Two particle correlation
measurements in ALICE at
LHC
Yaxian Mao
(for the ALICE collaboration)
IOPP/CCNU, Wuhan, China
Jet Symposium, 21/07/2011, Hangzhou
1
Outline
• The context of the study
- HIC@LHC
- Probing the medium with jets
- 2-particle correlations
• Experiment and analysis
• Comprehensive study of 2-particle correlations
in pp collisions
• Modification of the jet particle yields
Jet Symposium, 21/07/2011, Hangzhou
2
Objective
LHC
•
RHIC
•
Explore the physics of
strongly interacting
QCD matter under
extreme conditions of
energy density (Quark
Gluon Plasma)
Establish its equation
of state, degrees of
freedom and transport
properties
Jet Symposium, 21/07/2011, Hangzhou
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Heavy Ion Collisions at LHC
(√sNN=2.76TeV)
• Energy density > 15 GeV/fm3
• Volume ~ 300 fm3
• Life time ~ 10 fm/c
Phys. Rev. Lett. 105, 252301
(2010)
RHIC
+ 40%
RHIC
x2
Phys. Lett. B 696 (2011) 328
Volume
at decoupling
Phys. Lett. B 696 (2011) 328
Lifetime: from collision to
‘freeze-out’ (hadron
decoupling)
MultiplicityJet Symposium, 21/07/2011, HangzhouMultiplicity1/3
4
QGP at LHC
• Still a perfect liquid !
arXiv:1102.3010
•The medium is opaque
PLB 696 (2011) 30-39
f
Z
Y
X
Jet Symposium, 21/07/2011, Hangzhou
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Hard Scattering
phadron
z=
p jet
Jet fragmentation function (FF): hadron distribution as a
function of z, defined as the momentum fraction taken by
hadron from the jet
Jet Symposium, 21/07/2011, Hangzhou
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Jets
•
hard scattered parton looses energy while traversing the
medium
• di-jet imbalance (Δϕ and Ejet )
→Challenge in heavy ion
collisions due to large
Jet Symposium, 21/07/2011, Hangzhou
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Associated
Di-hadron correlations
Δ
•
Trigge
r
Intra-jet properties (near side jT ∝
σnear)
jT
Near side
Away side
σnear
xE = -
σaway
pTt × pTa
pTt
2
• Inter-jet properties (away side
xE, kT ∝ σaway)
»z
Jet Symposium, 21/07/2011, Hangzhou
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xE kinematics
pTt
near side zt = near
p jet
p
away side za = Ta
p away
jet
• Df = ftrigger - fassociate ~ p
Þ
• di-jet balance (no kT)
Þ
o
xE = -
xE »
pTt × pTa
2
p Tt
za × p
pTa
=cos(Df )
pTt
away
jet
near
jet
zt × p
za
xE »
zt
Charged/neutral trigger: z <1
t
Þ
Jet Symposium, 21/07/2011, Hangzhou
xE ¹ za
9
Isolation
IP
η
Φ
R
trigger
Enrich the trigger sample with <zt>
→1
•
•
estimate hadronic (charged only)
activity around the trigger
o R (= 0.4) of the cone
o ε(= 10%) fraction of trigger pT
Isolatio
isolated> ~
<
z
n
t
trigger (pTt) is isolated if < zt > ~ 0.5
0.8
h±
∑cone pT < ε* pTt
Jet Symposium, 21/07/2011, Hangzhou
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xE kinematics
pTt
near side zt = near
p jet
xE = -
p
away side za = Ta
p away
jet
• Df = ftrigger - fassociate ~ p
Þ
• di-jet balance (no kT)
Þ
o
o
xE »
2
p Tt
za × p
pTa
=cos(Df )
pTt
away
jet
near
jet
zt × p
za
xE »
zt
Charged/neutral trigger: z <1
t
Isolated trigger:
pTt × pTa
Þ
zt ®1 Þ
