Ridge correlation structure in high
multiplicity pp collisions with CMS
Dragos Velicanu
for the CMS Collaboration
Quark Matter 2011, Annecy, May 23-29 2011
Results from High Multiplicity pp
Dragos Velicanu (MIT)
pp Ridge for Quark Matter 2011
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CMS Experiment
Hadron Calorimeter (HCAL)
EM Calorimeter (ECAL)
Beam Scintillator Counters (BSC)
Forward Calorimeter (HF)
Tracker
(Pixels and Strips)
Large coverage (|η|<5.0)
50 μm vertex resolution
Dragos Velicanu (MIT)
Muon system
pp Ridge for Quark Matter 2011
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Trigger on High Multiplicity pp
Level-1:
CMS trigger and DAQ
ET> 60 GeV
in calorimeters
High-Level trigger:
number of tracks with pT>0.4 GeV/c, ||<2
from a single vertex
Dragos Velicanu (MIT)
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Trigger on High Multiplicity pp
JHEP 1009:091, 2010
Total integrated luminosity: 980nb-1
Two HLT thresholds:
• Nonline > 70
• Nonline > 85
Nonline > 85 trigger
un-prescaled for
full 980nb-1 data set
~350K top multiplicity events (N>110) out of 50 billion collisions
Dragos Velicanu (MIT)
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Angular Correlation Technique
Signal distribution:
Event 1
Background distribution:
1 d 2 N mix
B( ,  ) 
Ntrig dd
1 d 2 N same
S ( ,  ) 
Ntrig dd
Event 2
same event pairs
mixed event pairs
trig
  
