The Double Ridge Phenomenon in p-Pb Collisions Measured with ALICE Jan Fiete Grosse-Oetringhaus, CERN for the ALICE Collaboration Moriond QCD 2013

The Double-Ridge in p–Pb Collisions Measured with ALICE – Jan Fiete Grosse-Oetringhaus2 Motivation Proton-nucleus collisions are studied to access cold nuclear matter effects and assess a baseline for heavy- ion studies At RHIC d-Au collisions, a modification of the away side is seen in mid-forward correlations for central collisions (PRL 107 (2011) ) CMS has observed a near side ridge in high-multiplicity pp and p-Pb collisions (PLB718 (2013) 795) PHENIX, PRL 107 (2011) CMS, PLB718 (2013) %60-88%pp

The Double-Ridge in p–Pb Collisions Measured with ALICE – Jan Fiete Grosse-Oetringhaus3 Not shown: ZDC (at ±114m) L3 Magnet A Large Ion Collider Experiment T0/VZERO Trigger/Centrality TPC Tracking, PID (dE/dx) ITS Low p T tracking PID + Vertexing MUON μ-pairs TOF PID TRD Electron ID (TR) HMPID PID high p T PHOS γ, π 0, jets PMD γ multiplicity ACORDE Cosmic trigger FMD Charged multiplicity Dipole EMCAL γ, π 0, jets

The Double-Ridge in p–Pb Collisions Measured with ALICE – Jan Fiete Grosse-Oetringhaus4 Two-Particle Correlations Correlation between a trigger and an associated particle in certain p T intervals (p T,assoc < p T,trig ) Signal S contains correlation within same event Background B contains "correlation" between different events 1.7M p-Pb events from 4 hour test run in Sep/2012  s NN = 5.02 TeV [4 TeV p beam / 1.58 TeV per nucleon Pb beam] Rapidity shift of in p direction Quantities given here in the lab system trigger particle associated particle Mixed event  (rad) 

The Double-Ridge in p–Pb Collisions Measured with ALICE – Jan Fiete Grosse-Oetringhaus5 Event Classes Centrality in "heavy-ion terms" of impact parameter is not used  separate, non-trivial, topic in p-A collisions We define four multiplicity event classes in multiplicity ranges with a forward scintillator detector (VZERO) –2.8 <  < 5.1 and -3.7 <  < -1.7 –Denoted by 0-20% (highest multiplicity), 20-40%, 40-60%, % (lowest multiplicity)

The Double-Ridge in p–Pb Collisions Measured with ALICE – Jan Fiete Grosse-Oetringhaus6 The Ridge The near-side long-range ridge observed by CMS in pp and p-Pb can also be seen with ALICE [JHEP 09 (2010) 091, PLB718 (2013) 795] (zoomed) Near-side ridge (  ~ 0, elongated in  ) Near-side jet (  ~ 0,  ~ 0) Away-side jet (  ~ , elongated in  ) 2 < p T,trig < 4 GeV/c 1 < p T,assoc < 2 GeV/c 20% highest multiplicity  (rad)  1/N trig d 2 N assoc /d  d 

The Double-Ridge in p–Pb Collisions Measured with ALICE – Jan Fiete Grosse-Oetringhaus7 Projection to  Shifted to same baseline by subtracting the value at  = 1.3 Low multiplicity class agrees with results from pp collisions Increase of the yield on the near-side and away-side towards higher event multiplicity classes 0-20% 20-40% 40-60% % pp

The Double-Ridge in p–Pb Collisions Measured with ALICE – Jan Fiete Grosse-Oetringhaus8 Subtraction Procedure Can we separate the jet and ridge components? –No ridge seen in % and similar to pp  what remains if we subtract %? A double ridge! – 0-20%60-100% =  (rad)   (rad)   (rad) 

The Double-Ridge in p–Pb Collisions Measured with ALICE – Jan Fiete Grosse-Oetringhaus9 The Double Ridge Projections to  Ridges are flat in |  | < 2 Slight excess on the near side around  ~ 0 Could be residual jet: change of jet contribution as a function of multiplicity (fragmentation bias)? Exclude |  | < 0.8 on near side Away side? Bias evaluated and added to the systematic uncertainty by –Subtracting near side excess also from away side –Scaling % such that no near side excess remains |  | <  /3 |  -  | <  /3 Remaining   (rad)  1/N trig d 2 N assoc /d  d 

The Double-Ridge in p–Pb Collisions Measured with ALICE – Jan Fiete Grosse-Oetringhaus10 The Double Ridge Projections to  Modulation mostly of cos 2  type Small but significant cos 3  term needed Fit allows to extract v n coefficient –Baseline from 0-20% event class to be used! Same procedure applied on HIJING simulated events  no significant modulation remains  (rad)  1/N trig d 2 N assoc /d  d 

The Double-Ridge in p–Pb Collisions Measured with ALICE – Jan Fiete Grosse-Oetringhaus11 v 2 and v 3 Coefficients v 2 and v 3 as a function of p T for different event classes (each % subtracted) v 2 –Strong increase with p T –Mild increase with multiplicity v 3 –Increase with p T within large uncertainties v2v3v2v3

The Double-Ridge in p–Pb Collisions Measured with ALICE – Jan Fiete Grosse-Oetringhaus12 Ridge Yields Integrating near side and away side above baseline allows to extract ridge yields Increase with trigger p T and multiplicity Despite significant change in absolute values, remarkable agreement of near side and away side ridge yields –Common underlying physical origin for near side and away side ridge? line = diagonal (no fit!) Near side Away side

The Double-Ridge in p–Pb Collisions Measured with ALICE – Jan Fiete Grosse-Oetringhaus13 Symmetric Ridge? What would the assumption of a symmetric ridge give us? Determine near-side ridge in 1.2 < |  | < 1.8 Mirror to away side and subtract In addition to symmetric double ridge, no significant other structures  (rad) 0-20% 20-40% 40-60% % pp

The Double-Ridge in p–Pb Collisions Measured with ALICE – Jan Fiete Grosse-Oetringhaus14 Interpretation Flow? 3+1 viscous hydro (arXiv: ) Saturation? Color glass condensate (arXiv: ) Boxes: our values for 0-20% arXiv: Ridge Yields per  0-20% 20-40% 40-60% arXiv: Band: Calculation Points: ALICE data

The Double-Ridge in p–Pb Collisions Measured with ALICE – Jan Fiete Grosse-Oetringhaus15 Summary An intriguing double ridge structure in high- multiplicity p-Pb collisions has been observed The ridge on near side and away side has identical yield and is mostly symmetric (plus a v 3 component) regardless of p T and multiplicity Qualitative agreement of the p T and multiplicity dependence of the observed ridges with hydrodynamic calculations and color-glass condensate models Find more details in PLB719 (2013) 29 Thank you for your attention!