Central exclusive production at LHCb

Abstract

The LHCb collaboration has measured central exclusive production of $J/\psi$, $\psi(2S)$, and $\Upsilon$ mesons as well as $J/\psi J/\psi$, $J/\psi\psi(2S)$, $\psi(2S)\psi(2S)$, and $\chi_c\chi_c$ meson pairs in proton-proton collisions. The analyses of $\Upsilon$ and charmonium pairs are performed at the centre-of-mass energies of 7 TeV and 8 TeV, and those of $J/\psi$ and $\psi(2S)$ are done at 7 TeV and 13 TeV. The analysis at 13 TeV involves the use of new shower counters. These allow a reduction in the background by vetoing events with activity in an extended region in rapidity. The measurements of central exclusive production at LHCb are sensitive to gluon distributions for Bjorken-$x$ values down to $2\times10^{-6}$ (at 13 TeV). An overview of the LHCb results is presented and compared to existing measurements of other experiments and theoretical calculations.