Post on 15-Jan-2016
description
1MBUE working group 2012Yen-Jie Lee (CERN)
Yen-Jie Lee (CERN)
for the CMS Collaboration
MBUE working group
CERN
3rd Dec, 2012
Two-particle correlations in pPb collisions from the CMS Collaboration
2MBUE working group 2012Yen-Jie Lee (CERN)
p p
IntroductionTwo-particle correlation: study of particle production mechanism
3MBUE working group 2012Yen-Jie Lee (CERN)
Two-particle correlation: study of particle production mechanism and possible collective effects in high particle density environment created at the LHC energies
p pPb Pb
Introduction
4MBUE working group 2012Yen-Jie Lee (CERN)
p pPb Pb
?
p Pb
IntroductionTwo-particle correlation: study of particle production mechanism and possible collective effects in high particle density environment created at the LHC energies
5MBUE working group 2012Yen-Jie Lee (CERN)
Very large coverage for full tracking
(|| up to 5.0)!
Compact Muon Solenoid
Inner tracker:charged particles
primary vertexsolenoid
EM and Hadron calorimetersphotons, jet
MuonMuon
HCALHCAL
ECALECAL
TrackerTracker |η|< 2.5
|η|< 3.0
|η|< 5.2
|η|< 2.4
proton
Pb
6MBUE working group 2012Yen-Jie Lee (CERN)
High multiplicity events recorded by CMS
pp PbPb
pPb
7MBUE working group 2012Yen-Jie Lee (CERN)
Defining two-particle correlation
Event 1
Event 2
same event pairs
mixed event pairs
Signal pair distribution: Background pair distribution:
Δη = η1-η2
Δφ = φ1-φ2
8MBUE working group 2012Yen-Jie Lee (CERN)
Defining two-particle correlation
Event 1
Event 2
same event pairs
mixed event pairs
Signal pair distribution: Background pair distribution:
JHEP 09 (2010) 091
Associated hadron yield per trigger:
Δη = η1-η2
Δφ = φ1-φ2
9MBUE working group 2012Yen-Jie Lee (CERN)
Understanding the correlation function
“Near-side” (, ~ 0) correlations from single jets
JHEP 09 (2010) 091
p p
10MBUE working group 2012Yen-Jie Lee (CERN)
Understanding the correlation function
“Near-side” (, ~ 0) correlations from single jets
“Away-side” ( ~ ) back-to-back jet correlations
JHEP 09 (2010) 091
p p
Z axis adjusted to reveal the detail of the correlation function (peak truncated)
11MBUE working group 2012Yen-Jie Lee (CERN)
Understanding the correlation function
Striking “ridge-like” structure extending over at ≈0
In high multiplicity events, N≥110 where:
N ≡ number of offline tracks with pT>0.4 GeV/c
JHEP 09 (2010) 091
p p
12MBUE working group 2012Yen-Jie Lee (CERN)
Understanding the correlation function
pTtrig: 4–6 GeV/c
pTassoc: 2–4 GeV/c
CMS PbPb 2.76 TeV 35-40%
PbPbPbPb 35-40% centrality
arXiv:1201.3158
CMS PbPb 2.76 TeV
EPJC 72 (2012) 2012
Pb Pb
13MBUE working group 2012Yen-Jie Lee (CERN)
pTtrig: 4–6 GeV/c
pTassoc: 2–4 GeV/c
CMS PbPb 2.76 TeV 35-40%
PbPbPbPb 35-40% centrality
arXiv:1201.3158
CMS PbPb 2.76 TeV
EPJC 72 (2012) 2012
Understanding the correlation functionPb Pb
Particle azimuthal distributions:
dN/dΣ vn
cos(n())
14MBUE working group 2012Yen-Jie Lee (CERN)
Any guesses for pPb correlations?
What do we expect to see in(high-multiplicity) pPb?
p Pb
N=235 event
15MBUE working group 2012Yen-Jie Lee (CERN)
A ridge!p Pb
N ≡ number of offline tracks with pT>0.4 GeV/c
What do we expect to see in(high-multiplicity) pPb?
arXiv 1210.5482Accepted by PLB
16MBUE working group 2012Yen-Jie Lee (CERN)
A (relatively big) ridge!p Pb
JHEP 09 (2010) 091
pp 7 TeV
Much bigger than in pp!
