Central exclusive production of $\chi_c$ mesons in $\sqrt{s}=13$ TeV proton-proton collisions

A measurement is made of the central exclusive production of $\chi_{c1}$ and $\chi_{c2}$ mesons in proton-proton collisions at a centre-of-mass energy of $\sqrt{s}=13$ TeV, using data collected by the LHCb experiment corresponding to a total single-interaction integrated luminosity of $711\rm{pb}^{-1}$. The $\chi_c$ candidates are reconstructed through their radiative decay into $J/\psi\gamma$, where we use photons that have converted into an electron pair, $\gamma\rightarrow e^+e^-$, and reconstruct the $J/\psi$ through its dimuon final state. The High Rapidity Shower Counter sub-detector HeRSCheL is used to reduce inelastic background. The product of the cross section and the branching fractions, where the $J/\psi$ muons are measured to be within the pseudorapidity region $2<\eta<4.5$, are measured to be, \begin{align} \sigma_{\chi_{c1}\to J/\psi[\mu^+\mu-]\gamma}^{(2<\eta_{\mu^+mu^-}<4.5)} = 19.5\pm15.0(\rm{stat})\pm15.4(\rm{syst}) \rm{pb} & \ \ [\text{90\% C.L.}] \nonumber\\ \nonumber\\ \sigma_{\chi_{c2}\to J/\psi[\mu^+mu^-]\gamma}^{(2<\eta_{\mu^+mu^-}<4.5)} = 99.6\pm12.7(\rm{stat})\pm27.7 (\rm{syst})\rm{pb}, & \nonumber \end{align} where the first uncertainty is statistical and the second is systematic. The product of the cross section and the branching fractions of $\chi_{c1}$ mesons is quoted at a 90\% confidence level (C.L.). These results are found to be in agreement with theoretical predictions.