Bremsstrahlung photon contributions to parton energy loss at high virtuality ($Q^2$) : a perturbative calculation at $O(α_{s} α_{em})$

Abstract: In this work, real photon production scattering kernels from jet-medium interactions in the QCD medium are perturbatively calculated using the higher-twist (HT) formalism. Focus is given towards real photon production from a highly virtual (and highly energetic) quark, taking into account heavy-quark mass scales [Phys. Rev. C 94, 054902 (2016)], fermion-boson conversion processes [Nucl. Phys. A 793, 128-170 (2007)], as well as coherence effects [Phys. Rev. C 105, 024908 (2022)]. A generalized factorization procedure, such as that used in e-A deep-inelastic scattering, is employed to derive an improved single-scattering medium-induced photon emission kernels that go beyond the traditional in-medium gluon exchange approximation. Diagrams with real-photon emission from the hard quark are classified based on the final-state particles, and include two types of scattering kernels at $O(\alpha_{em}\alpha_{s})$ giving the following final states: (i) real photon and real quark, (ii) real photon and real gluon. The collisional kernels, thus derived, include full phase factors from all non-vanishing diagrams and complete second-order derivative terms in the transverse momentum gradient expansion. Moreover, the calculation includes heavy-quark mass effects, thus exploring heavy-quark energy loss. The in-medium parton distribution functions and the related jet transport coefficients have a hard transverse momentum dependence (of the emitted gluon or photon) present within the phase factor. It is observed that the jet transport coefficients resemble the transverse-momentum-dependent parton distribution functions.