Physical origin still unclear
arXiv 1210.5482Accepted by PLB
N ≡ number of offline tracks with pT>0.4 GeV/c
17MBUE working group 2012Yen-Jie Lee (CERN)
A (relatively big) ridge!p Pb
Initial-state geometry +
collective expansion
arXiv 1210.5482Accepted by PLB
N ≡ number of offline tracks with pT>0.4 GeV/c
18MBUE working group 2012Yen-Jie Lee (CERN)
Multiplicity Evolution
Divide into 4 multiplicity bins:
p Pb
Low multiplicity Low transverse density
arXiv 1210.5482Accepted by PLB
N ≡ number of offline tracks with pT>0.4 GeV/c
19MBUE working group 2012Yen-Jie Lee (CERN)
Multiplicity Evolution
Divide into 4 multiplicity bins:
p Pb
Increasing multiplicity Increasing transverse density
arXiv 1210.5482Accepted by PLB
N ≡ number of offline tracks with pT>0.4 GeV/c
20MBUE working group 2012Yen-Jie Lee (CERN)
Multiplicity Evolution
Divide into 4 multiplicity bins:
p Pb
Increasing multiplicity High transverse density
arXiv 1210.5482Accepted by PLB
N ≡ number of offline tracks with pT>0.4 GeV/c
21MBUE working group 2012Yen-Jie Lee (CERN)
Multiplicity Evolution
Divide into 4 multiplicity bins:
p Pb
Increasing multiplicity Highest transverse density
arXiv 1210.5482Accepted by PLB
N ≡ number of offline tracks with pT>0.4 GeV/c
22MBUE working group 2012Yen-Jie Lee (CERN)
Quantitative evolution of ridge effect
Want to use the same approach as in pp ridge paper for “apples-apples” comparison
23MBUE working group 2012Yen-Jie Lee (CERN)
Quantitative evolution of ridge effect
Average over ridge region(2<|Δη|<4)
Want to use the same approach as in pp ridge paper for “apples-apples” comparison
24MBUE working group 2012Yen-Jie Lee (CERN)
Quantitative evolution of ridge effect
Average over ridge region(2<|Δη|<4)
Want to use the same approach as in pp ridge paper for “apples-apples” comparison
25MBUE working group 2012Yen-Jie Lee (CERN)
Multiplicity and pT dependence
Multiplicity
N<35
35≦N<90
90 ≦ N<110
N≧110
0 – 1 1 – 2 2 – 3 3 – 4 pT (GeV/c)
N ≡ number of offline tracks with pT>0.4 GeV/c
26MBUE working group 2012Yen-Jie Lee (CERN)
Multiplicity and pT dependence
0 – 1 1 – 2 2 – 3 3 – 4 pT (GeV/c)
N ≡ number of offline tracks with pT>0.4 GeV/c
Multiplicity
N<35
35≦N<90
90 ≦ N<110
N≧110
27MBUE working group 2012Yen-Jie Lee (CERN)
Multiplicity and pT dependence
0 – 1 1 – 2 2 – 3 3 – 4 pT (GeV/c)
N ≡ number of offline tracks with pT>0.4 GeV/c
Multiplicity
N<35
35≦N<90
90 ≦ N<110
N≧110
28MBUE working group 2012Yen-Jie Lee (CERN)
No ridge in pPb MC
Compare to AMPT and HIJING pPb
AMPT pPb, N>=100
HIJING pPb, N>=120 Generator-level
No ridge in these pPb MCs!
29MBUE working group 2012Yen-Jie Lee (CERN)
Ridge Associated Yield
ZYAM example
N ≡ number of offline tracks with pT>0.4 GeV/c
arXiv 1210.5482Accepted by PLB
30MBUE working group 2012Yen-Jie Lee (CERN)
Ridge Associated Yield
In the signal (N>110) region, the strength of the effect rises and falls with pT
ZYAM example
N ≡ number of offline tracks with pT>0.4 GeV/c
arXiv 1210.5482Accepted by PLB
31MBUE working group 2012Yen-Jie Lee (CERN)
Ridge Associated Yield
In the pT range where the yield is the strongest, the ridge turns on at N≈50
In the signal (N>110) region, the strength of the effect rises and falls with pT
ZYAM example
N ≡ number of offline tracks with pT>0.4 GeV/c
arXiv 1210.5482Accepted by PLB
32MBUE working group 2012Yen-Jie Lee (CERN)
Correlations from planar particle production
Correlations from back to back jets
Correlations from planar particle production
Summary and Conclusions
• A significant ridge is observed in high multiplicity (central) pPb collisions at 5 TeV
– strong mechanism to produce particles in a plane
– much larger than in pp
33MBUE working group 2012Yen-Jie Lee (CERN)
Summary and Conclusions
• A significant ridge is observed in high multiplicity (central) pPb collisions at 5 TeV
– strong mechanism to produce particles in a plane
– much larger than in pp
• Effect turns on slightly above average minimum bias multiplicity
• Effect rises and falls with pT
– similar trend as observed in both PbPb and pp ridge
before
34MBUE working group 2012Yen-Jie Lee (CERN)
Outlook
• All this came from a few hours of LHC pPb test running, only one fill! Thank you LHC!
• Several questions to be asked:
– What is the physics origin of the ridge?
• Collective effect?
• Modification of jet structure?
– Have we created a medium in pPb collision?
• Elliptic flow measurement (vn) ?
• Jet quenching ?
• Hope for more surprises from the full pPb run coming up in January!
35MBUE working group 2012Yen-Jie Lee (CERN)
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