1 surface versus volume emissions in photon-hadron correlations han-zhong zhang institute of...
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Surface versus volume emissions in photon-hadron correlations
Han-Zhong Zhang
Institute of Particle Physics, Huazhong Normal University, China
Collaborators: J. Owens, E. Wang and X.-N. Wang
ATHIC 2008, Tsukuba Oct. 14, 2008
I. IntroductionII. Isolated photonIII. Gamma-tagged jet emissions in A+A collisionsIV. Conclusions
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I. Introduction
Jet quenching:
The hard jet loses a significant amount of its energy
via gluon radiation induced by multiple scattering.
hadrons
q
q
hadrons
leadingparticle
leading particle
N-N collision
hadrons
q
q
hadrons
Leading particle suppressed
leading particle suppressed
A-A collision
X.-N.Wang and M.Gyulassy, Phys.Rev.Lett.68,1480(1992)
What happens for a jet propagating inside QGP?
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Three kinds of hard probes of QGP
1) Single jet Single hadron spectra
2) Dijet Hadron-triggered away-side hadron spectra
3) Gamma-jet Photon-triggered away-side hadron spectra
Single jet Dijet Gamma-jet
?H.Z. Zhang, J.F. Owens, E. Wang and X.-N. Wang , PRL 98(2007)212301
Motivation
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Gamma-jet by NLO pQCD parton model
LO : 22 tree level: NLO corrections: 22 one loop
+23 tree level
FFsdPDFsTd ABAA
GamT
JetT pp
GamTp
JetTp
GamT
JetT
GamT
JetT
GamT
JetT
pp
pp
pp
,
,GamTp
1JetTp
2JetTp
),( 21 JetT
JetT
JetT ppofOnep
therefore leading to hadrons with transverse
momentum larger than that of the photons
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FFs modification due to jet quenching
The jet energy loss in a 1D expanding system:
),,(0
000
0
1
nrbd
dL
dEE g
L
dc
)/5.7/()6.1/( 02.1
001
EEdL
dE
d
Energy loss parameter
(a parameterization form of theory calculations) Enke Wang , X. -N. Wang , PRL87(2001)142301)
(X. -N. Wang , PRC70(2004)031901)
r
),()],(/),()[1(),,( 20/
/2'0/
'2'0
/
'/2
/ cch
Lggh
c
gcch
c
cLccch zDezD
z
zLzD
z
zeEzD
,//),/( ''cTgcTcTc EpLzEppz
With energy loss without loss
0000
/1),,,(0
0
nrb
dLg
L
The modified fragmentation functions:
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II. Isolated photon
Direct photon sources in p+p:
1) Prompt photon from hard scattering
Annihilation Compton 23 tree level 22 one-loop Annihilation Compton
LO NLO
22 tree level
2) Fragmentation or e. g.
bremsstrahlung contributions
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7J. F. Owens, Rev. Mod. Phys. 59, 465(1987);
H. Baer, J. Ohnemus, and J. F. Owens, Phys. Rev. D. 42, 61(1990)
Most accompanying hadrons arewithin a cone of half-angle coneR
Isolation Cut (IC)
Generally, the bremsstrahlung photons are always accompanied by nearly collinear hadrons on the same side.
22 )()( coneR
Jet
GammaAn “isolation” cut (IC) are often applied on the electromagnetic signal to separate the prompt photons from other sources.
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NLO results for prompt photons in p+p
Because of IC selected at
RHIC, most fragmentation
contributions from parton
jets are taken out.
Data with IC:
1.0/.,5.0 TT
cone pEradR
PHENIX, PRL 98 (2007) 012002
“the measured photon samples … are expected to be isolated from parton jet activity.”
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If we only consider the events where the photon has no nearly collinear hadrons accompanying on the same side, high p_T photon/photon-hadron will be dominated by annihilation and Compton processes.
Focus on isolated photons now
In order to give a simple and clear jet quenching picture in A+A, we will only consider such gamma-jet events in which the photons are isolated or only contributed by annihilation and Compton processes.
Theory prompt photons Ex. IC photons
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Turbide, Gale, Jeon, Moore, Phys. Rev. C. 72 (2005) 014906
High p_T prompt photon dominates in central Au+Au
Other sources in AA:conversion photons;induced photons;thermal photons
With isolation cut applied, the “clean” photons are separated from the “unclean” EM signals in A+A collisions.
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III. Gamma-tagged jet emissions
D_AA gives such a fragmentation function of the final parton jets, which is just weighted with the invariant cross sections of the correlated photons.
