Exclusive production of $J/\psi$ and $\phi$ mesons at the LHCb experiment

Abstract The LHCb detector, which is one of the experiments at the Large Hadron Collider (LHC), is a forward arm spectrometer that was built specifically with studies of CP violation and b-physics in mind. This analysis uses data from proton-proton collisions at a center of mass energy of 13 TeV taken during runs of the LHC from 2016 to 2018. Central Exclusive Production (CEP), in which the centrally produced system is separated from the intact outgoing protons by rapidity gaps, is of interest because of its clean signature. It is also possible to predict CEP processes in theory which can be tested experimentally or vice versa. The low number of collisions per bunch crossing inside the LHCb detector creates an ideal environment for the detection of CEP. Additionally, LHCb is equipped with great track reconstruction and particle identification systems which in combination with its triggers allow for the isolation of low multiplicity events. LHCb has in the past analysed inclusive $J/\psi\phi$ production, although its cross section was not measured. Abstract $J/\psi$ and $\phi$ candidates are selected in low multiplicity events through their decay to two muons and two kaons respectively. This means the selection of events with exactly four well-defined, i.e. fully reconstructed and identified, tracks in the final state and no further activity in the detector. With these selection requirements 1360$\pm$37 $J/\psi\phi$ events are found in the sample. Background contributions to the mass spectrum due to kaon misidentification as well as the signal purity are determined from data. The detector efficiencies are calculated using MC simulation and data and the single interaction Luminosity is determined from data using the fact that the average number of interactions per event is known. The cross section for $J/\psi\phi$ production is determined inside the limits in which LHCb is sensitive in which requires all tracks to have a pseudorapidity of $2<\eta<5$ and transverse momenta for the muons and kaons of $p_{T_{\mu}}>400$ MeV and $p_{T_{K}}>100$ MeV. The cross section result is $\sigma_{J/\psi\phi\rightarrow\mu^{+}\mu^{-}K^{+}K^{-}}=6.0\pm0.2(\textrm{stat})\pm1.0(\textrm{syst})\ \mathrm{pb}$ Abstract The fraction of exclusively produced events, i.e. events where no proton dissociation took place, which is usually difficult to determine because the protons move closely along the beam line, is determined using LHCb's new forward shower counter HeRSCheL. This shows that the sample is 14$\pm$2% exclusive and hints at a production mechanism that regularly induces proton dissociation, such as Double Pomeron Exchange. The mass spectrum is analysed and shown to be inconsistent with a pure continuum production.