First Physics Results from Heavy Ion Run 2010 at LHC/ALICE

1. First Physics ResultsfromHeavy Ion Run 2010 atLHC/ALICE Kenta Shigaki 10th “Heavy Ion Pub” Inter-University Seminar December 17, 2010 Hiroshima University

2. First Heavy Ion Run at LHC/ALICE • 11/04: end of p+p run at s = 7 TeV • 11/07: first Pb+Pb collisions at sNN = 2.76 TeV • 11/08: stable Pb+Pb beams declared • 11/18: first ALICE Pb+Pb papers submitted • 12/06: end of run 2010 First Physics Results from Heavy Ion Run 2010 at LHC/ALICE – 10th HIP – K.Shigaki

3. Luminosity, Uptime, Collected Events • slightly below 10 mb-1 • 36% time with stable beam (as of 12/02) • peak luminosity close to 3×1025 cm-2s-1 (ditto) • 89 M total triggers (~20 M nuclear interactions) • ALICE uptime ~ 80% • electro-magnetic events ~ 80% First Physics Results from Heavy Ion Run 2010 at LHC/ALICE – 10th HIP – K.Shigaki

4. References • “first results from HI collisions” seminar, 12/02 • http://indico.cern.ch/conferenceDisplay.py?confId=114939 • ATLAS B. Cole Columbia ATLAS/PHENIX • CMS B. Wyslouch MIT CMS/PHOBOS • ALICE J. Schukraft CERN ALICE(/CERES) • Pb+Pb papers as of today • 3 from ALICE • particle multiplicities, elliptic flow, jet quenching • 1 from ATLAS • jet quenching First Physics Results from Heavy Ion Run 2010 at LHC/ALICE – 10th HIP – K.Shigaki

5. Menu de Jour • entrees:jet quenching in calorimetric view • from ATLAS and CMS • plats:“standard” heavy ion stories • from ALICE, based on J.Schukraft’s slides • some details behind the papers • not really insider information :) • e.g. trigger and event classes • garnitures:new eyes for heavy quarkonia, Z0, … • desserts:“bleeding edge” report • pâtissiers:H.Torii/J.Midori/H.Obayashi First Physics Results from Heavy Ion Run 2010 at LHC/ALICE – 10th HIP – K.Shigaki

6. Observation of a Centrality-Dependent Dijet Asymmetry in Lead-Lead Collisions with the ATLAS Detector • Brian A. Cole • Columbia University • on behalf of the ATLAS Collaboration • December 2, 2010

7. The paper: arXiv:1011.6182 STAR • Paper submitted on Nov 25, accepted by PRL Direct quenching @ LHC? Indirect jet quenching @ RHIC

8. ATLAS luminosity, data-taking • ATLAS luminosity profile vs day • Data-taking efficiency > 95% • Paper used runs corresponding to 1.7 μb-1(Nov 8 - 17) Fraction of data passing data quality selection

9. Measurements over Δη = 9.8 More central

10. A (more) symmetric dijet event Peripheral, symmetric dijet event

11. An asymmetric event More central, asymmetric dijet event

12. Another asymmetric event Even more central collision, more asymmetric dijet

13. Yet another asymmetric event Central event, with split dijet + additional activity

14. Jet reconstruction (2) • Take maximum advantage of ATLAS segmentation • Underlying event estimated and subtracted for each longitudinal layer and for 100 slices of Δη = 0.1 • ρ is energy density estimated event-by-event • From average over 0 < φ < 2π • Avoid biasing ρ due to jets • Using anti-kt jets: • Exclude cells from ρ if • Cross check • Sliding Window algorithm • NO jet removal on basis of D, or any other quantity 0.1x0.1 towers

15. Dijet event before & after • Before subtraction • ΣET in ΔηxΔφ = 0.1x0.1 towers • After subtraction, underlying event at zero • Event structure, topology unchanged by subtraction.

16. Dijet analysis • Use R = 0.4 anti-kt jets • calibrated using energy density cell weighting • Select events with leading jet, ET1 > 100 GeV, |η| < 2.8 • 1693 events after cuts in 1.7 μb-1 • Sub-leading: highest E T jet in opposite hemisphere, Δφ > π/2 with ET2 > 25 GeV, |η| < 2.8 • 5% of selected have no sub-leading jet • Introduce new variable to quantify dijetimbalance • Not used before in jet quenching literature: • Asymmetry: • Robust variable: • Residual subtraction errors cancel in numerator • Absolute jet energy scale errors cancel in ratio. 15

