Transcript heavy quark energy loss

Gluon Radiation of the
Heavy Quark
in Nuclei
Ben-Wei Zhang
Institute of Particle Physics
Central China Normal Univeristy
Weihai, Shandong --- Augest 4, 2004
Outline
Motivation
 Twist expansion approach
 Modified heavy quark fragmentation function
 Heavy quark energy loss
------ Heavy VS. Light
 Conclusions

Ben-Wei Zhang @ Weihai
2
Jet Quenching
parton
E
hadrons
ph
Jet quenching has been observed as a
new nuclear phenomenon at RHIC.
Ben-Wei Zhang @ Weihai
3
From Light to Heavy
Theory:


A lot of theoretical studies have been devoted to the
calculations of the energy loss of a light quark.
For massive quarks, the formalism of medium-induced
gluon radiation is not developed to the same extent.
Experiment:

Open charm suppression, which can now be measured at
RHIC by comparing pt distributions of D-mesons in D-Au
and Au-Au collisions, is a novel probe of QGP dynamics.
Ben-Wei Zhang @ Weihai
4
Heavy Quark Energy Loss

There are several theoretical calculations to
obtain the heavy quark energy loss:
Dokshitzer and Kharzeev
Djordjevic and Gyulassy
Armesto, Salgado and Wiedemann
……
 Twist Expansion approach in pQCD
B.W. Zhang, E. Wang and X.N. Wang, Phys. Rev. Lett. 93(2004) in press
Ben-Wei Zhang @ Weihai
5
Jet quenching with pQCD
How to measure the parton energy loss?
 Direct measurement is impossible
 Particle distributions within a jet
 Modification to Fragmentation Function
 Obtain the energy loss indirectly from
measuring the modification of FF

Ben-Wei Zhang @ Weihai
6
Heavy Quark

In parton model, for the heavy quark production we obtain :
Ben-Wei Zhang @ Weihai
7
Fragmentation Function

Charm quark fragmentation function in Peterson form:
Ben-Wei Zhang @ Weihai
8
Mutiple Scattering in Nuclei
In order to calculate the heavy quark energy loss
induced by gluon radiation in pQCD, we should
separate the ‘hard’ part from the ‘soft’ part:
factorizing.
Ben-Wei Zhang @ Weihai
9
Factorization of Twist-4
Luo, Qiu, Sterman, PLB 279(1992)377;
Luo, Qiu, Sterman, PRD 49(1994)4493; PRD 50(1994)1951
Ben-Wei Zhang @ Weihai
10
Generalized Factorization
Ben-Wei Zhang @ Weihai
11
Quark-Gluon Double Scattering
Ben-Wei Zhang @ Weihai
12
Collinear Approximation

Make a collinear approximation:

The tensor structure can be factorized:
Ben-Wei Zhang @ Weihai
13
LPM Effect
LMP interference effect.
The formation time of gluon radiation.
Ben-Wei Zhang @ Weihai
14
Other Processes
Ben-Wei Zhang @ Weihai
15
Hard Partonic Part
Dead-Cone effect of heavy quark propagating
Ben-Wei Zhang @ Weihai
16
Modified FF of Heavy Quark

With the generalized factorization theorem we
obtain the semi- inclusive hadronic tensor:
The modified effective fragmentation function is
Ben-Wei Zhang @ Weihai
17
Twist-4 Parton Correlation
Ben-Wei Zhang @ Weihai
18
Two-parton Correlation Function
Twist-4 parton correlation
function is in principle
not calculable.
With some
approximations
we can
estimate
Ben-Wei Zhang @ Weihai
J. Osborne and Xin-Nian Wang, NPA 710(2002)281
19
Heavy Quark Energy Loss
Twist-4 correlation functions approximated as:
Heavy quark energy loss can be derived as:
Ben-Wei Zhang @ Weihai
20
Two Limits
(1) When
Q2
Q^2
(2) For large values of Q2 or small xB, we have
Ben-Wei Zhang @ Weihai
21
Nuclear Size Dependence
Ben-Wei Zhang @ Weihai
22
Heavy Quark VS. Light Quark
Energy loss of heavy quark is significantly suppressed
due to mass effect, in particular, the dead-cone effect.
Ben-Wei Zhang @ Weihai
23
Conclusions



Heavy quark energy loss induced by gluon radiation
is derived in terms of Modified FF with pQCD.
Medium size dependence of heavy quark enegy loss
is found to change from a linear to a quadratic form
when the initial energy and momentum scale are
increasing.
Heavy quark energy loss is significantly suppressed
relative to a light quark due to mass effect.
Thanks to my collaborators: Xin-Nian Wang and Enke Wang
Ben-Wei Zhang @ Weihai
24
Thank you very
much!
谢谢!
Ben-Wei Zhang @ Weihai
25