Jet Symposium, 21/07/2011, Hangzhou
xE ¹ za
xE ® za
11
Photon trigger
Photons come from every phase of the expanding
system, carry undistorted information about the medium
q
conditions at their production points. q
γ
γ
•LO pQCD direct photons (Eγ = Ejetg,
isolated)
• NLO pQCD fragment photons (Eγ <
Ejet, non-isolated)
•Medium induced thermal photons
(temperature)
• Medium induced bremsstrahlung and
conversion (chemical composition)
• Decay photons from mesons (jet
Jet Symposium, 21/07/2011, Hangzhou
q
g
q
γ
g
q
g
q
π0
q
q
γ
γ
q
12
Direct photon
• Do not interact with the
medium
•Provide reference for
hard process
RAA
Megan Connors for PHENIX
ln(1/xE)
Jet Symposium, 21/07/2011, Hangzhou
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xE kinematics
pTt
near side zt = near
p jet
p
away side za = Ta
p away
jet
• Df = ftrigger - fassociate ~ p
Þ
• di-jet balance (no kT)
Þ
o
xE = -
xE »
Charged/neutral trigger: z <1
t
Isolated trigger:
2
p Tt
za × p
pTa
=cos(Df )
pTt
away
jet
near
jet
zt × p
za
xE »
zt
zt ®1
o Direct photon trigger : zt =1
o
pTt × pTa
Þ xE ¹ za
Þ xE ® za
Þ
Jet Symposium, 21/07/2011, Hangzhou
xE » za
14
EMCAL
γ, π0, jets
T0/V0
Trigger
A Large Ion Collider
Experiment
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
Jet Symposium, 21/07/2011, Hangzhou
MUON
μ-pairs
Not shown: ZDC (at
±114m)
The ALICE detectors
(2010)
•
4/10
•
Photons are detected
in the EM calorimeters
-
PHOS ( ⎜η⎜< 0.12,
ΔΦ = 60°)
-
EMCAL ( ⎜η⎜< 0.7,
ΔΦ = 40°)
Charged particles are
detected by the
central tacking system
Jet Symposium, 21/07/2011, Hangzhou
ITS+TPC ( ⎜η⎜<
0.9, ΔΦ = 2π)
16
The ALICE detectors
(2011)
•
10/10
•
Photons are detected
in the EM calorimeters
-
PHOS ( ⎜η⎜< 0.12,
ΔΦ = 60°)
-
EMCAL ( ⎜η⎜< 0.7,
ΔΦ = 100°)
Charged particles are
detected by the
central tacking system
Jet Symposium, 21/07/2011, Hangzhou
ITS+TPC ( ⎜η⎜<
0.9, ΔΦ = 2π)
17
ALICE detectors: DCAL
(>2011)
•Same towers than the
EMCAL, shorter Super
Modules in h.
•Including PHOS, acceptance
is Δη=1.4, ΔΦ=60º
•To be installed for 2013 runs
•Focus on γ/π0/jet-jet
correlation
Jet Symposium, 21/07/2011, Hangzhou
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Strategy of measurements
• Reconstruct and identify trigger
(request leading) ➜ pTt
• Reconstruct associate charged
tracks ➜ pTa
• Azimuthal correlation between
trigger and charged hadrons
➜ ΔΦ = Φtrigger−Φassociate
• Calculate kT ➜ kT∝ width, ΔΦ = π
• Construct the fragmentation function
➜ xE= -pTa · pTt / |pTt|2, ΔΦ = π
• Estimate and subtract background
Jet Symposium, 21/07/2011, Hangzhou
Near side
Away side
19
pp events at s = 7 TeV
4
4 < p < 5 GeV/c
5 < p < 6 GeV/c
Tt
Tt
leading cluster trigger
3
1/Nt dN/dDf (rad-1)
ΔΦ correlation
EMCAL
PHOS
2
ALICE Performance
22/05/2011
1
0
4
6 < p < 7 GeV/c
Neutral trigger
associate charged
hadrons:
pTa > 1 GeV/c
7 < p < 8 GeV/c
Tt
Tt
3
2
1
0
0
2
4
0
2
4
Df (rad)
• Azimuthal correlation
independent of the calorimeters
• leading cluster leads similar
correlation as leading π0
Jet Symposium, 21/07/2011, Hangzhou
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pp events at s = 7 TeV
4