17. Dijets: comparison to p+p, HIJING + PYTHIA • Pb+Pb di-jet asymmetry (AJ), acoplanarity (Δφ) • Compare to p+p data • And PYTHIA (7 TeV) dijet events embedded in HIJING • No HIJING quenching, flow added in afterburner • Data agrees with p+p, MC in peripheral Pb+Pb. Peripheral 40-100% 16

18. Pb+Pb, 40-100% - Peripheral

19. Pb+Pb 20-40% - semi-central

20. Pb+Pb, 10-20% - more central

21. Pb+Pb, 0-10% - central

22. Full centrality range: paper plots • For more central collisions, see: • Reduced fraction of jets with small asymmetry • Increased fraction of jets with large asymmetry • For all centralities, Δφ strongly peaked at π • Possible small broadening in central collisions

23. Cross checks • We have carried out a number of cross checks to test for detector or analysis sources of asymmetry. • A partial list to be shown below: • Problems in calorimeter • Background subtraction • Different jet sizes • Jet shape • Energy loss to muons • Missing ET • Many more, subset in backup

24. Data-driven check on subtraction • Evaluate jet edge ET in region 0.2 < r < 0.4 • More susceptible to background subtraction errors. • Expect to be worse in central collisions • No centrality dependence except where the asymmetry effects are largest. 0.4 r 0.2

25. Cross-check: cone size dependence • Compare results for 3 different jet radii • Asymmetry increases for R = 0.2 • Opposite trend from what would be expected if asymmetry were due to background problems

26. Jet shapes • Peripheral events agree with Monte-Carlo • Monte Carlo and data agree well for leading jet • Systematic decrease of sub-leading Ψ(0.2) in more central collisions • But: beware, sub-leading jets are softer. Calculate core/total ratio for leading, second jets in Compare to HIJING + PYTHIA (7 TeV) MC

27. Energy, pT flow analysis • Independent check without jet algorithm and no subtraction • Asymmetry seen in calorimeter data • And also in tracks Calorimeter ET, offset subtracted Tracks, pT > 4 GeV

28. PbPb collisions in CMS Bolek Wyslouch École Polytechnique Massachusetts Institute of Technology on behalf of CMS Collaboration CERN, December 2, 2010

29. Triggering on collisions • Maximize efficiency for high pT probes • Muons • Jets • Photons/Electrons • Record large Minimum Bias sample • Additional triggers • Ultra Peripheral Collisions • Background monitoring • Minimize backgrounds

30. Dijet event candidates in CMS • First hours of LHC running • We see dijet events • We see dijets with unbalanced energy: is this real?

31. Background subtraction

32. Dijet selection • Leading jet is required to have at least 120 GeV • Trigger fully efficient • Sub-leading jet is required to have at least 50 GeV • Above background fluctuations • Leading and sub-leading jets with |h|<2 • Select back-to-back jets Df>2.5 • To study jet quenching effects use jet energy asymmetry

33. Jet Trigger Efficiency CMS Preliminary CMS Preliminary |ηjet| < 3 |ηjet| < 3 Corrected Jet Energy Raw Jet Energy

34. Aj in proton-proton collisions at 7 TeV Excellent agreement with PYTHIA+CMS simulation We will use PYTHIA as a reference at 2.76 TeV

35. Leading Jet ET Distributions Pb Pb Pb Pb Pb Pb Semi-Peripheral Semi-Central Central Leading Jet ET (GeV) Leading jet ET distribution shape well reproduced by simulations

36. Azimuthaldijet correlation Pb Pb Pb Pb Pb Pb Semi-Peripheral Semi-Central Central Df [rad] Select back-to-back dijets with Df>2.5 for further study

37. Dijet energy imbalance Pb Pb Pb Pb Pb Pb Semi-Peripheral Semi-Central Central A significant dijet imbalance, well beyond that expected from unquenched MC, appears with increasing collision centrality

38. Quantifying the imbalance:fraction of unbalanced dijets Pb Pb Pb Pb Pb Pb • Fraction of jets with imbalance larger than 0.24 • Plot as a function of number of participating nucleons (volume) averaged over centrality bin

39. Dijet imbalance and jet energy selection cut Varying the leading jet cutoff (ET = 120, 130, 140 GeV) CMS Preliminary (ET1-ET2)/(ET1+ET2) Varying the sub-leading jet cutoff (ET = 35, 50, 55 GeV) CMS Preliminary (ET1-ET2)/(ET1+ET2)

