Initial transverse-momentum broadening of Breit-Wheeler process in relativistic heavy-ion collisions

Abstract

We calculate the cross section and transverse-momentum (P⊥) distribution of the Breit-Wheeler process in relativistic heavy-ion collisions and their dependence on collision impact parameter (b). To accomplish this, the Equivalent Photon Approximation (EPA) was generalized in a more differential way compared to the approach traditionally used for inclusive collisions. In addition, a lowest-order QED calculation with straightline assumption was performed as a standard baseline for comparison. The cross section as a function of b is consistent with previous calculations using the equivalent one-photon distribution function. Most importantly, the P⊥ shape from this model is strongly dependent on impact parameter and can quantitatively explain the P⊥ broadening observed recently by RHIC and LHC experiments. This broadening effect from the initial QED field strength should be considered in studying possible trapped magnetic field and multiple scattering in a Quark-Gluon Plasma (QGP). The impact-parameter sensitive observable also provides a controllable tool for studying extreme electromagnetic fields.