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The Double Ridge Phenomenon in p-Pb Collisions Measured with ALICE

The Double Ridge Phenomenon in p-Pb Collisions Measured with ALICE. Jan Fiete Grosse-Oetringhaus, CERN for the ALICE Collaboration Moriond QCD 2013. 0-20% 60-88% pp. Motivation.

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The Double Ridge Phenomenon in p-Pb Collisions Measured with ALICE

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

  2. 0-20% 60-88%pp 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) 172301) • CMS has observed a near side ridge in high-multiplicity pp and p-Pb collisions (PLB718 (2013) 795) PHENIX, PRL 107 (2011) 172301 CMS, PLB718 (2013) 795 The Double-Ridge in p–Pb Collisions Measured with ALICE – Jan Fiete Grosse-Oetringhaus

  3. L3 Magnet T0/VZEROTrigger/Centrality ACORDE Cosmic trigger EMCAL γ, π0, jets HMPID PID (RICH) @ high pT TRD Electron ID (TR) TOF PID PMD γ multiplicity TPC Tracking, PID (dE/dx) ITS Low pT tracking PID + Vertexing MUON μ-pairs FMD Charged multiplicity PHOS γ, π0, jets ALarge Ion Collider Experiment Dipole Not shown: ZDC (at ±114m) The Double-Ridge in p–Pb Collisions Measured with ALICE – Jan Fiete Grosse-Oetringhaus

  4. Two-Particle Correlations • Correlation between a trigger and an associated particle in certain pT intervals (pT,assoc < pT,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 • sNN = 5.02 TeV [4 TeV p beam / 1.58 TeV per nucleon Pb beam] • Rapidity shift of 0.465 in p direction • Quantities given here in the lab system trigger particle associated particle Mixed event Dh Dj (rad) The Double-Ridge in p–Pb Collisions Measured with ALICE – Jan Fiete Grosse-Oetringhaus

  5. 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 < h < 5.1 and -3.7 < h < -1.7 • Denoted by 0-20% (highest multiplicity), 20-40%, 40-60%, 60-100% (lowest multiplicity) The Double-Ridge in p–Pb Collisions Measured with ALICE – Jan Fiete Grosse-Oetringhaus

  6. The Ridge 2 < pT,trig < 4 GeV/c1 < pT,assoc < 2 GeV/c20% highest multiplicity • 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 jet(Dj ~ 0, Dh ~ 0) 1/Ntrig d2Nassoc/dDhdDj Away-side jet(Dj ~ p, elongated in Dh) Near-side ridge(Dj ~ 0, elongated in Dh) Dh Dj (rad) The Double-Ridge in p–Pb Collisions Measured with ALICE – Jan Fiete Grosse-Oetringhaus

  7. Projection to Dj 0-20%20-40%40-60%60-100%pp • Shifted to same baseline by subtracting the value at Dj = 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 The Double-Ridge in p–Pb Collisions Measured with ALICE – Jan Fiete Grosse-Oetringhaus

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

  9. The Double RidgeProjections to Dh • Ridges are flat in |Dh| < 2 • Slight excess on the near side around Dh ~ 0 • Could be residual jet: change of jet contribution as a function of multiplicity (fragmentation bias)? • Exclude |Dh| < 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 60-100% such that no near side excess remains Dh Dj (rad) |Dj| < p/3 |Dj - p| < p/3 Remaining Dj 1/Ntrig d2Nassoc/dDhdDj Dh The Double-Ridge in p–Pb Collisions Measured with ALICE – Jan Fiete Grosse-Oetringhaus

  10. The Double RidgeProjections to Dj • Modulation mostly of cos 2Dj type • Small but significant cos 3Dj term needed • Fit allows to extract vn coefficient • Baseline from 0-20% event class to be used! • Same procedure applied on HIJING simulated events  no significant modulation remains Dh Dj (rad) 1/Ntrig d2Nassoc/dDhdDj Dj (rad) The Double-Ridge in p–Pb Collisions Measured with ALICE – Jan Fiete Grosse-Oetringhaus

  11. v2 and v3 Coefficients • v2 and v3 as a function of pT for different event classes (each 60-100% subtracted) • v2 • Strong increase with pT • Mild increase with multiplicity • v3 • Increase with pTwithin large uncertainties v2 v3 The Double-Ridge in p–Pb Collisions Measured with ALICE – Jan Fiete Grosse-Oetringhaus

  12. Ridge Yields • Integrating near side and away side above baseline allows to extract ridge yields • Increase with trigger pT 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 sideAway side The Double-Ridge in p–Pb Collisions Measured with ALICE – Jan Fiete Grosse-Oetringhaus

  13. Symmetric Ridge? • What would the assumption of a symmetric ridge give us? • Determine near-side ridge in 1.2 < |Dh| < 1.8 • Mirror to away side and subtract 0-20%20-40%40-60%60-100%pp Dj (rad) Dj (rad) In addition to symmetric double ridge, no significant other structures The Double-Ridge in p–Pb Collisions Measured with ALICE – Jan Fiete Grosse-Oetringhaus

  14. Flow? 3+1 viscous hydro (arXiv:1112.0915) Saturation? Color glass condensate (arXiv:1302.7018) Interpretation Band: Calculation Points: ALICE data arXiv:1112.0915 Ridge Yields per Dh arXiv:1302.7018 0-20% 20-40% 40-60% Boxes: our values for 0-20% The Double-Ridge in p–Pb Collisions Measured with ALICE – Jan Fiete Grosse-Oetringhaus

  15. 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 v3 component) regardless of pT and multiplicity • Qualitative agreement of the pT and multiplicity dependence of the observed ridges with hydrodynamic calculations and color-glass condensate models Thank you for your attention! Find more details in PLB719 (2013) 29 The Double-Ridge in p–Pb Collisions Measured with ALICE – Jan Fiete Grosse-Oetringhaus

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