Coherent photons induced high energy reactions in ultraperipheral heavy ion collisions

Abstract

We review the recent progress in the studies of coherent photons induced high energy reactions in ultraperipheral heavy ion collisions. The strong electromagnetic field created by a fast moving charged heavy ion can be effectively viewed as a flux of quasi-real coherent photons. In this paper, we mainly discuss two different type processes that coherent photons take part in: lepton pair production via photon fusion and diffractive vector meson production in UPCs. We focus on investigating the impact parameter dependent effect and the final state soft radiation effect. On the other hand, a series of recent work have revealed that coherent photons are highly linearly polarized with its polarization vector being parallel to its transverse momentum. It has been shown that the linearly polarized photons can lead to <inline-formula><tex-math id="M2">\begin{document}$\cos 4\phi$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="7-20230074_M2.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="7-20230074_M2.png"/></alternatives></inline-formula> azimuthal asymmetries in di-lepton production. This theoretical predication soon has been confirmed by the STAR measurement. With this new development from both theory and experiment sides, the linearly polarized photons provide a new experimental avenue to explore novel QCD phenomenology. For example, the linearly polarized photons can give rise to various different azimuthal asymmetries in diffractive vector meson production. These observables provide us unique chance to study two source interference effect in high energy scatterings, Coulomb-Nuclear interference effect as well as extracting gluon Wigner distribution. We will discuss these novel phenomenology studies and the possible future developments.