Photoproductions of dileptons, photons, and light vector mesons in p-p, p-Pb, and Pb-Pb collisions at Large Hadron Collider energies

TL;DR

This paper develops an exact method for photoproduction processes in heavy ion collisions at LHC energies, highlighting the significance of ultra-incoherent photons and limitations of the EPA in certain kinematic regions.

Contribution

An exact treatment for photoproduction in heavy ion collisions is introduced, improving accuracy over the equivalent photon approximation in wide kinematic ranges.

Findings

Photoproduction processes are significant at large $p_{T}$ and $y_{r}$.

Ultra-incoherent photon emission is a key channel in photoproduction.

EPA errors are substantial beyond small $y$ and $Q^{2}$ regions.

Abstract

The photoproduction of large $p_{T}$ dileptons, photons and light vector mesons in p-p, p-Pb and Pb-Pb collisions at LHC energies is studied, where the fragmentation processes and the ultra-incoherent photon channel are included. An exact treatment is developed for photoproduction processes in heavy ion collisions, which recovers the equivalent photon approximation (EPA) in the limit $Q^{2}\rightarrow0$ and can avoid double counting effectively. The full kinematical relations are also achieved. We present the results as the distributions in $Q^{2}$, $p_{T}$ and $y_{r}$, the total cross sections are also estimated. The numerical results indicate that: the contribution of photoproduction processes is evident in the large $p_{T}$ and $y_{r}$ regions, and starts to play a fundamental role in p-Pb collisions. The ultra-incoherent photon emission is a important channel of photoproduction processes, which can provide the meaningful contributions. EPA is only applicable in small $y$ and $Q^{2}$ domains, and is very sensitive to the values of $y_{\mathrm{max}}$ and $Q^{2}_{\mathrm{max}}$. The EPA errors appear when $y>0.29$ and $Q^{2}>0.1~\mathrm{GeV}^{2}$, and are rather serious in p-Pb and Pb-Pb collisions. When dealing with widely kinematical regions, the exact treatment needs to be adopted.