References - Bogoliubov Laboratory of Theoretical Physics - JINR

POLARIZED HADRON STRUCTURE IN THE VALON MODEL AND
THE NUCLEON AXIAL COUPLING CONSTANTS: a3 AND a8
F. Taghavi Shahri 1 † ,F.Arash 2
(1) School of Particles and Accelerators, Institute for Research in Fundamental Sciences (IPM)
P.O. Box 19395-5531, Tehran, Iran
(2) Physics Department, Tafresh University, Tafresh, Iran
† E-mail: f taghavi@ipm.ir
Abstract
We have utilized the concept of valon model to calculate the spin structure
function of the nucleon. The structure of the valon itself developed through the
perturbative dressing of a valence quark in QCD, which is independent of the hosting
hadron. In this paper we calculate the nucleon axial coupling constants, a3 and a8
in the valon framework. We compare the results with experimental data and find
good agreement between them.
1 Polarized hadron structure in the Valon model
One of the fundamental properties of the nucleon is its spin. The spin structure of
nucleon has been the subject of heated debates over the past twenty years. The key
question is that how the spin of the nucleon is distributed among its constituent partons.
Any determination of parton distribution function in a nucleon in the framework of QCD
always involves some model-dependent procedures.
We utilized the valon model [1] to study polarized nucleon structure. In the valon model,
it is assumed that a nucleon is composed of three dressed valence quarks: the valons.
Each valon has its own internal structure which can be probed at high enough Q 2 . At
low Q 2 , a valon behaves as a valence quark. The valons play a role in scattering problems
as the constituents do in bound state problems. It is assumed that the valons stand at a
level in between hadrons and partons and that the valon distributions are independent of
the probe or Q 2 . In this representation a valon is viewed as a cluster of its own partons.
The evolution of the parton distributions in a hadron is effected through the evolution of
the valon structure, as the higher resolution of a probe reveals the parton content of the
valon. The valon model is essentially a two component model. In this framework, the
helicity distributions of various partons in a hadron are given by:
δq h i (x, Q2 )= � � 1
dy
x
y δGhvalon (y)δqvalon i
( x
y ,Q2 ) (1)
where δGh valon (y) is the helicity distribution of the valon in the hosting hadron. It is
related to unpolarized valon distribution by:
δGj(y) =δFj(y)Gj(y). (2)
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