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

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

• Jan Fiete Grosse-Oetringhaus, CERN
• for the ALICE Collaboration
• Moriond QCD 2013

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Motivation
0-20 60-88 pp

• 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
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A Large Ion Collider Experiment
Dipole
Not shown ZDC (at 114m)
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Two-Particle Correlations

• Correlation between a trigger and an associated
  particle in certain pT intervals (pT,assoc lt
  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)
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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 lt h lt 5.1 and -3.7 lt h lt -1.7
• Denoted by 0-20 (highest multiplicity), 20-40,
  40-60, 60-100 (lowest multiplicity)

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The Ridge
2 lt pT,trig lt 4 GeV/c1 lt pT,assoc lt 2 GeV/c20
highest multiplicity
(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 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

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Projection to Dj
0-2020-4040-6060-100pp

• 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

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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
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The Double RidgeProjections to Dh

• Ridges are flat in Dh lt 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 lt 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 lt p/3 Dj - p lt p/3 Remaining Dj
1/Ntrig d2Nassoc/dDhdDj
Dh
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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)
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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 pT within large uncertainties

v2 v3
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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
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Symmetric Ridge?

• What would the assumption of a symmetric ridge
  give us?
• Determine near-side ridge in 1.2 lt Dh lt 1.8
• Mirror to away side and subtract

0-2020-4040-6060-100pp
Dj (rad)
Dj (rad)
In addition to symmetric double ridge, no
significant other structures
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Interpretation

• Flow?
• 31 viscous hydro (arXiv1112.0915)

• Saturation?
• Color glass condensate (arXiv1302.7018)

Band Calculation Points ALICE data
arXiv1112.0915
Ridge Yields per Dh
arXiv1302.7018
0-20 20-40 40-60
Boxes our values for 0-20
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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

Find more details in PLB719 (2013) 29
Thank you for your attention!