The Double Ridge Phenomenon in
p-Pb Collisions Measured with ALICE
Jan Fiete Grosse-Oetringhaus, CERN
for the ALICE Collaboration
Moriond QCD 2013
60-88% pp
PHENIX, PRL 107 (2011) 172301
Motivation
0-20%
CMS, PLB718 (2013) 795
• Proton-nucleus collisions are
studied to access cold
nuclear matter effects and
assess a baseline for heavyion 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)
The Double-Ridge in p–Pb Collisions Measured with ALICE – Jan Fiete Grosse-Oetringhaus
2
A Large Ion Collider Experiment
EMCAL
γ, π0, jets
T0/VZERO
Trigger/Centrality
L3 Magnet
ACORDE
Cosmic trigger
HMPID
PID (RICH) @ high pT
TRD
Electron ID (TR)
TOF
PID
PMD
γ multiplicity
Dipole
TPC
Tracking,
PID (dE/dx)
PHOS
γ, π0, jets
FMD
Charged
multiplicity
ITS
Low pT tracking
PID + Vertexing
MUON
μ-pairs
Not shown: ZDC (at ±114m)
The Double-Ridge in p–Pb Collisions Measured with ALICE – Jan Fiete Grosse-Oetringhaus
3
Two-Particle Correlations
• Correlation between a trigger and an associated
particle in certain pT intervals (pT,assoc < pT,trig)
1
2
d N assoc
N trig d    

S ( ,  )
B ( ,  )
trigger particle
associated particle
• Signal S contains correlation within same event
• Background B contains "correlation"
Mixed event
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
The Double-Ridge in p–Pb Collisions Measured with ALICE – Jan Fiete Grosse-Oetringhaus
4
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 Double-Ridge in p–Pb Collisions Measured with ALICE – Jan Fiete Grosse-Oetringhaus
5
The Ridge
1/Ntrig d2Nassoc/dd
2 < pT,trig < 4 GeV/c
1 < pT,assoc < 2 GeV/c
20% highest multiplicity
(zoomed)
Near-side jet
( ~ 0,  ~ 0)
Away-side jet
( ~ p, elongated in )
Near-side ridge
( ~ 0, elongated in )
• 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]
The Double-Ridge in p–Pb Collisions Measured with ALICE – Jan Fiete Grosse-Oetringhaus
6
Projection to 
• Shifted to same
baseline by
subtracting the
value at  = 1.3
0-20%
20-40%
40-60%
60-100%
pp
• 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
7
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%?
0-20%
60-100%
–
=
• A double ridge!
The Double-Ridge in p–Pb Collisions Measured with ALICE – Jan Fiete Grosse-Oetringhaus
8
The Double Ridge
Projections to 
– Subtracting near side excess
also from away side
– Scaling 60-100% such that
no near side excess remains
|| < p/3
| - p| < p/3
Remaining 
1/Ntrig d2Nassoc/dd
• 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

The Double-Ridge in p–Pb Collisions Measured with ALICE – Jan Fiete Grosse-Oetringhaus
9
The Double Ridge
Projections to 
– Baseline from 0-20%
event class to be used!
• Same procedure applied
on HIJING simulated
events  no significant
modulation remains
1/Ntrig d2Nassoc/dd
• Modulation mostly of cos 2 type
• Small but significant cos 3 term
needed
• Fit allows to extract
an
vn 
b
vn coefficient
 (rad)
The Double-Ridge in p–Pb Collisions Measured with ALICE – Jan Fiete Grosse-Oetringhaus
10
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
v2
v3
• v3
– Increase with pT
within large
uncertainties
The Double-Ridge in p–Pb Collisions Measured with ALICE – Jan Fiete Grosse-Oetringhaus
11
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 side
Away side
The Double-Ridge in p–Pb Collisions Measured with ALICE – Jan Fiete Grosse-Oetringhaus
12
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
 (rad)
0-20%
20-40%
40-60%
60-100%
pp
 (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
13
Interpretation
Flow?
3+1 viscous hydro (arXiv:1112.0915)
Saturation?
Color glass condensate
Band: Calculation
Points: ALICE data
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
14
arXiv:1302.7018
Ridge Yields per 
arXiv:1112.0915
(arXiv:1302.7018)
Summary
• An intriguing double ridge structure in highmultiplicity 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 in p–Pb Collisions Measured with ALICE – Jan Fiete Grosse-Oetringhaus
15