Sometimes we call D_AA as the photon-triggered hadron fragmentation functions
abABbaAB
abABbahAB
ThTT
TABT
hhTT
hABThT
T
hAB
ABTAB
dPDFsTdxrdxbddd
FFsdPDFsTdxrdxbddd
ppzwhere
dydpddydp
ddydydpdpdpddydydp
dz
dN
NzD
ˆ
,ˆ
,/
/
/1)(
22
22
The per-trigger photon-hadron spectra
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12Data from “Matthew Nguyen for PHENIX, talk at QM2008”
dydp
dNdy
dydpdydp
dNdydpdy
T
pp
hhTT
hpp
hhT
Per-trigger yield as a
function of the p_T of
the triggered photon:
NLO pQCD results describe
the behavior of the data for
photon-hadron production
in p+p collisions.
Gamma-triggered hadrons in p+p:
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Gamma-triggered hadrons in A+A:
Data from “A. Hamed for STAR, talk at QM2008 and HP2008”
Within the same energy loss formalism as that in our previous studies on single/dihadron spectra in A+A collisions.
Simultaneous fit of single, dihadron and photon-hadron productions.----Another evidence of jet quenching!
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Qualitatively, Iaa in small z_Tregion is slightly more sensitiveto epsilon_0 than Iaa in large z_Tregion. LO
Per-trigger yield for photon-hadron in central Au+Au
NLO
T
hAA
AATAA
Tpp
TAATAA dz
dN
NzDwhere
zD
zDzI
1)(,
)(
)()(
Nuclear modification factor:
Energy loss parameter,which is introduced to describe the uncertainty of the medium density.
00
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T
hTT ppz
NLO Nh > 0 at z_T>1: surface emissionnolossch
Llossch
LAAch DeDeD /
//
// )1(
At large z_T:the contributions with energy loss vanish due to jet quenching,dominated by the contributions without energy loss.
9.0Tz
0
For LO, the jet’s energy can’t exceed the gamma’s energy, no contributions for z_T>1 region.
For NLO, because of 2->3 processes, have z_T>1 contributions.
However 2->2 (tree level + one loop) dominate.
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T
hTT ppz
For small z_T: Volume emission
nolossch
Llossch
LAAch DeDeD /
//
// )1(
At small z_T: both contribute.The jets near the center with energy loss dominate.
3.0Tz
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The averaged distance <L> for the gamma-triggered parton jets passing through the quark matter.
Surface versus Volume emission
Small zt probes the matter deeper than large zt, so more sensitive.
Surface emission
Volume emission
3.0Tz
9.0Tz
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Single hadron Dihadron Photon-hadron
More sensitive probe?
NLO
Comparing small-zt gamma-jets to single jets and dijets
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Comparing small-zt gamma-jets to single jets
Gamma-jet Single jetsmall zT
Gam-jets for small zt probes the matter deeper than single jets.
3.0Tz 9.0Tz
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Comparing small-zt gamma-jets to dijets
Gamma-jet Dijetsmall zT
Because of punch-through jets for dihadrons, it is not sure whether small-zt gam-jets are more sensitive than dijets.
3.0Tz
9.0Tz3.0Tz 9.0Tz
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Hadron-triggered FFs are greater than gamma-triggered FFs
)()(
)()(
ThAuAuT
hhAuAu
ThppT
hhpp
zDzD
zDzD
e.g. Trig=8GeV, zt=0.5
hadr:8 jet:12 jet:12 assoc: 4
gamm:8 jet:8 assoc: 4
p+p: Per trigger
Why hadron-triggered FFs are greater than gamma-triggered FFs in pp/AA?
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)()(
)()(
ThAuAuT
hhAuAu
ThppT
hhpp
zDzD
zDzD
e.g. Trig=8GeV, zt=0.5
hadr:8 12 12 assoc: 4
gamm:8 8 6 assoc: 4
Au+Au: Per trigger
Volume emission
Tangential ~pp
Why hadron-triggered FFs is greater than gamma-triggered FFs in pp/AA?
Hadron-triggered FFs are greater than gamma-triggered FFs
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IV. Conclusions
1) With isolation cut applied, the “clean” photons are separated from the “unclean” EM signals in A+A collisions. With the “clean” photons tagged, the back-to-back jets are “clearly” stared for checking their energy loss.
2) Within the same energy loss formalism, our numerical results for photon-hadron fit data well, providing another evidence of jet quenching.
3) The suppression factor for hadrons with large z_T is controlled mainly by the surface emission of the gamma-jet events, while small z_T region will be volume emission bias.
4) Gamma-jets for small z_T region probe the dense matter deeper than those for large z_T region, so the gamma-jets for small z_T region are slightly more sensitive to the dense matter properties.
Thanks for your attention!