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Measurement of J/ y production in Pb-Pb and pp collisions at the LHC with the ALICE experiment

Measurement of J/ y production in Pb-Pb and pp collisions at the LHC with the ALICE experiment. M. Gagliardi (Università degli Studi e Sezione INFN, Torino) for the ALICE Collaboration The 11 th International Conference on Nucleus-Nucleus collisions San Antonio, TX, USA – May 31 st , 2012.

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Measurement of J/ y production in Pb-Pb and pp collisions at the LHC with the ALICE experiment

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  1. Measurement of J/y production in Pb-Pb and pp collisions at the LHC with the ALICE experiment M. Gagliardi (Università degli Studi e Sezione INFN, Torino) for the ALICE Collaboration The 11th International Conference on Nucleus-Nucleus collisionsSan Antonio, TX, USA – May 31st, 2012

  2. Outline • Motivation • J/y detection in ALICE • Results in pp collisions • Results in Pb-Pb collisions • Conclusions

  3. Motivation • pp collisions • Insight on quarkonium production at LHC energies:hadronisation of heavy quark pairs into a bound colourless state is highly non-perturbative -> challenge for models • Reference for the study of nuclear modifications in heavy ion collisions • Pb-Pb collisions • Resonance melting by colour screening in aQuark Gluon Plasma: one of the first proposed signatures of deconfinement (T.Matsui, H. Satz, Phys. Lett. B 178 (1986) 416) • Suppression beyond cold nuclear matter effects • observed at SPS and RHIC (but similar magnitude • in spite of different energy densities). • J/y regeneration by statistical hadronisation of cc • pairs might play a roleand even become dominant at LHC energies(e.g., Andronic et al, Phys. Lett. B 652 (2007) 659) • Detailed discussion by P. Zhuang and P. Braun-Munzinger in Plenary 2 and 3 Perturbative vacuum Colour screening

  4. ALarge Ion Collider Experiment • Central Barrel |h| < 0.9 • Solenoidal magnet • Time Projection Chamber:-Tracking • PID via dE/dx • Inner Tracking System (Silicon Pixel, Drift, Strip Detectors) • Vertexing • Tracking • Triggering (SPD ) • Time of Flight MRPCs • PID • VZERO scintillators • Triggering • Centrality in Pb-Pb • Luminosity Muon spectrometer -4 < h < -2.5 Dipole magnet Front absorber 10 lI 10 tracking planes (Cathode Pad Chambers) Trigger system (Resistive Plate Chambers) Muon filter in front of the trigger chambers (7 lI) Only detectors used in this analysis are discussed

  5. J/y detection in ALICE Three sources of J/y (fractions refer to the pt-integrated yield) Directly produced ~50% Feed down from y’, cc ~40% Prompt :~90% From B-decay ~10% Inclusive Separation possibile in the electron channel in pp Central barrel:J/y -> e+e- |y| < 0.9, pt > 0 Muon spectrometer:J/y -> m+m- 2.5 < y < 4, pt > 0

  6. pp collisions

  7. J/y cross section measurement 7 TeV: Phys. Lett. B 704 (2011) 442 2.76 TeV: arXiv:1203.3641v1 [hep-ex] • Measurement down to zero transverse momentum in a broad rapidity range • pt-differential cross section at 2.76 and 7 TeV well reproduced by Non-Relativistic QCD (Colour Singlet +Colour Octet) • 2.76 TeV results used as reference in Pb-Pb analysis

  8. J/y polarisation measurement • Polarisation is a crucial observable for the comparison between data and models • Measured via polar (q) and azimuthal (φ) angle distributions of decay muons, analysed in two reference frames: Collins-Soper and Helicity W (cosq, φ) µ 1 + lq cos2q + lφ sin2q cos2φ + lqφsin2q cosφ 1 transverse polarisation 0 no polarisation -1 longitudinal polarisation lq =

  9. J/y polarisation lq arXiv:1201.3862v1 [hep-ex] lφ Phys.Rev.Lett. 108 (2012) 082001 Data suggest weak or no polarisation Extended pt coverage will provide a more stringent test of the models

  10. Multiplicity dependence of J/y production • - Highest multiplicity in this data-sample: dNch/dh ~30 (i.e. 5 times the minimum bias multiplicity): comparable to semi-central Cu-Cu collisions at RHIC • Measurement of J/y production in high multiplicity collisions provides insights on the interplay between hard and soft regime in multi-partonic interactions • A linear increase of the relative J/y yield with the multiplicity is observed • Behaviour not reproduced by Pythia (v. 6.4) Phys. Lett. B 712 (2012) 165

  11. J/y production from B-hadron decay • Unique measurement at mid-rapidity and low pt • - Good impact parameter resolution (σrφ< 75 μm for pt>1 GeV/c) • -> contribution from B estimated via the pseudo-proper decay length TO BE UPDATED WITH ARXIV VERSION Fraction of J/y from B decay in pt> 1.3 GeV/c, |y|<0.9: fB= 0.149 ±0.037(stat)-0.027+0.018(syst) -0.021+0.025 (syst polar.) arXiv:1205.5880v1 [hep-ex] Good agreement among LHC (and Tevatron) experiments

  12. J/y production from B-hadron decay • Unique measurement at mid-rapidity and low pt • - Good impact parameter resolution (σrφ< 75 μm for pt>1 GeV/c) • -> contribution from B estimated via the pseudo-proper decay length Fraction of J/y from B decay in pt> 1.3 GeV/c, |y|<0.9: fB= 0.149 ±0.037(stat)-0.027+0.018(syst) -0.021+0.025 (syst polar.) arXiv:1205.5880v1 [hep-ex] Extrapolation to pt = 0 to get prompt J/y cross section at mid-rapidity

