Photon parton distributions in nuclei and the EMC effect

TL;DR

This paper evaluates the photon contribution to nuclear structure functions and demonstrates that it accounts for most of the EMC effect at x ≤ 0.5, with minimal hadronic mechanism contribution, refining understanding of nuclear modifications.

Contribution

It provides a model-independent calculation of photon contributions to the EMC effect and clarifies their significance compared to hadronic mechanisms.

Findings

Photon contributions explain most of the EMC effect for x ≤ 0.5.

Hadronic mechanism contribution is less than 3% for all nuclei at these x values.

The A-dependence of the EMC effect at x > 0.5 is significantly altered.

Abstract

Photons as well as quarks and gluons are constituents of the infinite momentum frame (IMF) wave function of an energetic particle. They are mostly equivalent photons whose amplitude follows from the Lorentz transformation of the particle rest frame Coulomb field into the IMF and from the conservation of the electromagnetic current. We evaluate in a model independent way the dominant photon contribution \propto \alpha_{em}(Z^2/A^{4/3})\ln(1/R_{A}m_{N}x) to the nuclear structure functions as well as the term \propto \alpha_{em}Z/A. In addition we show that the definition of x consistent with the exact kinematics of eA scattering (with exact sum rules) works in the same direction as the nucleus field of equivalent photons. Combined, these effects account for the bulk of the EMC effect for x\le 0.5 where Fermi motion effects are small. In particular for these x the hadronic mechanism contribution to the EMC effect does not exceed \sim 3% for all nuclei. Also the A-dependence of the hadronic mechanism of the EMC effect for x > 0.5 is significantly modified.