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Ultrarelativistic Nucleus-Nucleus Collisions at the LHC

Ultrarelativistic Nucleus-Nucleus Collisions at the LHC. Federico Antinori INFN, Padova, Italy and CERN, Geneva, Switzerland 中国合肥 - 中国科学技术大学 – 2012 年 3 月 29 号. Contents. Introduction Multiplicity Correlations High p T suppression Identified particles Strangeness enhancement

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Ultrarelativistic Nucleus-Nucleus Collisions at the LHC

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  1. Ultrarelativistic Nucleus-Nucleus Collisions at the LHC Federico Antinori INFN, Padova, Italy and CERN, Geneva, Switzerland 中国合肥 - 中国科学技术大学 –2012年3月29号

  2. Contents Introduction Multiplicity Correlations High pT suppression Identified particles Strangeness enhancement Quarkonia Jets Heavy Flavours Conclusions Federico Antinori – 科大 – 2012年3月29号

  3. Introduction

  4. Ultrarelativistic AA Collisions basic idea: compress large amount of energy in a very small volume • produce a “fireball” of hot matter: temperature O(1012 K) • ~ 105 x T at centre of Sun • ~ T of universe @ ~ 10 µs after Big Bang • how does matter behave under such extreme conditions? • study the fireball properties • QCD predicts state of deconfined quarks and gluons (Quark-Gluon Plasma) Federico Antinori – 科大 – 2012年3月29号

  5. Lattice QCD 3 flavours; (q-q)=0 • in lattice QCD, non-perturbative problems are treated by discretization on a space-time lattice  cure ultraviolet divergences • zero baryon density, 3 flavours • e changes rapidly around Tc • Tc = 170 MeV:  ec = 0.6 GeV/fm3 • at T~1.2 Tce settles at about 80% of the Stefan-Boltzmann value for an ideal gas of q,q g (eSB) Federico Antinori – 科大 – 2012年3月29号

  6. ~ 10 ms after Big Bang LHC Early Universe RHIC Quark-Gluon Plasma Tc ~ 170 MeV SPS AGS Hadron gas Temperature Nuclear Neutron Star matter ~ 5 - 10 nuclear r Baryon density ec ~ 1 GeV/fm3 QCD phase diagram Federico Antinori – 科大 – 2012年3月29号

  7. Geometry of a Pb-Pb collision • central collisions • small impact parameter b • high number of participants high multiplicity • peripheral collisions • large impact parameter b • low number of participants low multiplicity for example: sum of the amplitudes in the ALICE V0 scintillators reproduced by Glauber model fit (red): • random relative position of nuclei in transverse plane • Woods-Saxon distribution inside nucleus • deviation at very low amplitude expected due to non-nuclear (electromagnetic) processes b peripheral central Federico Antinori – 科大 – 2012年3月29号

  8. Multiplicity

  9. Particle multiplicity ALICE: PRL105 (2010) 252301 log extrapolationfails (finally!) 2.2 x central Au+Au (√sNN=0.2 TeV) 1.9 x pp (NSD) (√sNN=2.36 TeV) √sNN=2.76 TeV Pb+Pb, 0-5% central, |η|<0.5 2 dNch/dη / <Npart> = 8.3  0.4 (sys.) for the most central collisions: ~ 1600 charged particles per unit of η Federico Antinori – 科大 – 2012年3月29号

  10. Centrality dependence ALICE: PRL 106 (2011) 032301 Helena Santos – ATLAS (EPS 2011) • model comparisons • DPMJET (with string fusion) • HIJING 2.0 (no quenching) • centrality-dependent gluon shadowing • tuned to multiplicity in 0-5% • saturationmodels • very similar centrality dependence at LHC & RHIC • once corrected for difference in absolute values Federico Antinori – 科大 – 2012年3月29号

