Hadron Correlations Measured with ALICE Jan Fiete GrosseOetringhaus

Hadron Correlations Measured with ALICE Jan Fiete Grosse-Oetringhaus, CERN for the ALICE Collaboration Hard Probes, Cagliari, 29. 05. 12 Hadron Correlations Measured with ALICE - Jan Fiete Grosse-Oetringhaus 1

In-Medium Modification at LHC • Strong quenching in central HI collisions at LHC – RAA ~ 0. 14 [ALICE, ATLAS, CMS] – IAA ~ 0. 5 (away side) IAA ~ 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, IAA also outside the jet cone [CMS] • Study the low p. T region where the energy reappears 0 -5% 60 -90% – 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/p ratio in jet and bulk, high p. T flow PRL 108, 092301 p. T, assoc (Ge. V/c) See talks by • Andreas Morsch (Mon 15: 40) • Misha Veldhoen (Mon 16: 30) Hadron Correlations Measured with ALICE - Jan Fiete Grosse-Oetringhaus 2

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 Dh = 0 • Estimate Dh-independent effects (e. g. flow) by studying the long-range correlation region (|Dh| > 1) • Remove from short-range region (|Dh| < 1) |Dh| < 1 1 < |Dh| < 1. 6 Signal Dh Dj Hadron Correlations Measured with ALICE - Jan Fiete Grosse-Oetringhaus Dj 3

Analysis Details Same event • Event sample – 15 M Pb-Pb events at s. NN = 2. 76 Te. V – 55 M pp events at s = 2. 76 Te. V • Track selection benefits from uniform j acceptance of TPC – |h| < 0. 9 – Two-track efficiency cut on distance of closest approach of a track pair in the TPC volume Mixed event • 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) Hadron Correlations Measured with ALICE - Jan Fiete Grosse-Oetringhaus 4

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

Peak Characterization • Quantification with rms (~ s) and excess kurtosis K – K = m 4 / m 22 – 3 (mn nth 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 2 D Gaussians (h, j)) • 4 shape parameters: s. Dh, s. Dj, KDh, KDj Data Dh Dj Dh Fit details in talk by Andreas Residual. Morsch More Dh (Mon 15: 40) Dj Dj Hadron Correlations Measured with ALICE - Jan Fiete Grosse-Oetringhaus 6

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

• Significant increase of s. Dh towards central events s. Dj , s. Dh (fit) Peak Deformation 2 < p. T, t < 3 4 < p. T, t < 8 1 < p. T, a < 2 Ge. V/c 2 < p. T, a < 3 Ge. V/c s. Dh s. Dj – s. Dh > s. Dj (eccentricity ~ 0. 2) • Armesto, Salgado, Wiedemann suggested that longitudinal flow can deform the conical jet shape (PRL 93, 242301 (2004)) Centrality | 100 = pp Calculation + STAR prel: h rms (calc. ) j rms (calc. ) • Interplay of flow with the jet? PRL 93, 242301 (2004) Hadron Correlations Measured with ALICE - Jan Fiete Grosse-Oetringhaus d. Nch/dh 8

AMPT Comparison • AMPT (A Multi. Phase 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 2 < p. T, t < 3 3 < p. T, t < 4 4 < p. T, t < 8 1 < p. T, a < 2 Ge. V/c 2 < p. T, a < 3 Ge. V/c Lines: AMPT 2. 25 and Pythia P-0 (for pp) Centrality | 100 = pp s. Dh (fit) (rad. ) s. Dj (fit) (rad. ) – Interplay of jet and flow in AMPT via parton and hadron scattering Centrality | 100 = pp Hadron Correlations Measured with ALICE - Jan Fiete Grosse-Oetringhaus 9

– Near-side is less peaked towards central collisions – Described rather well with AMPT Kurtosis Dj • Excess kurtosis shows clear evolution with centrality for j and h Kurtosis Dh Kurtosis 2 < p. T, t < 3 3 < p. T, t < 4 4 < p. T, t < 8 1 < p. T, a < 2 Ge. V/c 2 < p. T, a < 3 Ge. V/c Lines: AMPT 2. 25 and Pythia P-0 (for pp) Centrality | 100 = pp Hadron Correlations Measured with ALICE - Jan Fiete Grosse-Oetringhaus 10

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

p/p Ratio in Jet and Bulk • Baryon over meson ratios differ significantly between AA and pp collisions Pb-Pb 0 -5% pp – 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/p ratio in the peak and the bulk Bulk I Dh Peak region Bulk II Dj (rad. ) Hadron Correlations Measured with ALICE - Jan Fiete Grosse-Oetringhaus 12

Analysis Details • 0 -10% centrality; trigger particles: 5 < p. T, trig < 10 Ge. V/c • Associated particles: 1. 5 < p. T, assoc < 4. 5 Ge. V/c t. TOF - <t. TOF>p (ps) – Combined particle identification with specific energy loss in the TPC and time of flight in the TOF p K Data Fit p d. E/dx - <d. E/dx>p (a. u. ) t. TOF - <t. TOF>p (ps) Hadron Correlations Measured with ALICE - Jan Fiete Grosse-Oetringhaus d. E/dx - <d. E/dx>p (a. u. ) 13

Particle Yields 1/Ntrig d. N/dp. T, assoc (c/Ge. V) • 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 p+ + p. Peak Bulk p + pbar Peak Bulk p. T, assoc (Ge. V/c) Hadron Correlations Measured with ALICE - Jan Fiete Grosse-Oetringhaus p. T, assoc (Ge. V/c) 14

p/p Ratio (p+pbar)/(p++p-) ratio • p/p ratio in the bulk is consistent with inclusive p/p ratio – NB. Inclusive ratio in 0 -5% and feeddown corrected • p/p ratio in peak - bulk is significantly smaller Bulk Peak-Bulk Pythia – Consistent with ratio from Pythia (6. 4 default tune) Bulk Inclusive • No evidence for mediuminduced modification of jet fragmentation (R ~ 0. 40. 5) in this p. T regime not feeddown corrected p. T, assoc (Ge. V/c) See talk by Misha Veldhoen (Mon 16: 30) Hadron Correlations Measured with ALICE - Jan Fiete Grosse-Oetringhaus 15

Flow at High p. T • v 2, v 3, v 4 was measured to 20 Ge. V/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 0 -5% v 2{EP} (filled) v 2{4} (open) v 3{EP} v 4/ 4{EP} v 4/ 2{EP} 20 -30% • v 3 shows weak centrality dependence • v 4 consistent with 0 within (large) uncertainties for p. T > 8 Ge. V/c ar. Xiv: 1205. 5761 Hadron Correlations Measured with ALICE - Jan Fiete Grosse-Oetringhaus p. T (Ge. V/c) 16

Flow Fluctuations • v 2{2} and v 2{4} allow an estimate of flow fluctuations non-flow d small 0 -5% (open) 5 -10% (filled) 10 -20% 30 -40% 20 -30% 40 -50% • Relative flow fluctuations only weakly dependent on p. T common origin of flow fluctuations ar. Xiv: 1205. 5761 p. T (Ge. V/c) Hadron Correlations Measured with ALICE - Jan Fiete Grosse-Oetringhaus 17

Conclusions • Asymmetric near-side peak in central Pb-Pb collisions – The Dh rms shows a strong centrality dependence, contrary to Dj – rms and excess kurtosis are well reproduced by AMPT – Interplay of the jet with flow? • Associated p/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/p ratio at different distances from trigger particle in particular in the tails of the distribution Hadron Correlations Measured with ALICE - Jan Fiete Grosse-Oetringhaus 18