1. Hadron Correlations Measured with ALICE Jan Fiete Grosse-Oetringhaus, CERN for the ALICE Collaboration Hard Probes, Cagliari, 29.05.12

2. Hadron Correlations Measured with ALICE - Jan Fiete Grosse-Oetringhaus2 In-Medium Modification at LHC Strong quenching in central HI collisions at LHC –R AA ~ 0.14 [ALICE, ATLAS, CMS] –I AA ~ 0.5 (away side) I AA ~ 1.2 (near side) [ALICE] –Strong di-jet energy asymmetry, but jet fragmentation (of the quenched parton) only minimally modified [ATLAS, CMS] –Quenched energy reappears at low p T, also outside the jet cone [CMS] Study the low p T region where the energy reappears –Interactions of remnants of a high energetic parton interact with the collectivity-dominated bulk? Two-particle correlations allow to extract a small signal over a background dominated by combinatorics and collectivity –Near-side peak shapes, p/  ratio in jet and bulk, high p T flow See talks by Andreas Morsch (Mon 15:40) Misha Veldhoen (Mon 16:30) p T,assoc (GeV/c) I AA PRL108, 092301 0-5% 60-90%

3. Hadron Correlations Measured with ALICE - Jan Fiete Grosse-Oetringhaus3 Near-Side Peak Shapes Can we see modification of the near-side peak at low p T ? On the near-side the jet peak is concentrated around  = 0 Estimate  -independent effects (e.g. flow) by studying the long-range correlation region (|  | > 1) Remove from short-range region (|  | < 1) |  | < 1 1 < |  | < 1.6 Signal     

4. Hadron Correlations Measured with ALICE - Jan Fiete Grosse-Oetringhaus4 Analysis Details Event sample –15M Pb-Pb events at  s NN = 2.76 TeV –55M pp events at  s = 2.76 TeV Track selection benefits from uniform  acceptance of TPC –|  | < 0.9 –Two-track efficiency cut on distance of closest approach of a track pair in the TPC volume Event mixing corrects for two-track acceptance [in bins of centrality and vertex position] Per-trigger yields corrected for tracking efficiency and contamination (no influence on shapes) Mixed event Same event

5. Hadron Correlations Measured with ALICE - Jan Fiete Grosse-Oetringhaus5  Shape Evolution pTpT  Wider peak in central collisions Peripheral and pp similar Strong p T dependence Aim: Characterize the peak  0-10%60-70%pp 2 < p T,trig < 3 1 < p T,assoc < 2 4 < p T,trig < 8 2 < p T,assoc < 3

6. Hadron Correlations Measured with ALICE - Jan Fiete Grosse-Oetringhaus6 Peak Characterization Quantification with rms (~  ) and excess kurtosis K –K =  4 /  2 2 – 3 (  n n th moment) = measure of the peakedness Laplace: K=3, Gaussian: 0, semi-circle: -1, uniform -1.2 rms determined with two independent methods: fitting and projections –Fit with a suitable function (sum of 2 2D Gaussians ( ,  )) 4 shape parameters:  ,  , K , K  ResidualDataFit       More details in talk by Andreas Morsch (Mon 15:40)

7. Hadron Correlations Measured with ALICE - Jan Fiete Grosse-Oetringhaus7 Near-Side Peak:  (fit) No significant centrality dependence of   –Dependence on p T,assoc governed by j T ~ p T,assoc ·   Significant increase of   towards central events Smooth continuation from peripheral to pp Centrality | 100 = pp 2 < p T,t < 3 1 < p T,a < 2 GeV/c 3 < p T,t < 4 2 < p T,a < 3 GeV/c 4 < p T,t < 8 2 < p T,a < 3 GeV/c   (fit) (rad.)   (fit) Centrality | 100 = pp 2 < p T,t < 3 1 < p T,a < 2 GeV/c 3 < p T,t < 4 1 < p T,a < 2 GeV/c 3 < p T,t < 4 2 < p T,a < 3 GeV/c 4 < p T,t < 8 1 < p T,a < 2 GeV/c 4 < p T,t < 8 2 < p T,a < 3 GeV/c

8. Hadron Correlations Measured with ALICE - Jan Fiete Grosse-Oetringhaus8 Peak Deformation Significant increase of   towards central events –   >   (eccentricity ~ 0.2) Armesto, Salgado, Wiedemann suggested that longitudinal flow can deform the conical jet shape (PRL 93,242301 (2004)) Interplay of flow with the jet? PRL 93,242301 (2004) dN ch /d   rms (calc.)  rms (calc.) Calculation + STAR prel: 2 < p T,t < 3 1 < p T,a < 2 GeV/c 4 < p T,t < 8 2 < p T,a < 3 GeV/c Centrality | 100 = pp  ,   (fit)    

9. Hadron Correlations Measured with ALICE - Jan Fiete Grosse-Oetringhaus9 AMPT Comparison AMPT (A MultiPhase Transport Code) describes collective effects (e.g. v 2, v 3, v 4 ) in HI collisions at LHC –Here version with string melting (2.25) is shown It also does rather well for the rms of the near-side peak –Interplay of jet and flow in AMPT via parton and hadron scattering Centrality | 100 = pp   (fit) (rad.)   (fit) (rad.) Centrality | 100 = pp Lines: AMPT 2.25 and Pythia P-0 (for pp) 2 < p T,t < 3 1 < p T,a < 2 GeV/c 3 < p T,t < 4 2 < p T,a < 3 GeV/c 4 < p T,t < 8 2 < p T,a < 3 GeV/c