  11. Correlations

  12. Azimuthal asymmetry • transfer of this asymmetry to momentum space provides a measure of the strength of collective phenomena • Large mean free path • particles stream out isotropically, no memory of the asymmetry • extreme: ideal gas (infinite mean free path) • Small mean free path • larger density gradient -> larger pressure gradient -> larger momentum • extreme: ideal liquid (zero mean free path, hydrodynamic limit) •  quantified by second coefficient (v2) of Fourier expansion of azimuthal distribution … in the transverse momentum distribution of produced particles • why is it important? • non-central collisions are asymmetric in azimuth azimuth = angle in the plane of the screen Federico Antinori – 科大 – 2012年3月29号

  13. v2 • v2 still large at the LHC • system still behaves very close to ideal liquid (low viscosity) • v2(pT) very similar at LHC and RHIC  same hydrodynamical behaviour? ALICE: PRL 105 (2010) 252302 Federico Antinori – 科大 – 2012年3月29号

  14. Structures in (Δη,Δφ) two shoulders on away side (at 120 and 240 ) aka “the Mach cone” near side jet peak long range structure in η on near side aka “the ridge” Federico Antinori – 科大 – 2012年3月29号

  15. Fluctuations  v3 Matt Luzum (QM 2011) v2 v3 ALICE: PRL 107 (2011) 032301 • “ideal” shape of participants’ overlap is ~ elliptic • in particular: no odd harmonics expected • participants’ plane coincides with event plane • but fluctuations in initial conditions: • participants plane  event plane • v3 (“triangular”) harmonic appears [B Alver & G Roland, PRC81 (2010) 054905] • and indeed, v3  0 ! • v3 has weaker centrality dependence than v2 • when calculated wrt participants plane, v3 vanishes • as expected, if due to fluctuations… Federico Antinori – 科大 – 2012年3月29号

  16. Long-η-range correlations Andrew Adare – ALICE (QM2011) • “ultra-central” events: dramatic shape evolution in a very narrow centrality range • double hump structure on away-side appears on 1% most central • visible without any need for v2 subtraction! • first five harmonics describe shape at 10-3 level • “ridge” and “Mach cone” • explanations based on medium response to propagating partons were proposed at RHIC • Fourier analysis of new data suggests very natural alternative explanation in terms of hydrodynamic response to initial state fluctuations Federico Antinori – 科大 – 2012年3月29号

  17. High pT suppression

  18. Nuclear modification factor (RAA) CMS: PAS HIN-10-005 • RAA: particle yield in AA relative to pp scaled with number of binary nucleon-nucleon collisions • RAA(pT) for charged particles produced in 0-5% centrality range • minimum (~ 0.14) for pT ~ 6-7 GeV/c • then slow increase at high pT • still significant suppression at pT ~ 100 GeV/c ! • essential quantitative constraint for parton energy loss models! Federico Antinori – 科大 – 2012年3月29号

  19. Suppression vs event plane • significant effect, even at 20 GeV! • further constraints to energy loss models  path-length dependence of energy loss (L2, L3, …) Alexandru Dobrin – ALICE (QM2011) Federico Antinori – 科大 – 2012年3月29号

  20. Identified particles

  21. pT spectra vs hydrodynamics Michele Floris – ALICE (QM2011) • comparison of identified particle spectra with hydro predictions • (calculations by C Shen et al.: arXiv:1105.3226 [nucl-th]) • OK for π and K, but p seem to “misbehave” (less yield, flatter spectrum) Federico Antinori – 科大 – 2012年3月29号

  22. v2 vs hydrodynamics Raimond Snellings ALICE (QM2011) • comparison of identified particles v2(pT) with hydro prediction • (calculation by C Shen et al.: arXiv:1105.3226 [nucl-th])  again, protons… Federico Antinori – 科大 – 2012年3月29号

  23. Strangeness enhancement

  24. Strangeness enhancement Maria Nicassio – ALICE (SQM2011) • enhancement decreases with increasing √s  strange/non-strange increases with √s in pp Federico Antinori – 科大 – 2012年3月29号

