Gamma-jet tomography of high-energy nuclear collisions in NLO pQCD
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Transcript of Gamma-jet tomography of high-energy nuclear collisions in NLO pQCD
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Gamma-jet tomography of high-energy nuclear collisions in NLO pQCD
Han-Zhong Zhang
Institute of Particle Physics, Huazhong Normal University, China
Collaborators: Enke Wang, J. Owens and X.-N. Wang
Weihai, 08-14 August, 2009
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H. Z. Zhang, J. Owens, E. Wang and X. –N. Wang, Phys. Rev. Lett, 103(2009)032302; 98(2007)212301
OUTLINE
Jet Quenching Gamma-jet in NLO pQCD Same (energy loss) formalism for
calculating gam-hadr, single/dihadron Gamma-jet tomography: volume vs
surface emission Conclusions
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Jet quenching:
The hard jet loses a significant amount of its energy
via radiating gluon 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)
A powerful tool for the study of Quark Matter
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Three kinds of hard probes of Quark Matter
1) Single jet Single hadron spectra
2) Dijet Hadron-triggered away-side hadron spectra
3) Gamma-jet Photon-triggered away-side hadron spectra
Single hadron Dihadron Gamma-hadron
STAR Pre.
Experimental studies of jet quenching, suppression observed
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Single and di-jet tomography
Single HadronDihadronH. Z. Zhang, J. Owens, E. Wang and X. –N. Wang,
PRL98(2007)212301
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Gamma-jet probe
GamTp
1JetTp
2JetTp
GamTp
JetTp
LO (tree level)
GamT
JetT pp Gam
TJetT pp
NLO corrections: (e.g. 23)
PQCD parton model:
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H. Baer, J. Ohnemus, and J. F. Owens, Phys. Rev. D. 42, 61(1990)
Most accompanying hadrons arewithin a cone of angle radius coneR
Try to eliminate fragmentation photon
An “isolation” cut (IC) is often applied on the electromagnetic signal to separate the direct photons from other sources.
22 )()( coneR
Jet
Gamma
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With IC, the fragm. only 10%.
Data :
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.”
Inclusive photons
we will focus mainly on photons with isolation cuts
Direct vs. fragmentation photons
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The effective fragmentation functions
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
),(,11
11),,( 20
//20
//2
/ cch
Lcch
Lccch zDe
z
zD
zeEzD
Tcc pEz /
in medium in vacuum
0000
/1),,,(0
0
nrb
dLg
L
q̂0
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Jet energy loss is dependent on its transverse momentum and initial space origination, a function of the azimuthal angle, the passing distance, the medium particle density along the trajectory
Track any jet inside the medium
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dihadronsingle hadron
determined in a same energy loss formalism 0in most central Au+Au
fmGeV /1.25.10
Actually we will choose epsilon_0 1.68GeV/fm for gam-hadr
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Hard sphere versus Woods-Saxont(
r)
r/fm
dze
rt
RrR
Art
crzr
WSA
HSA
/)(
0
222
022
1)(
/12
3)(
A hard sphere overlap geometry differs at most about10% from a Wood-Saxon geometry.
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1) Same model: NLO pQCD parton model
2) Same geometry: hard sphere
3) Same PDFs in vacuum: CTEQ6M
4) Same FFs in vacuum: KKP
5) Same energy loss parameterization
and =1.68GeV/fm
Calculate gamma-hadron in the same formalism as for single/dihadron in HIC
0
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D_AA is a sum of FF’s of the away-side jets (quark and gluon) weighted with the fractional gamma-jet cross sections.
Sometimes we call D_AA as
the photon-triggered fragmentation functions.
ThTT
TABT
hhTT
hABThT
T
hAB
ABTAB
ppzwhere
dydpddydp
ddydydpdpdpddydydp
dz
dN
NzD
/
/
/1)(
The per-trigger photon-hadron spectra
Xba
Xbach
AAd
dDD
ˆ
ˆ~
~/
Gamma-triggered hadron spectra
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1) Fit data very well in p+p for different values of p_T^trig.
2) Fit data well in Au+Au. Agreement is not nontrivial. It reinforces the success of the parton energy loss picture.
3) Hadron-triggered FF’s > Photon-triggered FF’s.
H.Z. Zhang, J.F. Owens, E. Wang
and X.-N. Wang ,
Phys. Rev. Lett, 103(2009)032302
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Mainly because the fraction of hadron-triggered gluon jets is larger than the fraction of photon-triggered gluon jets at same pt^trig, And the hadron yield of gluon jets is larger than that of quarks.
50%
10%
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More stronger dependence of on
1) In LO energy balance limits for gam-hadr
2) NLO radiative correction permits for G-hadr. The two effects cause NLO very different from LO . NLO needed
3) Compared with h-h , Gam-hadr has more dependence of on .
)(/)()( TppTAATAA zDzDzI
1Tz
1Tz
TzhAAI
hhAAI
Quantify suppressions of hadron- and gamma-triggered FF’s in A+A relative to p+p collisions due to JQ
hAAI
hAAI
TzhAAI
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Large : more susceptible to Eloss --- Surface emission
1) Even a small amount energy loss can greatly suppress the large- gam-hadr yield.
2) Only those jets originating near and escaping through the surface will contribute without energy loss.
3) Large- is mainly determined by the thickness of the corona of the surface emission.
Tz
9.0Tz
Tz
hAAI
Tz
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1) For contributing to small- gam-hadr, high energy jet is “wealthy” enough to lose finite energy, and can therefore originate near the center region, --- volume emission.
2) Intermediate- gam-hadr are determined by the competition of the two emission mechanism.
Tz
Tz
Small : encounter more Eloss --- Volume emissionTz
3.0Tz
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Different centrality dependence of the suppression for different values of z_T
1) z_T < 0.6, volume emissions dominate, weaker dependence.
2) z_T > 1, surface emissions dominate, stronger dependence
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Conclusions
• Gamma-hadron correlation described well by NLO pQCD• Gamma-hadron suppression consistent with single hadron and dihadron suppression• Volume & surface emission in different region of kinematic (small and large z_T)• Gamma-jet study toward true tomography of dense matter
Thank for your attention!
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Gamma-jet by NLO pQCD parton model
LO pQCD
FFsdPDFsTd ABAA
NLO pQCD
J. F. Owens, Rev. Mod. Phys. 59, 465(1987)