Exclusive production of large invariant mass pion pairs in ultraperipheral ultrarelativistic heavy ion collisions

TL;DR

This paper calculates the cross sections for exclusive large invariant mass pion pair production in ultraperipheral heavy ion collisions at LHC energies, considering photon-photon interactions and advanced QCD mechanisms.

Contribution

It introduces a detailed calculation of pion pair production using impact-parameter EPA and incorporates recent experimental data into the QCD modeling of the elementary process.

Findings

Significant cross sections predicted for LHC measurements.

Inclusion of realistic nuclear charge distributions enhances accuracy.

Comparison of different QCD mechanisms for the elementary process.

Abstract

The cross section for exclusive production of $\pi^+ \pi^-$ and $\pi^0 \pi^0$ meson pairs in ultrarelativistic heavy ion collisions is calculated for LHC energy $\sqrt{s_{NN}} =$ 3.5 TeV taking into account photon-photon mechanism. We concentrate on the production of large two-pion invariant masses where the mechanism of the elementary $\gamma \gamma \to \pi \pi$ process is not fully understood. In order to include a size of nuclei we perform calculation in the impact-parameter equivalent photon approximation (EPA). Realistic charge densities are used to calculate charged form factor of $^{208}$Pb nucleus and to generate photon fluxes associated with ultrarelativistic heavy ions. Sizeable cross sections are obtained that can be measured at LHC. The cross section for elementary $\gamma \gamma \to \pi \pi$ is calculated in the framework of pQCD Brodsky-Lepage (BL) mechanism with the distribution amplitude used to descibe recent data of the BABAR collaboration on pion transition form factor, using hand-bag mechanism advocated to describe recent Belle data as well as $t$ and $u$-channel meson/reggeon exchanges. We present distributions in two-pion invariant mass as well as the pion pair rapidity for the nuclear process.