Parton distributions and cos2ϕh asymmetry induced by anomalous photon-quark coupling

Abstract

In the spectator models of the nucleon with scalar and axial-vector diquarks, we show the effect of Pauli coupling in the 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 an obvious contribution to unpolarized and polarized PDFs and also leads to a cos 2ϕ 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 2ϕ 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 as the Cahn effect.

Semi-inclusive deep-inelastic scattering in Wandzura-Wilczek-type approximation

“…hT /Q 2 , which is not negligible in fixed-target experiments as shown in phenomenological [113] and model [161] studies. In the phenomenological works [157,162,163], an attempt was made to estimate and correct for this contribution in order to obtain a picture of the Boer-Mulders function undistorted from Cahn effect.…”

Semi-inclusive deep inelastic scattering in Wandzura-Wilczek-type approximation

“…in fixed-target experiments as shown in phenomenological [114] and model [160] studies. In the phenomenological works [156,161,162], an attempt was made to estimate and correct for this contribution in order to obtain a picture of the Boer-Mulders function undistorted from Cahn effect.…”

“…Especially for COMPASS one can see that calculation and data are of opposite signs. In fact, it is suspected that this observable receives a significant contribution from the Cahn effect [159], a term of higher-twist character of the type P 2 hT /Q 2 , which is not negligible in fixed-target experiments as shown in phenomenological [114] and model [160] studies. In the phenomenological works [156,161,162], an attempt was made to estimate and correct for this contribution in order to obtain a picture of the Boer-Mulders function undistorted from Cahn effect.…”