Bremsstrahlung of a Quark Propagating through a Nucleus

TL;DR

The paper analyzes how quantum coherence affects the transverse-momentum distribution of photons and gluons radiated by a quark propagating through a nucleus, focusing on the regime where the radiation formation time vastly exceeds the nuclear size. Using a light-cone, dipole-based approach, the authors derive k_T-dependent radiation cross sections for both electromagnetic and gluonic bremsstrahlung, including the nuclear modification via eikonalized dipole interactions. They find a suppression (shadowing) at small k_T and a surprising enhancement (antishadowing) for k_T around 1–2 GeV, with the effect weakening at very high k_T (~10 GeV), and they connect these results to observable processes like Drell–Yan, prompt photon production, and hadroproduction. The work provides a framework to quantify nuclear effects on k_T distributions and sets the stage for applications to relativistic heavy-ion collisions at RHIC and LHC.

Abstract

The density of gluons produced in the central rapidity region of a heavy ion collision is poorly known. We investigate the influence of the effects of quantum coherence on the transverse momentum distribution of photons and gluons radiated by a quark propagating through nuclear matter. We describe the case that the radiation time substantially exceeds the nuclear radius (the relevant case for RHIC and LHC energies), which is different from what is known as Landau-Pomeranchuk-Migdal effect corresponding to an infinite medium. We find suppression of the radiation spectrum at small transverse photon/gluon momentum k_T, but enhancement for k_T>1GeV. Any nuclear effects vanish for k_T > 10GeV. Our results allow also to calculate the k_T dependent nuclear effects in prompt photon, light and heavy (Drell-Yan) dilepton and hadron production.

Figures (5)

• Figure 1: Feynman graphs for bremsstrahlung.
• Figure 2: Feynman graphs for gluon bremsstrahlung of an interacting quark.
• Figure 3: The exponent (\ref{['3.4']}) of the atomic number dependence parameterized as $A^n$ versus $k_T$ and $\alpha$ for transversely (left figure) and longitudinally (right figure) polarized virtual photons.
• Figure 4: The same as in Fig. \ref{['fig3']}, but for real photons.
• Figure 5: The same as in Fig. \ref{['fig3']}, but for gluons at $\alpha=0$ for different effective gluon masses.