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

The Double-Ridge in p–Pb Collisions Measured with ALICE – Jan Fiete Grosse-Oetringhaus 2

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)

PH

EN

IX,

PR

L 10

7 (2

011

) 1

7230

1C

MS

, P

LB7

18 (

2013

) 79

5

0-20% 60-88% pp


Not shown: ZDC (at ±114m)

L3 Magnet

A Large Ion Collider ExperimentT0/VZEROTrigger/Centrality

TPCTracking, PID (dE/dx) ITS

Low pT trackingPID + Vertexing

MUON μ-pairs

TOFPID

TRDElectron ID (TR)

HMPIDPID (RICH) @ high pT

PHOSγ, π0, jets

PMDγ multiplicity

ACORDECosmic trigger

FMDCharged multiplicity

Dipole

EMCALγ, π0, jets


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

),(

),(1 2

B

S

d

Nd

Nassoc

trig

trigger particleassociated particle

Mixed event

(rad)


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%,

60-100% (lowest multiplicity)


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 < pT,trig < 4 GeV/c1 < pT,assoc < 2 GeV/c20% highest multiplicity

(rad)

1/N

trig d

2 Nas

soc/

d

d


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%60-100%pp


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%

=

(r

ad)

(rad)

(rad)


The Double RidgeProjections 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 60-100% such that no near side excess remains

|| < /3| - | < /3Remaining

(r

ad)

1/N

trig d

2 Nas

soc/

d

d


The Double RidgeProjections to

• Modulation mostly of cos 2 type• Small but significant cos 3 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

(rad)

bav n

n (r

ad)

1/N

trig d

2 Nas

soc/

d

d


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


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


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) (rad)

0-20%20-40%40-60%60-100%pp


Interpretation

Flow? 3+1 viscous hydro (arXiv:1112.0915)

Saturation?Color glass condensate (arXiv:1302.7018)

Boxes: our values for 0-20%

arX

iv:1

112.

0915

Rid

ge

Yie

lds

per

0-20% 20-40% 40-60%

arX

iv:1

302.

7018

Band: Calculation

Points: ALICE data


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!

the double ridge phenomenon in p-pb collisions measured with alice

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