40. Jet Quenching – ATLAS/CMS’ View • di-jet calorimetric energy asymmetry • new approach with hermetic detectors • first observation of single jets in single events • direct, not statistical (!) • little change in azimuthal correlation (!) • currently limited by minimum pT of sub-leading jet • need higher pT leading jet, i.e. statistics • g-jet correlation foreseen • advantageous large acceptances First Physics Results from Heavy Ion Run 2010 at LHC/ALICE – 10th HIP – K.Shigaki

41. Future studies: g-jet ?

42. ‘Little Bang’The first 3 weeks … • Heavy Ion Physics at LHC • First look at Pb-Pb • Final (published) results • Ongoing Analysis

43. Role of LHC after RHIC/SPS • Search for the ‘QGP’ is essentially over • Discovery of QGP is well under way (with fantastic results & surprises at RHIC) • Measuring QGP parameters has just begun • 1) Quantitative differences • significantly different state of QGP in terms of energy density, lifetime, volume • large rate for ‘hard probes’ : jets, heavy quark states (b,c,U,J/Y ),… • 2) Test & validate the HI ‘Standard Model’ (< 10 years old !)QGP = very strongly interacting (almost) perfect liquid • Test predictions/extrapolations from RHIC to LHC • examples: flow (‘soft’) Quarkonia suppression (‘hard’) • 3) ‘Precision’ measurements of QGP parameters • Quantitative and systematic study of the new state of matter • Equation-of-Statef(e,p,T), viscosityh (flow), transport coefficientq (jet quenching), Debye screening mass (Quarkonia suppression), … • Confront data with Theory and Models: • standard tools: Lattice QCD, pQCD, Thermo- and Hydrodynamics, … • new tools: AdS/CFT (‘duality’), Classical QFT (‘Colour Glass Condensate’) • 4) Surprises ? • we are dealing with QCD in the strong coupling limit ! > 10 year program where are we after 3 weeks ? ^

44. Jets in ALICE (TPC) we see qualitatively a similar effect quantitative analysis is ongoing small acceptance (statistics), => need full 2010 data try to include low pt (study pt-cut off dependence of imbalance) 10-20% peripheral 168 GeV 192 GeV Dh Df 0-10% central 102 GeV 47 GeV Df Dh Charged Jets bin size: 0.1x0.1

45. Minimum-Bias (and Other) Triggers • 3 minimum-bias trigger detectors • innermost 2 layers of silicon vertex detector (SPD) • -2.0 < h < 2.0 • “V0A” and “V0C” trigger paddle detectors • 2.8 < h < 5.1; -3.7 < h < -1.7 • increasingly tightened with higher luminosity • from “OR” to “2 out of 3” to “AND” • high multiplicity and ultra-peripheral triggers, too First Physics Results from Heavy Ion Run 2010 at LHC/ALICE – 10th HIP – K.Shigaki

46. Centrality Determination • 4 (or more) centrality detectors • zero-degree calorimeter measuring break-up neutrons • V0, SPD, TPC measuring particle multiplicities • Npart and Ncoll from Glauber model • detector responses fitted with “aNpart+ (1-a) Ncoll” convoluted with NBD • valid fitting up to 90% centrality • electro-magnetic interaction dominance thereafter First Physics Results from Heavy Ion Run 2010 at LHC/ALICE – 10th HIP – K.Shigaki

47. Data driven Interpolation 900 GeV & 7 TeV or using NLO for change in shape 7 TeV * NLO (2.76 TeV)/NLO(7 TeV) Including CDF data 0.9 TeV * NLO (2.76 TeV)/NLO(0.9 TeV) ‘Jet Quenching’ as seen by pt spectra • Suppression of high pt particles ( ~ leading jet fragments) • Minimum RAA ~ 1.5 – 2 x smaller than at RHIC • Rising with pt ! (ambiguous at RHIC !) • accuracy limited by pp reference => need pp at 2.76 TeV ! RAA = 1 for (very) hard QCD processes in absence of nuclear modifications Paper to be submitted today

48. High pT Suppression at LHC and RHIC LHC-ALICE charged particles RHIC-PHENIX p0 First Physics Results from Heavy Ion Run 2010 at LHC/ALICE – 10th HIP – K.Shigaki

49. Associated pTt Δf Trigger High pT Particle Correlations ‘near’ side ‘away’ side UE Trigger Particle Trigger Particle: highest pT particle in event (pTt) Associate Particle: all the others (pTa)

50. q q Star@RHIC pT,trig 8-15 GeV Jet Quenching seen by High pT Correlations • classic ‘jet quenching signal’ • away side correlation in central Pb-Pb washed out up to pT,trig > 10 GeV PT associated 2 – 6 GeV ‘near’ side ‘away’ side Df Df Df