QCD Fragmentation Functions for Hard Hadron Production

(with emphasis on charged-hadron/pion ratio)

HIC03, Montreal , Canada, Jun 27, 2003

Xiaofei ZhangXiaofei Zhang

Kent State UniversityKent State University

X. F. Zhang, et al. , PRL, 89,272301 (2002)

X. F. Zhang , et al , hep-ph/0306227 (2003)

based on work:

ddpdT

ddpdNR

Tpp

AB

TAB

AB /

/

New phenomena at RHIC

Charged hadron and pion are different!

Enhancement of p/ π, h/π

Uncertainty in Fragmentation Functions (FF-s)

A simple model to explain h/π enhancement from pQCD

this talk:

0-10%

60-92%

pQCD for hard hadron production

p

hadron

hadron

FF-s are important for hadron production

Phase space of fragmentation in pp collisions

8GeV

4GeV

16GeV

X. Zhang, et. al. , PRL, 89,272301 (2002)

91.2 GeVfull

light q

charm

bottom

β=0.8βKKP

27.8 GeV

β= βKKP

β=0.8βKKP

β=βKKP

β=0.5βKKP

Large z part of FF-s and p/π ratio in pp

Large z part of FF-s have significant effect on p/π

FF-s and p/π ratio at RHIC

pQCD

x~0.01, gluon PDF-s become very important Gluon FF-s dominate

Large z part of gluon FF-s are even less certain, especially for proton!

FF

sets

Charged-hadron to pion ratio in pp

mimic higher twist effect

• using Kretzer FF-s• • h/π in pp at RHIC can be described by pQCD

* Croinin effect in p+A : RpA>1, for pT>2 GeV* At fixed target energies, Cronin effect is not universal ; heavier particle, larger enhancement p>k>π

J. W. Cronin et al., Phys. Rev. D 11,3105 (1975)

* Cronin effects in pA collisions kT broadening

* kT effects: mimic power correction -- power correction is larger for proton than pion -- more phase space for dynamic broadening

A simple model for h/π enhancement in Au+Au

KT broadening is non-universal for different particles

Simple model

•Assume suppression factors cancel approximately

L(b)

place of hard collisions

h/π enhancement in Au+Au

, more central, larger L(b),

more enhancement of charged-haron/pion

X.F. Zhang and G. Fai , hep-ph/0306227

Cronin effects in dAu

• Difference of RdAu for charged-hadron and pion are reasonably described using the same parameters

EKS anti-shadowing

Intrinsic kT mimics power corrections, systematic study of power correction is needed

A simple model based on the non-universal kT

broadening of different particles can explain both

Direct prediction of this model: enhancement of h/ π in d+Au

• h/ π enhancement in central Au+Au collisions • Difference of RdAu for pion and charged hadron

Can it explain the difference of RAA betweencharged-hadron and pion?