4 < p < 5 GeV/c
ΔΦ correlation
5 < p < 6 GeV/c
Tt
Tt
leading trigger h±
3
Isolated charged
1/Nt dN/dDf (rad-1)
w/o isolation
2
ALICE Performance
22/05/2011
w/ isolation
R = 0.4, e = 10%
1
0
4
6 < p < 7 GeV/c
Charged hadron
trigger:
pTt Î [4, 5],[5,6], [6,7],[7,8] GeV/c
7 < p < 8 GeV/c
Tt
Tt
Associate charged
hadrons:
pTa >1 GeV/c
3
2
1
0
0
2
4
0
2
4
Isolation
R=0.4,
e =10%
Df (rad)
Jet Symposium, 21/07/2011, Hangzhou
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kT from away side width
PRD74
(2006)072002
pTpair = √2 kT
a
a
^T
-p
pTt
pT
p^T
p^Tt
jTy
√2kTy
^pT
Pout
partonic
hadronic
For isolated trigger: jTy ~ 0 zt ~ 1
Jet Symposium, 21/07/2011, Hangzhou
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pout from away width
pTpair = √2 kT
a
pT
a
Pout
p^ Tt = pTt
pp@7TeV
p^T
Fitted with
^
pT
^pT
pTtrig > 5 GeV/c
pTa > 1 GeV/c
IC: R=0.4, ε = 0.1, pTthres = 1 GeV
• Result independent of the
trigger: properties of the partonic
level
Jet Symposium, 21/07/2011, Hangzhou
Performanc
e
2/12/2010
23
kT : the result
• kT exaction from isolated trigger-hadrons gives the
same results from di-hadron measurement
Data pp @ 7 TeV
charged di-hadron correlatio
Jan Rak: HP2010
pTtrig > 5
GeV/c
pTa > 1
Jet Symposium, 21/07/2011, Hangzhou
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I had anticipated the result
at QM 2009 !
World data & MC
kT (GeV)
QM2009 poster
PRD 74 (2006) 072002
Preliminary
√s (GeV)
extrapolation to √s=7 TeV: <kT>=4.5±0.5 GeV/c,
<pT>pair≈6.3±0.7 GeV/c
Jet Symposium, 21/07/2011, Hangzhou
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Near side
xE distribution (S+B)
Away side
xE = -
pTt × pTa
p Tt
2
pTa
=cos(Df )
pTt
Background estimate for away side xE:
• Assumption: isotropic in Δϕ
• Estimate from
Df Î [ 0.325, 0.475] p
• Normalize to the away side
hemisphere
Df Î [ 0.5,1.5] p
Jet Symposium, 21/07/2011, Hangzhou
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xE distribution (bkg. subtracted)
dN
dxE
pTt
»< m > (n -1)
1
1
x̂h (1+ xx̂Eh )n
(n is slope of inclusive
dist. )
paway
jet
h
p near
jet
x̂ =
h
dN
= Ne-bxE
dxE
PHENIX, PRD 74 (2006)
072002
• Low xE (<0.4) → power law (away/near side jet momentum
za
imbalance)
xE »
zt
• High xE (>0.4) → exponential (
)
→ Extract xE exponential inverse slope in the range [0.4,
0.8]
Jet Symposium, 21/07/2011, Hangzhou
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dN
= Ne-bxE
dxE
pT dependence xE slope
zt =1.0
zt » 0.5
J. D. Bjorken, PRD8 (1973)
4098
DELPHI, EPJC, 13 (2000)
573
OPAL, ZPC, 69 (1996) 543
• data exhibits uniform xE slope over a wide pTt range
• away side xE distribution different from real jet FFzt ( » 0.5
)
Jet Symposium, 21/07/2011, Hangzhou
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xE distribution: w/ (w/o) isolation
pp events at s = 7 TeV
1/Ntrig dN/dxE
10
leading h±
8 < p < 10 GeV/c
Tt
w/o isolation
w/ isolation
R = 0.4, e = 10%
1
ALICE Performance
22/05/2011
10-1
10-2
0
0.2
0.4
0.6
0.8
1
1.2
1.4
xE
• Isolated trigger has steeper slope than non-isolated trigger
Jet Symposium, 21/07/2011, Hangzhou
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jet-h ± gluon
