Dijet

1
Dijet Gamma-jet correlations in NLO pQCD
Han-Zhong Zhang Institute of Particle Physics,
Huazhong Normal University, China Collaborators
J. Owens, E. Wang and X.-N. Wang

• Introduction
• Dijet correlations
• Gamma-jet correlations
• Conclusion

Workshop on Photon and Jet with ALICE , CCNU,
Wuhan Dec. 4-6 , 2008
2
I. Introduction
What happens for a jet propagating inside QGP?
Jet quenching
X.-N.Wang and M.Gyulassy, Phys.Rev.Lett.68,1480(19
92)
The hard jet loses a significant amount of its
energy via radiating gluon induced by multiple
scattering.
3
Three kinds of hard probes of QGP

• Single jet ? Single hadron spectra
• Dijet ? Hadron-triggered away-side hadron spectra
• 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
?
4
Motivation gamma-jet probe

• Gamma-jets had been suggested for studying
  jet energy loss in dense matter. X. -N. Wang, Z.
  Huang, and I. Sarcevic, PRL 77(1996) 231-234.
• The NLO study of the photon-triggered hadron
  spectra will help to obtain the detailed picture
  of jet quenching in the whole region of the
  hadron transverse momentum.
• The sensitivity of Gamma-jets to probe the dense
  matter.

5
Gamma-jet by NLO pQCD parton model
LO (tree level) NLO
corrections (e.g. 2?3)
therefore leading to hadrons with transverse
momentum larger than that of the photons
6
The fragmentation of the jets off the dense matter
in medium in vacuum
(X. -N. Wang , PRC70(2004)031901)
The jet energy loss in a 1D expanding system
Energy loss parameter
(a parameterization form of theory calculations)
Enke Wang , X. -N. Wang , PRL87(2001)142301)
7
II. Dijet correlations
Single jet
Single hadron suppression factor
Dijet
Correlated away-side dihadron suppression factor
8
Simultaneous fits of single/dihadron productions
single hadron
dihadron
In most central AuAu collisions
9
-fit to both single/dihadron spectra
in most central AuAu
for dihadron
Sharp concave Flat concave
for single
H.Z. Zhang, J.F. Owens, E. Wang and X.-N. Wang ,
PRL 98(2007)212301
1) Jet quenching consistent in single/dihadron 2)
Iaa robust when Raa fragile.
10
Spatial transverse distribution of the initial
parton production points that contribute to the
single and dihadron along a given direction
Color strength single/dihadron yield from the
jets originating from the square
Dominated by dijets close and tangential to the
surface and the punch-through dijets
Dominated by jets close and perpendicular to the
surface
single hadron
dihadron
Thickness of the outer corona
25 contribution
11
III. Gamma-jet correlations
(1). Isolated photons
Direct photon sources in pp a) Prompt photon
from hard scattering
LO
NLO
Annihilation Compton
2?3 one-loop
b) Fragmentation (bremsstrahlung)
contributions (accompanied by nearly collinear
hadrons on the same side)
12
Isolation Cut (IC)
An isolation cut (IC) is often applied on the
electromagnetic signal to separate the prompt
photons from other sources.
Gamma
Most accompanying hadrons are within a cone of
angle radius
J. F. Owens, Rev. Mod. Phys. 59, 465(1987) H.
Baer, J. Ohnemus, and J. F. Owens, Phys. Rev. D.
42, 61(1990)
Jet
13
NLO results for direct photons in pp
Data
PHENIX, PRL 98 (2007) 012002
Inclusive photons
the measured photon samples are expected to be
isolated from parton jet activity.
With IC, the fragm. are only 10. So IC photons
mainly belong to prompt photons.
14
High p_T prompt photon dominates in central AuAu
Other sources in AA conversion photons induced
photons thermal photons
Turbide, Gale, Jeon, Moore, Phys. Rev. C. 72
(2005) 014906
15
Focus on isolated photons now
So the high p_T IC photons observed in pp/AA
collisions can be well described by annihilation
and Compton processes in pQCD theory. In the
following studies for photon-hadron correlations
we will only consider the contribution from
annihilation and Compton processes. Such an
approximation could give a simple and clear jet
quenching picture in AA collisions because the
parton jets can be well calibrated by triggered
photons.
16
(2) Gamma-triggered jet emissions
The per-trigger photon-hadron spectra
X. -N. Wang, Z. Huang, and I. Sarcevic, PRL
77(1996) 231-234.
D_AA gives such a fragmentation function for the
final parton jets, which is just averaged with
the invariant cross sections for the correlated
photons. Sometimes we call D_AA as the
photon-triggered hadron fragmentation functions.
17
Photon-triggered hadrons in pp
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 produced in pp at 200GeV
Data from Matthew Nguyen for PHENIX, talk at
QM2008
18
Photon-triggered hadrons in AA
Within the energy loss formalism same as that in
our previous studies on single/dihadron spectra
in AA collisions.
Data from A. Hamed for STAR, talk at QM2008 and
HP2008
Simultaneous fits of single, dihadron and
photon-hadron productions.----Another convincing
evidence of jet quenching!
19
Check Gamma-jet sensitivity to QGP in central
AuAu
Energy loss parameter, the ability for a jet to
heat the medium. It can reflect the medium
properties, such as density and temperature,
etc.
Nuclear modification factor
NLO
LO results stop at z_T1. NLO cover all z_T
region, more reliable.
Qualitatively, Iaa in small z_T region is
slightly more sensitive to epsilon_0 than Iaa in
large z_T region.
LO
Why?
20
NLO Ngh gt 0 at z_Tgt1 surface emission
At large z_T First term contributions vanish due
to jet quenching, dominated by the second term
contributions.
For LO, the jets energy cant exceed the gammas
energy, no contributions for z_Tgt1 region. For
NLO, because of contributions from 2-gt3
processes, the jets energy can exceed the
gammas energy, have z_Tgt1 contributions.
21
For small z_T Volume emission
At small z_T both contribute
Volume emission means the jet probes the matter
deeper than the surface emission. Thats why
small z_T jet is more sensitive to the matter.
22
Surface versus Volume emission
The averaged distance ltLgt for the gamma-triggered
parton jets passing through the quark matter.
Volume emission
Surface emission
Small zt probes the matter deeper than large zt,
so more sensitive.
23
IV. Conclusions

• Punch-through jets are found to result in the
  dihadron slightly more sensitive to medium
  properties than the single hadron.
• With isolation cut applying, the clean photons
  can be separated from the unclean EM signals in
  HIC. With the clean photons tagged, the parton
  jets are clearly stared for their energy loss.
• Within the same energy loss formalism,
  simultaneous fits of single hadron, dihadron and
  photon-hadron give a convincing evidence of jet
  quenching.
• The suppression factor for photon-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.
• 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.

Thank for your attention!