  13. Pb-Pb collisions

  14. J/y in Pb-Pb: trigger, centrality, track selection • Trigger • Electrons (min bias) • VZERO-A AND VZERO-C AND SPD Muons • VZERO-A AND VZERO-C AND di-muon trigger (pt > 1 GeV/c) • Centrality selection • Glauber model fit of the VZERO amplitude • Track selection • Electrons: - identified via dE/dx in TPC: |nse|<3, |nσp,π|>3.5- pairs with|yee|< 0.9 • Muons • muon tracks are requested to have hits in the trigger chambers -> efficient hadron rejection due to the iron wall before the trigger system • pairs with 2.5<ymm<4 PRL 106, (2011) 032301

  15. J/y in Pb-Pb: signal extraction ~40000 J/y ~2000 J/y • Muons • - Several shapes for background • Double Crystal Ball function for signal • Fit of signal + background • Electrons • - Background from event-mixing • Scaling to same-event spectrum and subtraction • Signal from bin counting in 2.92 < M < 3.16 GeV/c2

  16. J/y in Pb-Pb: Axe correction Computed from MC simulations with realistic detector configuration Electrons: HIJING events enriched with J/y -> ee Muons: J/y -> mm embedded in real Pb-Pb events Weak centrality dependence for both electrons (7.8% -8.9%) and muons (13.3%-14.5): Less than 10% loss of efficiency from peripheral to central events

  17. J/y in Pb-Pb: nuclear modification factor RAA • YJ/yPbPb = NJ/yPbPb / (A e BR NPbPb) • RAAi = YJ/yPbPb,i / (TAAisJ/ypp) (ith centrality bin) sJ/ypp: pp data at √s = 2.76 TeV TAAi: Glauber MC Integrated RAA Forward rapidity: RAA0-90% = 0.497 ± 0.006 (stat.) ± 0.078 (syst.) (uncertainty is dominated by pp reference) Mid-rapidity: RAA0-80% = 0.66 ± 0.10 (stat.) ± 0.24 (syst.)(uncertainty is dominated by signal extraction and pp reference) Red: 2.5<y<4Blue: |y|<0.9 ClearJ/y suppression at forward rapidity Data show weak centrality dependence for Npart > 100 Similar pattern at mid-rapidity, but larger uncertainties

  18. J/y in Pb-Pb: nuclear modification factor RAA Red: ALICE - pt>0, 2.5<y<4 Black: PHENIX - pt > 0, 1.2 < |y| <2.2 ALICE 2.5 < y < 4 Blue: CMS - pt > 6.5 GeV/c, |y| < 2.4 Transport models: 50% of regenerated J/y in the most central collisions Statistical hadronisation:all J/y produced at hadronisation PHENIX and CMS (at high pt): RAA decreasing with centrality 18

  19. J/y RAA vs pt at forward rapidity RAA decreases with pt

  20. J/y RAA vs pt at forward rapidity Red: ALICE 2.5 < y < 4Blue: CMS |y| < 2.4 Black: PHENIX 1.2 < |y| < 2.2 New behaviour at low pt?

  21. J/y RAA vs pt at forward rapidity Reproduced by models with regenerated J/y component

  22. J/y RAA vs rapidity RAA decreases with rapidity (by 40% from y = 2.5 to y =4)

  23. J/y elliptic flow: v2 Pressure gradients in a thermalised medium convert initial spatial anisotropy in momentum anisotropy dN/dDφµ 1+v2 cos (Dφ) Dφ = φ-yRP , RP = reaction plane The anisotropy is quantified by the v2 coefficient J/y flow at LHC energies may be driven by regeneration or recombination of charm quarks; low and mid pt ranges are the most interesting J/y signal extracted in 6 Dφ = φ-yRPbins for 4 pt bins in 20%-60% centrality Event plane determined using three different subsets of detectors with large h gaps v2 extracted from Dφ distributions

  24. J/y v2 vs pt Hint for non-zero v2 (2.2s significance) for 2 GeV/c < pt < 4 GeV/c

  25. J/y v2 vs pt Hint for non-zero v2 (2.2s significance) for 2 GeV/c < pt < 4 GeV/c

  26. J/y v2 vs pt Hint for non-zero v2 (2.2s significance) for 2 GeV/c < pt < 4 GeV/cCompatible with transport model predictions

  27. Conclusions and outlook • ALICE results on J/y production: • in pp collisions • cross section down to pt =0 - beauty feed-down contribution • weak or null polarisation • linear increase of the yield with multiplicity • Looking forward to p-Pb collisions (winter 2012), • to constrain shadowing of nuclear structure functions at LHC energies • in Pb-Pb collisions • weak or no centrality dependence of RAA at large Npart • RAA is larger at low pt • significant decrease of RAA at large rapidities • hint for non-zero elliptic flow at low pt

  28. Backup

  29. Non-prompt J/y and bb cross section in pp collisions

  30. J/y -> e+e- RAA: systematic uncertanties Uncorrelated in centrality: Correlated in centrality: - pp reference 26%

  31. J/y -> m+m- RAA: systematic uncertanties Uncorrelated in centrality: • Correlated in centrality: • - Trigger efficiency 6.4% • Tracking efficiency 6%- MC J/y distribution 5% • Trigger matching efficiency and relative trigger normalisation: 2.8% • - pp reference 8.2%

  32. J/y RAA vs Npart at high pt

  33. J/y RAA : more model comparisons

  34. J/y RAA : more model comparisons

  35. J/y RAA : more model comparisons

  36. Prompt J/y RAA

  37. J/y v2 vs centrality

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