10
zt =1.0
8
jet-h ± quark
E
inv xE slope in x Î (0.4,0.8)
xE slope for isolated trigger
zt » 0.5
6
4
pp events at s = 7 TeV
w/o isolation
w/ isolation
2
0
10
ALICE Preliminary
20
30
pTt (GeV/c)
• Exponential fit in the rangex = (0.4, 0.8)
E
→ Isolated trigger provides a better description for real jet
fragmentation (approach to <zt> →1)
Jet Symposium, 21/07/2011, Hangzhou
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HI collision: PbPb at
√sNN=2.76TeV
• Starting end of November 2010 we have accumulated
about 10M minimum bias PbPb collisions at √sNN = 2.76
TeV
• The analysis is focusing on two observables: jet energy
loss and kT broadening
Jet Symposium, 21/07/2011, Hangzhou
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Azimuthal correlation:
suppression
• classic ‘jet quenching signal’
-
away side correlation in central Pb+Pb suppressed
Correlation between ‘leading’ and soft jet particles
q
q
A. Adare,
QM2011
Jet Symposium, 21/07/2011, Hangzhou
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IAA: dihadron quenching
AdS/CFT,ASW,
Yajem(-D):
•LO pQCD
•WS matter dist.
•Ideal 2+1d hydro
•Different e-loss
scenarios
Jan-Fiete,
QM2011
X N Wang:
•Hard sphere
matter dist.
•NLO pQCD
•Avg. e-loss
•1D expansion
AdS/CFT,ASW,YaJEM(-D) simulations from T Renk: QM2
X N Wang [following calculation in PRL98:212301 (2007)
Near-side enhancement:
Away-side suppression:
- reproduced by AdS/CFT pQCD
- reproduced by AdS/CFT, ASW,
hybrid (L3 path length
YaJEM-D
dependence) and ASW (L2
- YaJEM too high (L dependence)
dependence)
X N Wang
slightly too low
Jet Symposium,- 21/07/2011,
Hangzhou
- YaJEM(-D) too high
33
Photon-hadron correlation:
Modified FF
EMCAL Physics Performance Report
qPYTHIA
qhat = 50 GeV2/fm
MC
pp
PbPb (qhat = 50)
XE = -pTa · pTt / |pTt
•
•
2
|
p-p and Pb-Pb correlation with isolated photons
Distributions show the expected shape in jet
fragmentation function modification!
Real data analysis is
Jet Symposium, 21/07/2011, Hangzhou
34
-
Azimuthal correlation:
A broadening of the
broadening
away side peak as
result of multiple
scattering of the probe
in the QCD medium
arXiv: 1008.0413v1 (EMCAL Physics Performance Report)
pp
PbPb
MC
qPYTHIA
qhat = 50 GeV2/fm
- The broadening can
be related to the
transport parameter of
the medium
(suggested by X. N.
Wang)
Real data analysis is
Jet Symposium, 21/07/2011, Hangzhou
35
Conclusion
&
Outlook
Two particle correlations are a suitable tool to study the jet
properties:
•
kT extracted at LHC (kT ~ 5 GeV/c), consistent with the √s
trends suggested from other measurements
•
Isolation introduces a trigger bias towards〈zt〉~ 0.8 providing
- a description closer to the fragmentation function
- the potential to distinguish quark and gluon jet fragmentation
• Isolated direct photons (dominated by quark jets from “Compton
scattering”) will allow to achieve 〈zt〉= 1
•Study how HI collisions will change this picture: k
T
broadening
and the modification of xE distribution
Jet Symposium, 21/07/2011, Hangzhou
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Thanks very much!
