1 / 44

Recent results from the ALICE experiment on p-p and Pb-Pb collisions

Recent results from the ALICE experiment on p-p and Pb-Pb collisions. Introduction The ALICE experiment The first LHC Pb-Pb run Selected pp highlights Prospects and conclusions. E. Scomparin (INFN Torino) for the ALICE Collaboration. Bormio , January 23-27, 2012

dwayne
Télécharger la présentation

Recent results from the ALICE experiment on p-p and Pb-Pb collisions

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. Recent results from the ALICE experimenton p-p and Pb-Pb collisions • Introduction • The ALICE experiment • The first LHC Pb-Pb run • Selected pp highlights • Prospects and conclusions E. Scomparin (INFN Torino) for the ALICE Collaboration Bormio, January 23-27, 2012 50th International Winter Meeting on Nuclear Physics

  2. Introduction • ALICE (A Large Heavy-Ion Collision Experiment): the dedicated heavy-ion experiment at the LHC • Main focus on Pb-Pb collisions  QGPstudies From the problem…. …to the solution • p-p collisionsstudied too (luminosity limited to a few 1030 cm-2s-1) • Reference for heavy-ion collision studies • Genuine p-p physics

  3. Size: 16 x 26 meters Weight: 10,000 tons Detectors:18

  4. ALICE: specific features • ALICE peculiarities among the LHC experiments • Focus on PID investigate chemical composition of the hot matter • Push acceptance down to pT=0 (low material budget, low B) •  many QGP-related features become more evident at low pT • Sustain very high hadronic multiplicities (up to dNch/d~8103)

  5. PID performance: selected plots ITS Silicon Drift/Strip dE/dx TPC dE/dx Ω ΛΚ TOF

  6. Analyzed data samples 2010 2011 • Triggers • MB: based on VZERO • (A and C) and SPD • SINGLE MUON: forward muon in coincidence with MB trigger

  7. Focus on Pb-Pb • Analysis of 2010 PbPb data well advanced. Results on: • Global event features • Collective expansion • Strangeness and chemical composition • Parton energy loss in the medium • Light flavours • Heavy flavours • Quarkonia dissociation/regeneration • in the medium See ALICE talks by: C. Perez Lara (Tue pm) S. Beole (Thu pm) A. Mischke (Thu pm) PRL106 (2011) 032301 • Centrality estimate: • standard approach • Glauber model fits • Define classes corresponding • to fractions of the inelastic • Pb-Pbcross section

  8. Charged multiplicity – Energy density PRL106 (2011) 032301 PRL105 (2010) 252301 • dNch/d = 1584  76 • (dNch/d)/(Npart/2) = 8.3  0.4 • ≈ 2.1 x central AuAuat √sNN=0.2 TeV • ≈ 1.9 x pp (NSD) at √s=2.36 TeV • Stronger rise with √s in AA w.r.t. pp • Stronger rise with √s in AA w.r.t. log extrapolation from lower energies • Very similar centrality dependence at LHC & RHIC, after scaling RHIC results (x 2.1) to the multiplicity of central collisions at the LHC

  9. System size • Spatial extent of the particle emitting source extracted from interferometry of identical bosons • Two-particle momentum correlations in 3 orthogonal directions -> HBT radii (Rlong, Rside, Rout) • Size: twice w.r.t. RHIC • Lifetime: 40% higher w.r.t. RHIC ALICE: PLB696 (2011) 328 ALICE: PLB696 (2011) 328

  10. Identified hadron spectra • Combined analysis (ITS, TPC and TOF) • Lines = blast-wave fits, extract • Integrated yields • Average pT • Parameters of the system at the thermal freeze-out, Tfoand  (radial flow)

  11. Radial expansion of the system Centrality STAR pp √s=200 GeV Blast-wave fit parameters • Common blast-wave fit to , K and p • Strong radial flow: b≈0.66 for most central collisions, 10% higher than at RHIC • Freeze-out temperature below 100 MeV • Significant change in mean pT between √sNN=200 GeV and 2.76 TeV harder spectra • For the same dN/dhhigher mean pT than at RHIC

  12. Hadrochemistry • Relative abundances of hadron species can be described by statistical distributions (Tch, B) A.Andronic et al., Nucl.Phys.A772(2006) • Description still not satisfactory • at LHC energy • Low Tch suggested by p spectra, but • excluded by  and  • If p excluded, Tch =164 MeV •  Tch (LHC) ~ Tch(RHIC) ~ Tc J.Cleymans et al., Phys.Rev.C73(2006)034905

  13. Elliptic flow • v2 (LHC) ~ 1.3 v2 (RHIC) (pTintegrated) • Increase consistent with increased radial expansion (higher pT) • System at LHC energy still behaves as a near-perfect fluid, not gas!

