Parton distributions and $\cos 2\phi_h$ asymmetry induced by anomalous photon-quark coupling

TL;DR

This paper investigates how anomalous photon-quark couplings, modeled via Pauli coupling in spectator models, influence parton distribution functions and induce a measurable $ ext{cos} 2 ext{phi}_h$ azimuthal asymmetry in unpolarized SIDIS, aligning with experimental observations.

Contribution

It introduces a novel effect of Pauli coupling on PDFs and azimuthal asymmetry, providing numerical predictions consistent with experimental data.

Findings

Pauli coupling significantly affects unpolarized and polarized PDFs.

The predicted $ ext{cos} 2 ext{phi}_h$ asymmetry is positive and a few percent.

Results are consistent with HERMES and COMPASS experimental regimes.

Abstract

In the spectator models of the nucleon with scalar and axial-vector diquarks, we show that the effect of Pauli coupling in photon-quark vertex to the parton distribution functions (PDFs) of nucleon and azimuthal asymmetry in the unpolarized semi-inclusive deep inelastic scattering (SIDIS). This anomalous coupling gives obvious contribution to unpolarized and polarized PDFs, and also leads to a $\cos 2\phi_h$ azimuthal asymmetry proportional to the squared Pauli form factor, due to the helicity flip of the struck quark. After determining the model parameters by fitting PDFs to the global fits, this new distribution for $\cos 2\phi_h$ asymmetry is given numerically. In the framework of transverse momentum dependence (TMD), we find that it is positive and of a few percent in the kinematical regime of HERMES and COMPASS collaborations, in the same order of magnitude with Cahn effect.