Jet Symposium, 21/07/2011, Hangzhou
Dihadron ⟨zt⟩≠1; xE ≠ FF
PHENIX, Phys. Rev D 74, 072002 (2006)
CCOR, Physica Scripta 19, 116 (1979)
xE ~ za/<zt> <zt>=0.85 measured
q (z)~e-6z
-5.3x
⇒D
a
e E
π
Measured dN/dxE compared
with dN/dxE calculated using
FF of quark Dq ∝ (−8.2 z )
(solid line) and gluon Dg ∝
(−11.4 z ) (dashed line) from
LEP.
Jet Symposium, 21/07/2011, Hangzhou
38
pp events at sNN = 7 TeV
4
ΔΦ correlation
5 < p < 6 GeV/c
4 < p < 5 GeV/c
Tt
Tt
leading trigger h±
3
Isolated cluster
1/Nt dN/dDf (rad-1)
w/o isolation
2
ALICE Performance
21/05/2011
w/ isolation
neutral cluster trigger:
pTt Î [4, 5],[5,6], [6,7],[7,8] GeV/c
R = 0.4, e < 10%
1
0
4
7 < p < 8 GeV/c
6 < p < 7 GeV/c
Tt
Tt
3
2
Associate charged
hadrons:
pTa >1GeV/c
1
0
0
2
4
0
2
4
Isolation
R=0.4, e =10%
Df (rad)
Jet Symposium, 21/07/2011, Hangzhou
39
Or Bjorken “parent-child
relation” and the “trigger bias”
Final state parton invar.
Dist.
f ( p̂ )
q
T
-n
æ pT ö dz
1 dN
A 1 q
q
n-2
» ò ADh (z) ç ÷ 2 »
D
(z)z
dz
h
n ò xT
xT
è z ø z
pT dpT
pT
1 dN
-n
fq ( p̂T ) µ p̂T »
where n ~ 6 @ 7TeV
pT dpT
1
Mean z
1
z =
N
Parent-child rel.
Jet cross section has
The “same” shape as
Particle cross section
-n
æ pT ö dz
ò xT zD (z)çè z ÷ø z 2
1
Trigger bias
Small z fragments
strongly suppressed
q
h
0.2 TeV ® z = 0.7 RHIC p 0
0.9 TeV ® z = 0.6 ± 0.05
7.0 TeV ® z = 0.55 ± 0.05
0.05 coming from Dhq (z)
Jet Symposium, 21/07/2011, Hangzhou
J. D. Bjorken, PRD8 (1973)
4098
ICP: dihadron
Jan-Fiete,
QM2011
IAA(0-5%) consistent with ICP with respect
to near-side enhancement and away-side
suppression
Jet Symposium, 21/07/2011, Hangzhou
Photon identification
• Photons are detected as clusters of cells in
the calorimeters
- clusters originated mainly from π0 decay
photons
- At low energy a cluster originates from a
single decay photon
- At higher energy (~25 GeV in PHOS and
~8 GeV in EMCAL) the 2 decay photons
merge into a single cluster
Jet Symposium, 21/07/2011, Hangzhou
42
Neutral cluster triggers
Photons are detected as clusters of cells in the calorimeters
No photon PID applied yet
Cluster dominated by π0 contribution
Frac. of the contribution
•
•
•
merged p 0 (Ecluster ~ Ep0)
incomplete merged p 0 (Ecluster < Ep0)
1
charged hadron
single photon (E
0.8
cluster
PYTHIA
~ 1/2*Ep0 / h)
0.6
0.4
0.2
0
0
2
4
6
8
10
12
14
PTcluster
p
16
T cluster
18 20
(GeV/c)
Jet Symposium, 21/07/2011, Hangzhou
PTtrue
43
Neutral cluster triggers
Photons are detected as clusters of cells in the calorimeters
No photon PID applied yet
Cluster dominated by π0 contribution
Frac. of the contribution
•
•
•
merged p 0 (Ecluster ~ Ep0)
incomplete merged p 0 (Ecluster < Ep0)
1
charged hadron
single photon (E
0.8
cluster
PYTHIA
~ 1/2*Ep0 / h)
0.6
0.4
0.2
0
0
2
4
6
8
10
12
14
PTcluster
p
16
T cluster
18 20
(GeV/c)
Jet Symposium, 21/07/2011, Hangzhou
PTtrue
44