10. Hadron Correlations Measured with ALICE - Jan Fiete Grosse-Oetringhaus10 Kurtosis Excess kurtosis shows clear evolution with centrality for  and  –Near-side is less peaked towards central collisions –Described rather well with AMPT Centrality | 100 = pp Kurtosis  Kurtosis  Lines: AMPT 2.25 and Pythia P-0 (for pp) 2 < p T,t < 3 1 < p T,a < 2 GeV/c 3 < p T,t < 4 2 < p T,a < 3 GeV/c 4 < p T,t < 8 2 < p T,a < 3 GeV/c

11. Hadron Correlations Measured with ALICE - Jan Fiete Grosse-Oetringhaus11 The lowest p T bin shows a structure with a flat top in  This feature is reproduced by AMPT Qualitative and quantitative agreement of peak shapes with AMPT compatible with hypothesis of interplay of jets with the flowing bulk Departure from Gaussian AMPTData  0-10% 2 < p T,t < 3 GeV/c 1 < p T,a < 2 GeV/c See talk by Andreas Morsch (Mon 15:40)

12. Hadron Correlations Measured with ALICE - Jan Fiete Grosse-Oetringhaus12 p/  Ratio in Jet and Bulk Baryon over meson ratios differ significantly between AA and pp collisions –Attributed to radial flow and coalescence/recombination How do these ratios behave in a jet in AA collisions? Two-particle correlations allow to disentangle the bulk from particles associated with a trigger particle We measure the p/  ratio in the peak and the bulk Pb-Pb 0-5% pp Peak region Bulk I Bulk II  (rad.) 

13. Hadron Correlations Measured with ALICE - Jan Fiete Grosse-Oetringhaus13 Analysis Details 0-10% centrality; trigger particles: 5 < p T,trig < 10 GeV/c Associated particles: 1.5 < p T,assoc < 4.5 GeV/c –Combined particle identification with specific energy loss in the TPC and time of flight in the TOF dE/dx -  (a.u.) p K  t TOF -  (ps) Data Fit t TOF -  (ps)

14. Hadron Correlations Measured with ALICE - Jan Fiete Grosse-Oetringhaus14 Particle Yields Particle yields are measured in peak and bulk region and corrected for tracking and PID efficiency Difference of peak and bulk is the yield associated to the trigger particle  + +  - Peak Bulk p T,assoc (GeV/c) 1/N trig dN/dp T,assoc (c/GeV) p T,assoc (GeV/c) 1/N trig dN/dp T,assoc (c/GeV) p + pbar Peak Bulk

15. Hadron Correlations Measured with ALICE - Jan Fiete Grosse-Oetringhaus15 Bulk Inclusive Bulk Peak-Bulk p/  Ratio p/  ratio in the bulk is consistent with inclusive p/  ratio –NB. Inclusive ratio in 0-5% and feeddown corrected p/  ratio in peak - bulk is significantly smaller –Consistent with ratio from Pythia (6.4 default tune) No evidence for medium- induced modification of jet fragmentation (R ~ 0.4- 0.5) in this p T regime (p+pbar)/(  + +  - ) ratio p T,assoc (GeV/c) Bulk Peak-Bulk Pythia See talk by Misha Veldhoen (Mon 16:30) not feeddown corrected

16. Hadron Correlations Measured with ALICE - Jan Fiete Grosse-Oetringhaus16 Flow at High p T v 2, v 3, v 4 was measured to 20 GeV/c in 6 centrality classes (0-5% to 40-50%) v 2 flattens at high p T, stays positive and increases towards mid-centrality –Path-length dependent quenching –Described by WHDG calculation v 3 shows weak centrality dependence v 4 consistent with 0 within (large) uncertainties for p T > 8 GeV/c v 2 {EP} (filled) v 2 {4} (open) v 3 {EP} v 4/  4 {EP} v 4/  2 {EP} p T (GeV/c) arXiv:1205.5761 0-5% 20-30%

17. Hadron Correlations Measured with ALICE - Jan Fiete Grosse-Oetringhaus17 Flow Fluctuations v 2 {2} and v 2 {4} allow an estimate of flow fluctuations Relative flow fluctuations only weakly dependent on p T  common origin of flow fluctuations non-flow  small p T (GeV/c) 0-5% (open) 5-10% (filled) 10-20% 20-30% 30-40% 40-50% arXiv:1205.5761

18. Hadron Correlations Measured with ALICE - Jan Fiete Grosse-Oetringhaus18 Conclusions Asymmetric near-side peak in central Pb-Pb collisions –The  rms shows a strong centrality dependence, contrary to  –rms and excess kurtosis are well reproduced by AMPT –Interplay of the jet with flow? Associated p/  ratio to a trigger particle much smaller than in the bulk and compatible with vacuum fragmentation –No evidence for medium-induced modification of jet fragmentation Relative v 2 fluctuations only weakly dependent on p T –Common origin of flow fluctuations These observations are intriguing and a combination of these studies seems promising –PID-dependent shape, p/  ratio at different distances from trigger particle in particular in the tails of the distribution