Twist-3 and quark mass contributions to the polarized nucleon structure function g2(x,Q2)

Abstract

Quark mass effects are clarified in the parton model approach to the transversely polarized nucleon structure function. The special propagator technique is employed to obtain manifestly gauge invariant results and extract the buried short-distance contributions inside the soft part after momentum factorization in the collinear expansion approach. A generalized massive special propagator for a massive quark is constructed. We identify the corresponding matrix elements of the transversely polarized structure function in deep inelastic scatterings by the massive special propagator technique.Typeset using REVT E X * Email address: kcyang@phys.sinica.edu.tw † Email address: PHHLYU@ccvax.sinica.edu.tw 1The naive parton model that built upon massless free partons had proved itself tremendously successful towards the understanding of spin averaged high energy processes. However, a lot of interesting physics and even possible spin dependent new physics are washed out during the averaging procedure. Following the advances of experimental techniques and facilities, both polarized probe and target become more popular. The EMC data [1] on longitudinally polarized deeply inelastic scattering (DIS) experiments had already provided us lots of surprises and insights into the nuclear structure in the past decade. Spin physics has therefore become one of the most fascinating subjects towards the understanding of the quark and gluon dynamics inside hadrons. High precision data along with state-of-the-art higher order perturbative QCD (pQCD) computation enable us to test the standard model to high accuracy. However, in DIS, where most spin data exist, much work on the subject has concentrated on the leading twist contributions which measure the helicity of the quark constituents. Recently, results in the DIS transversely polarized structure function g T (x, Q 2 ) were reported and have shown a non-negligible contribution [2]. g T contains a chiral even part which measures the quark transverse spin asymmetry and a chiral odd part which is described by the quark transversity distribution [3,4] in the nucleon. Extensive study is expected to be performed at DESY, CERN and SLAC.Quark transverse spin is famous for its conceptual difficulties and confusions in the literature [3,[5][6][7]. As emphasized in Ref. [6], quark transverse spin is a fundamental degree of freedom, and the transversity parton distribution which measures the quark helicity-flip in the helicity basis is well-defined even for massless partons. To see its partonic probabilistic interpretation one has to go to transverse spin eigenstate where transversity becomes diagonal. It simply measures the difference of oppositely transverse polarized quarks inside the nucleon. Using an anti-quark probe as in the Drell-Yan process, the transversity is a leading twist effect and is a naturally large quantity. It is important to note that helicity and chirality are identical for "good" light cone component of the Dirac field. Since in DIS, the virtual photon is a chirally inv...