  25. Comparison with thermal model Alexander Kalweit – ALICE (SQM2011) • Reasonable grand-canonical description with Tch=164 MeV  except for p/π ! Federico Antinori – 科大 – 2012年3月29号

  26. Quarkonia

  27. J/ψ @ LHC: central y, high pT • LHC: |y| < 2.4, pT > 6.5 GeV/c (CMS) prompt J/ψ • more suppressed than RHIC: |y| < 1. pT > 5 GeV/c (STAR) inclusive J/ ψ • LHC |y| < 2.5, pT > 3 GeV/c (ATLAS) CMS: PAS HIN-10-006 ATLAS: PLB 697 (2011) 294 Federico Antinori – 科大 – 2012年3月29号

  28. J/ψ @ LHC: forward y, low pT • LHC: 2.5 < y < 4, pT > 0 (ALICE) • less suppression than RHIC: 1.2 < y < 2.2, pT > 0 (PHENIX) • ~ as suppressed as RHIC: |y| < 0.35. pT > 0 (PHENIX) Federico Antinori – 科大 – 2012年3月29号

  29. Yen-Jie Lee – CMS Yen-Jie Lee – CMS (EPS-HEP 2011) Federico Antinori – 科大 – 2012年3月29号

  30. Jets

  31. Di-jet imbalance Frank Ma – CMS (EPS-HEP 2011) • Pb-Pb events with large di-jet imbalance observed by ATLAS & CMS • recoiling jet strongly quenched! Federico Antinori – 科大 – 2012年3月29号

  32. AJ • with increasing centrality: • enhancement of asymmetric di-jets • with respect to pp & HIJING + PYTHIA ATLAS: PRL105 (2010) 252303 Federico Antinori – 科大 – 2012年3月29号

  33. Di-jet Δφ CMS: arXiv:1102.1957 no visible angular decorrelation in Δφ wrt pp collisions Federico Antinori – 科大 – 2012年3月29号

  34. Jet nuclear modification factor • suppression of jet production • in central Pb-Pb wrt binary-scaled peripheral • very weak centrality dependence Helena Santos – ATLAS (EPS-HEP 2012) Federico Antinori – 科大 – 2012年3月29号

  35. Fragmentation function similar to fragmentation in vacuum? Federico Antinori – 科大 – 2012年3月29号

  36. Heavy Flavours

  37. c and b quenching @ LHC • substantial suppression of heavy flavour production • beauty, too! • study parton mass and colour charge dependence of interaction with medium! Federico Antinori – 科大 – 2012年3月29号

  38. D vs π • charm ~ as suppressed as pions • maybe a hint of RAAD > RAAπ at low pT …  to be continued with higher statistics… • quarks (CR = 4/3) expected to couple weaker than gluons (CR = 3) • at pT ~ 8 GeV, factor ~ 2 less suppression expected for D than for light hadrons in BDMPS-ADSW Andrea Dainese – ALICE (QM 2011) N Armesto et al., PRD 71 (2005) 054027 Federico Antinori – 科大 – 2012年3月29号

  39. D0 elliptic flow • First direct measurement of D flow in heavy-ion collisions • Yield extracted from invariant mass spectra of Kp candidates in 2 bins of azimuthal angle relative to the event plane Chiara Bianchin – ALICE (SQM 2011) Federico Antinori – 科大 – 2012年3月29号

  40. Conclusions • in November 2010, the field of ultrarelativistic nuclear collisions has entered a new era with the start of heavy ion collisions at the LHC • abundance of hard probes • state-of-the art collider detectors • exciting results already from 2010 data sample • death of ridge and Mach cone? • anomalies in proton yields & momentum distributions • pattern of jet and heavy flavour suppression  challenge to Eloss models • and the future looks bright! • ~ 150/µb delivered by LHC in 2011 • p-Pb run scheduled for 2012 • precision measurements + handle on cold nuclear matter effects • close in on dynamic and coupling properties of medium • and … look out for surprises!!! Federico Antinori – 科大 – 2012年3月29号

  41. 谢谢！

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