  14. Identified particle v2 • Elliptic flow mass dependence due to large radial flow • Magnitude and mass splitting predicted by viscous hydro • in all centrality bins •  and K v2(pT) well described, disagreement for p in central data • Radial flow too small from hydro, hadronicrescatteringsplay an • important role in flow development

  15. Charged hadron RAA Related to parton energy loss, in the BDMPS approach • RAA(pT) for charged particles : larger suppression wrt RHIC • Suppression increases with increasing centrality • Minimum for pT~ 6-7 GeV/c in all centrality classes • RAAincreases in the region pT>10 GeV/c • Hint of flattening above 30 GeV/c

  16. Identified particle RAA • Mesons vs baryons: different RAA at intermediate pT • Related to baryon enhancement, observed e.g. in /K ratio • At high pT(>8-10 GeV/c)RAA universality for light hadrons • For hadrons containing heavy quarks, smaller suppression expected: • dead cone effect, gluon radiation • suppressed for <mq/Eq

  17. ALICE pp data sample >100M muon triggers (MUS) (to get >2104 J/) >800M MB triggers (INT) >25M high-multiplicity triggers (SH1) • Interaction trigger reading all detectors: • SPD (min bias) or V0-A or V0-C • at least one charged particle in 8 -units • Single-muon trigger reading MUON, SPD, V0, FMD, ZDC : • single muon, low-pT threshold, in the muon arm in coincidence with interaction trigger • High Multiplicity trigger

  18. Open charm in ALICE • Analysis strategy • Invariant mass analysis of fully reconstructed decay topologies displaced from the primary vertex • Feed down from B (10-15 % after cuts) subtracted using FONLL • Plus in PbPb hypothesis on RAA of D from B K p D+K-p+p+

  19. D-meson RAA • pp reference from measured D0, D+ and D* pT differential cross-sections at 7 TeV scaled to 2.76 TeV with FONLL • Suppression of prompt D mesons in central (0-20%) PbPb collisions by a factor 4-5 for pT>5 GeV/c • Little shadowing at high pT suppression comes from hot matter • Similar suppression for D mesons and pions • Maybe a hint of RAAD > RAAπ at low pT

  20. Electrons from heavy-flavour decays e • Cocktail method • Inclusive electron pT spectrum • Electron PID from TOF+TPC • TRD used in pp • Subtract cocktail of known background sources • Impact parameter method (only ppfor now) • Track impact parameter cut to • select electrons from beauty

  21. RAA of cocktail-subtracted electrons • pp reference from measured heavy flavour electrons pT differential cross-sections at 7 TeV scaled to 2.76 TeV with FONLL • Analysis of pp data at 2.76 TeV ongoing (direct reference) • Suppression of cocktail-subtracted electrons • Factor 1.5 - 4 for pT>3.5 GeV/c in the most central (0-10%) events • Suppression increases with increasing centrality

  22. Heavy-flavor decay muons m • Single muonsat forward rapidity (-4<<-2.5) • Background from primary /K decay not subtracted • estimated with HIJING to be 9% in the most central class (0-10%) for pT>6 GeV/c • RCP for inclusive muonsin 6<pT<10 GeV/c • suppression increases with increasing centrality

  23. J/ suppression peripheral central • Inclusive J/y RAA • pp reference from pp data set at 2.76 TeV • Contribution from B feed-down not subtracted (very small effect) • J/y are suppressed with respect to pp collisions • J/y RAAalmost independent of centrality

  24. J/: comparison with RHIC ALICE, LHC, forward rapidity PHENIX, RHIC, mid-rapidity PHENIX, RHIC, forward rapidity • Less suppression than at RHIC at forward rapidity: • RAA(ALICE) > RAA(PHENIX, 1.2<y<2.2) • Similar suppression as at RHIC at midrapidity (not for central!) • RAA(ALICE) ≈ RAA(PHENIX, |y|<0.35) • Caveat:cold nuclear matter effects different at RHIC and LHC needs pPb running

