A Study of Central Exclusive Production at LHCb

This thesis presents an analysis of the central exclusive production (CEP) of $\chi_{\mathrm{c}}(1\mathrm{P})$ mesons in proton-proton collisions at a centre-of-mass energy of 8 TeV, using data collected with the LHCb detector corresponding to an effective integrated luminosity of $(126.3 \pm 4.3)$ pb$^{-1}$. Candidate $\chi_{\mathrm{c}}(1\mathrm{P})$ decays are reconstructed in the $K^+K^-$ and $\pi^+\pi^-$ final states, where the final state particles have pseudorapidities between 2.5 and 4.5. The products of the cross sections and branching fractions are measured as \begin{equation} \sigma\times\mathcal{B}(\chi_{\mathrm{c}0}(1\mathrm{P}) \rightarrow K^+K^-) = 28.0 \pm 2.8 \pm 13.9\,\mathrm{pb}, \end{equation} \begin{equation} \sigma\times\mathcal{B}(\chi_{\mathrm{c}0}(1\mathrm{P}) \rightarrow \pi^+\pi^-) = 20.3 \pm 2.4 \pm 8.1\,\mathrm{pb}, \end{equation} \begin{equation} \sigma\times\mathcal{B}(\chi_{\mathrm{c}2}(1\mathrm{P}) \rightarrow K^+K^-) < 12.1\,\mathrm{pb}, \end{equation} \begin{equation} \sigma\times\mathcal{B}(\chi_{\mathrm{c}2}(1\mathrm{P}) \rightarrow \pi^+\pi^-) < 10.5\,\mathrm{pb}, \end{equation} where the first uncertainties are statistical and the second systematic, and limits are set at the 95% confidence level. A major source of systematic uncertainty is the determination of the fraction of the selected data which is CEP rather than inelastic backgrounds. Also described is HERSCHEL, a new subdetector installed at LHCb during the first long shutdown of the LHC. This is a system of forward shower counters which will provide CEP analyses with event-by-event rejection of inelastic backgrounds. The pseudorapidity coverage and detection efficiency of HERSCHEL are determined using Monte Carlo simulations. The pseudorapidity coverage extends to $\left|\eta\right| \geq 10$ due to the showering of collision products at the aperture limit. With a photoelectron threshold yield of 0.2 times the yield in the mean minimum bias event, the detection efficiency for single diffractive events is predicted to be 89% in the forward direction and 91% in the backward direction.