Search for exclusively produced top quark pairs at the LHC

This thesis focuses on the search for exclusively produced top quark pairs at the Large Hadron Collider (LHC). This process, although quite rare, is expected to be sensitive to anomalous couplings of the top quark with the photon, and thus can be used to look for new physics. The expected Standard Model cross-section is of the order of 1fb, which might be beyond the current detecting capabilities of the LHC, however some Beyond the Standard Model (BSM) theories predict its enhancement. For the search, data collected in 2017 by the Compact Muon Solenoid (CMS) are used, including information collected by the Precision Proton Spectrometer (PPS), a forward proton detector located about 200 m from CMS. A study on the acceptance and performance of the latter detector is performed. Data collected in the central CMS detector and tracks recorded by the PPS detector are matched. As a first step, a cut-based selection is performed, and signal and control regions are defined. This approach allowed setting an expected upper limit for the cross-section of this process, of $1.1 \substack{+1.1\\ -0.5}$ pb. The observed limit is 7.2 pb. The search is then improved by using a Multivariate Analysis approach, which brought the expected limit down to $0.9\substack {+0.7\\-0.4}$ pb, and the observed cross-section upper limit to 1.4 pb. This is the first experimental result concerning the search for exclusive production of top quark pairs, and the first upper limit set using PPS data.