < pt < 15 GeV/c (at least) • study the mass and flavour dependence of QCD energy loss • And more: • beauty detection via semi-electronic decays • jet studies with charged tracks XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, 2004 Andrea Dainese

  18. BACK-UP SLIDES XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, 2004 Andrea Dainese

  19. # Jets in |h|<0.5 with Et > Etmin per central Pb-Pb event Jet Rates at the LHC 1/event 104/month (year) use High Level Trigger XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, 2004 Andrea Dainese

  20. Jet Quenching Studies (1) • High-pt suppression: RAA up to ~ 40–50 GeV/c in 1 year • Azimuthal correlation studies • What about “identified jets”? Quite unclear … • Energy plays a minor role as jet observable • Small effect of quenching on the energy inside a given cone (Salgado and Wiedemann (2003)) r(R): Energy contained in a cone of size R normalized to the energy contained in a cone of size 1. XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, 2004 Andrea Dainese

  21. Jet Quenching Studies (2) • Simple MC quenching model (which reproduces results of Salgado and Wiedemann) Most interesting signals from the radiated energy are at high R and low pt Very challenging due to the large background from the underlying event Good tracking in the region pt < 1–2 GeV/c is needed XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, 2004 Andrea Dainese

  22. Relevance of jet energy The Jet shape for a 50 GeV and a 100 GeV quark-lead jet which fragments in the vacuum (red line) or in a dense medium (blue line). • Energy plays a minor role as a jet observable: • Small effect of jet quenching on the energy inside a given cone (S.A. Salgado and U.A. Wiedemann (2003)). • Jet structure, i.e. normalized longitudinal momentum (z) and momentum perpendicular to jet axis (jT) of particles belonging to the jet show small energy dependence. 40-60 60-80 200-220 XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, 2004 Andrea Dainese

  23. Relevance of low-pT Different quenching scenarios result in differences in the low-pT region of jet particles. Tracking capabilities for pT < 1 GeV are essential. The large background from particles of the underlying event make this measurement challenging. The figures on the right show S/B and significance as a function of pT for DR < 0.7 and dN/dh = 4000. Simple quenching model: The energy loss of a 100 GeV jets is simulated by reducing the energy of the jet by 20% and replacing the missing energy by: 1 x 20 GeV gluon 2 x 10 GeV gluons 4 x 5 GeV gluons Jets have been simulated with Pythia. XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, 2004 Andrea Dainese

  24. Jet shapes Our simple MC quenching model reproduces qualitatively the results of Salgado and Wiedemann. The figure on the left shows that for quenching measurements with cone sizes between 0.3-0.4 good tracking in the region pT < 2 GeV is needed. The most interesting signals from the radiated energy are at high R and low pT which is very challenging due to the large background from the underlying event. r(r): Energy contained in a cone of size r normalized to the energy contained in a cone of size 1. XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, 2004 Andrea Dainese

  25. g p0 S/B  1 5.5 L/event Entries L Yield Reconstructed pt spectrum M(pp) (GeV/c2) High pt • Estimates of upper limit for 1 year ALICE run: • 107 central events Pb-Pb ~103 tracks with pt > 50 GeV/c • 109 min. bias events pp ~103 tracks with pt > 40 GeV/c • Identified particles: • D0 up to 15 GeV/c, B ~ 10 GeV/c (under study) • L up to 12 GeV/c • g/ p0 separation up to 100 GeV/c with PHOS XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, 2004 Andrea Dainese

  26. 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 XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, 2004 Andrea Dainese

  27. 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 XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, 2004 Andrea Dainese

  28. 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 XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, 2004 Andrea Dainese

  29. Tracking Tracking efficiency ~70% with dNch/dy=6000 pions kaons pt resolution = 1% at 1 GeV/c D0 invariant mass resolution: XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, 2004 Andrea Dainese

  30. < 60 mm for pt > 1 GeV/c Impact parameter resolution • Crucial for heavy-quark ID • Systematic study of resolution was carried out XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, 2004 Andrea Dainese

  31. TOF PID TOF Pb-Pb, dNch/dy=6000 Optimization for hadronic charm decays was studied: minimize probability to tag K as p XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, 2004 Andrea Dainese

  32. d0K d0 <<0 cos pointing 1 D0 K-p+:Selection of D0 candidates • Main selection: displaced-vertex selection • pair of tracks with large impact parameters • good pointing of reconstructed D0 momentum to the primary vertex XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, 2004 Andrea Dainese

  33. 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 XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, 2004 Andrea Dainese

  34. 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 % XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, 2004 Andrea Dainese

  35. 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) XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, 2004 Andrea Dainese

  36. vacuum medium kt w BDMPS model XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, 2004 Andrea Dainese

  37. 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) XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, 2004 Andrea Dainese

  38. 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%) XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, 2004 Andrea Dainese

  39. Comparison with pQCD for pp XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, 2004 Andrea Dainese

  40. 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 XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, 2004 Andrea Dainese

  41. Effect of shadowing XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, 2004 Andrea Dainese

  42. 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) XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, 2004 Andrea Dainese

  43. 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. XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, 2004 Andrea Dainese

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