  25. J/: comparison to models A.Andronic et al., arXiv:1106.6321 P.Braun-Munzinger et al.,PLB490(2000) 196 R.Rapp, X.Zhao, NPA859(2011)114 • Parton transport model • J/ dissociation in QGP • J/ regeneration by charm • quark pair recombination • Feed-down from B-decays • Shadowing • Statistical hadronizationmodel • Screening by QGP of all J/ • Charmonium production at • phase boundary by statistical • combination of uncorrelated • c-quarks

  26. A pp new result: J/ polarization • ALICE focusses on pp results mainly as reference for PbPb • On hard probes usually no competition with other LHC • experiments due to smaller luminosity in ALICE • Some notable exceptions, too  J/ polarization • (first LHC results from ALICE, arXiv:1111.1630) • Important measurement to discriminate among the different • theoretical models of J/ production • Long-standing puzzle with CDF results • J/ polarization measured via anisotropies in the angular • distributionsof J/ decay products • (polarization parameters    ) >0  transverse polarization, <0  longitudinal polarization

  27. J/ polarization results ALICE Coll., arXiv:1111.1630, accepted by PRL M.Butenschoen, A.Kniehl, arXiv:1201.3862 • First result: almost no polarization for the J/ • First theoretical calculation (NLO NRQCD) compared to data: • promising result, reasonable agreement with theory

  28. Prospects, 2011 Pb-Pb data • 2011 Pb-Pb data quite successful • Smooth running, much higher luminosity  ~10 times more • statistics (centrality and rare triggers) compared to 2010 • New, exciting results expected soon! A couple of performance plots Triggering on EMCAL Total 2011 statistics  40000 J/

  29. Conclusions • 2011 has been (another) memorable year for ALICE ! • After an already excellent start in 2010, with plenty of pp results, • focus in 2011 on the analysis of the first Pb-Pb run • First results • Medium with >3 times higher energy density than at RHIC • Abundance of hard probes • Soft observables • Smooth evolution of global event characteristics from RHIC to • LHC energies  better constraints for existing models • Move now towards precision differential measurements • Hard probes: novelties, surprises, challenges for theory • Strong suppression of high pT hadrons (factor 7 at pT=7 GeV/c) • Light and heavy quarks RAAsimilar • J/ is less suppressed than at RHIC • Pb-Pb 2011 running: success ! • Work on experiment upgrades is starting

  30. Backup

  31. 2010 data taking: detector configuration • ITS, TPC, TOF, HMPID, MUON, V0, To, FMD, PMD, ZDC (100%) • TRD (7/18) • EMCAL (4/10) • PHOS (3/5) • HLT (60%)

  32. Identified particle spectra Open symbols: ppbar Close symbols: pp

  33. More on strangeness Inverse slope increases with mass s do not follow this trend (limited statistics?) <pT> has almost no increase over a factor 36 in √s (ISRLHC)

  34. Still on HBT radii Increase with multiplicity both in p-p and A-A, but different features

  35. Analysis strategy • Require muon trigger signal to remove hadrons and low pt secondary muons • Remove residual decay muons by subtracting MC dN/dpt normalized to data at low pt • Alternative method: use muon distance-of-closest-approach to primary vertex • What is left are muons from charm and beauty • Apply efficiency corrections 35

  36. D meson reconstruction Analysis strategy: invariant-mass analysis of fully-reconstructed topologies originating from displaced vertices Build pairs/triplets/quadruplets of tracks with correct combination of charge signs and large impact parameters Particle identification from TPC and TOF to reject background (at low pt) Calculate the vertex (DCA point) of the decay tracks Require good pointing of reconstructed D momentum to the primary vertex • D0 K-π+ • D+ K-π+π+ • D*+ D0π+ • D0 K-π+π+π- • Ds K-K+π+ • Λc +  pK-π+ 36

  37. D0 K-p+ Signals from 108 events 7 pt bins in the range 1<pt<12 GeV/c Selection based mainly on cosine of pointing angle and product of track impact parameters (d0Kd0p) 37

  38. PID (ITS, TPC, TOF)

  39. MonteCarlo scoreboard

  40. Centrality vs models

  41. High pTelliptic flow Due to path length dependence of parton energy loss

  42. RAA – comparison with models

More Related