assoc

  
assoc

S ( ,  )
1 d 2 N pair
 B0,0 N
Ntrig dd
BN ( ,  )
trig
Divide signal by background
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pp Ridge for Quark Matter 2011
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Correlations in High Multiplicity pp
Intermediate pT: 1-3 GeV/c
High multiplicity pp (N>110)
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Correlations in High Multiplicity pp
Intermediate pT: 1-3 GeV/c
“Away-side” ( ~ )
back-to-back jet correlations
High multiplicity pp (N>110)
“Near-side” (,  ~ 0)
correlations from single jets
Dragos Velicanu (MIT)
pp Ridge for Quark Matter 2011
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Correlations in High Multiplicity pp
Intermediate pT: 1-3 GeV/c
Striking “ridge-like” structure extending over 
at  ~ 0
High multiplicity pp (N>110)
Dragos Velicanu (MIT)
pp Ridge for Quark Matter 2011
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Correlations in High Multiplicity pp
Intermediate pT: 1-3 GeV/c
Striking “ridge-like” structure extending over 
at  ~ 0
High multiplicity MC
(not observed before in hadron collisions or MC models)
High multiplicity pp (N>110)
Minbias pp
Dragos Velicanu (MIT)
pp Ridge for Quark Matter 2011
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Correlations in High Multiplicity pp
Intermediate pT: 1-3 GeV/c
Striking “ridge-like” structure extending over 
at  ~ 0
High multiplicity pp (N>110)
Dragos Velicanu (MIT)
pp Ridge for Quark Matter 2011
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Correlations in High Multiplicity pp
Intermediate pT: 1-3 GeV/c
Striking “ridge-like” structure extending over 
at  ~ 0
(Similarity to Heavy Ion)
arXiv:1105.2438
CMS PbPb 2.76 TeV
High multiplicity pp (N>110)
Dragos Velicanu (MIT)
pp Ridge for Quark Matter 2011
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Interpretations of the Ridge
CMS pp 7 TeV, N ≥ 110
?
CMS PbPb 2.76 TeV, 0-5%
Dragos Velicanu (MIT)
pp Ridge for Quark Matter 2011
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New Results
•
2x as much data
– |Δη| dependence
– pT dependence
– Multiplicity dependence
Dragos Velicanu (MIT)
pp Ridge for Quark Matter 2011
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New Results
•
2x as much data
– |Δη| dependence
– pT dependence
– Multiplicity dependence
Dragos Velicanu (MIT)
pp Ridge for Quark Matter 2011
Ridge goes away at high pT
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|| dependence of the ridge
Zero-Yield-At-Minimum (ZYAM)
Ridge is mostly flat in ||
Dragos Velicanu (MIT)
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 projections in various pT ranges
pTtrig
CMS Preliminary
2<||<4
pTassoc
CMS pp 7 TeV, N ≥ 110
Dragos Velicanu (MIT)
pp Ridge for Quark Matter 2011
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 projections in bins of pT
CMS pp 7 TeV, N ≥ 110
pT
Jet region (||<1)
Ridge region (2<||<4)
Dragos Velicanu (MIT)
pp Ridge for Quark Matter 2011
pT
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pT dependence of the ridge
Jet region (||<1)
Dragos Velicanu (MIT)
pp Ridge for Quark Matter 2011
Ridge region (2<||<4)
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Near-side yield vs pT
N
Jet region (||<1)
N
Ridge region (2<||<4)
Ridge first increases with pT, and then drops at high pT
Dragos Velicanu (MIT)
pp Ridge for Quark Matter 2011
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Near-side yield vs Multiplicity
Jet region (||<1)
Ridge region (2<||<4)
Ridge in pp turns on around N ~ 50-60 (4x MinBias) smoothly
(<N> ~ 15 in MinBias pp events)
Dragos Velicanu (MIT)
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Summary
Wei Li (Plenary session Thursday)
• Surprising new effect in pp
CMS pp 7 TeV, N ≥ 110
– Never before seen in pp
or pp MC
– Similar to HI
• New results provide more
detailed properties of ridge
– pt, |Δη|, multiplicity dependence
Jeremy Callner (Parallel Tuesday)
CMS PbPb 2.76 TeV, 0-5%
• New testing ground for high
density QCD physics
Dragos Velicanu (MIT)
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Backups
Dragos Velicanu (MIT)
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Understanding the Correlation Structure
pT inclusive
What was used in
PHOBOS, ISR, UA5
CMS 7TeV pp minimum bias
S (, ) 
R(, )  (N 1) N
1
BN (, )  N
Dragos Velicanu (MIT)
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Understanding the Correlation Structure
pT inclusive
“Away-side” (~ ) jet correlations:
Correlation of particles between backto-back jets
CMS 7TeV pp minimum bias
Bose-Einstein correlations:
,) ~ (0,0)
Momentum conservation:
~ -cos()
“Near-side” (~ 0) jet peak:
Correlation of particles
within a single jet
Dragos Velicanu (MIT)
Short-range correlations ( < 2):
Resonances, string fragmentation,
“clusters”
pp Ridge for Quark Matter 2011
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Comparing to various MC
PYTHIA8, v8.135
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More MC models
PYTHIA D6T MinBias, N>70
HERWIG++, N>110
Dragos Velicanu (MIT)
PYTHIA D6T, Dijet 80-120GeV
Madgraph, Dijet 100-250GeV, N>90
pp Ridge for Quark Matter 2011
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Cross Check: Event Pileup
Compare different run periods
Change in pileup fraction by factor 4-5
has almost no effect on ridge signal
Dragos Velicanu (MIT)
pp Ridge for Quark Matter 2011
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Cross Check: Event Pileup
Correlate tracks from high multiplicity vertex with tracks
from different collision (vertex) in same bunch crossing
N>110
1.0GeV/c<pT<3.0GeV/c
2.0<||<4.8
No background or noise effects
seen in cross-collision correlations
Dragos Velicanu (MIT)
pp Ridge for Quark Matter 2011
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Cross Check: Analysis Code
Control analysis II
Standard analysis
Control analysis I
N>110
1<pT<3GeV/c
||>2
Independent code
Same definition of R
Same input file (skim)
Independent code
Different definition of R
Different input file (skim)
Ridge is seen with three independent analysis codes
Dragos Velicanu (MIT)
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Cross Check: Trigger
Min-bias trigger vs high mult trigger
HLT 70 vs HLT 85 for N > 110
Min bias
trigger
1<pT<2GeV/c
||>2
N>110
1<pT<2 GeV/c
||>2
Ridge is seen using
min bias trigger + offline selection
Dragos Velicanu (MIT)
pp Ridge for Quark Matter 2011
No trigger bias seen from
comparison of trigger paths
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Cross Check: ECAL photons
Use ECAL “photon” signal
Mostly single photons from 0’s
No efficiency, and pT, smearing corrections
N>110
1<pT<3GeV/c
CMS preliminary
N>110
1<pT<3GeV/c
||>2
Track-photon correlations
Note: photons reconstructed using “particle flow”
event reconstruction technique
Dragos Velicanu (MIT)
pp Ridge for Quark Matter 2011
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Cross Check: ECAL photons
Use ECAL “photon” signal
Mostly single photons from 0’s
No efficiency, and pT, smearing corrections
N>110
1<pT<3GeV/c
CMS preliminary
N>110
1<pT<3GeV/c
||>2
Photon-photon correlations
Qualitative confirmation
Independent detector, independent reconstruction
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Particle density in high Mult pp
• Similar particle densities in these pp collisions
as were seen in CuCu at RHIC
CMS pp 7TeV, N>100
Dragos Velicanu (MIT)
PHOBOS CuCu 62.4GeV
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QM2011_ppRidge_DragosVelicanu_v9