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

AbstractThe cross section for exclusive production of π+π− and π0π0 meson pairs in ultrarelativistic heavy ion collisions is calculated for LHC energy sNN=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 γγ→ππ 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 208Pb 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 γγ→ππ process is calculated in the framework of pQCD Brodsky–Lepage (BL) mechanism with the distribution amplitude used to describe 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