A measurement of ultra-peripheral ρ0 meson production in heavy ion collisions using the LHCb detector

This thesis investigates the Central Exclusive Production (CEP) of the ρ0 vector meson using lead-lead ion collision data at a centre-of-mass energy of 5.02 TeV, collected by the LHCb detector during the LHC’s Run II in 2018. We present the results as functions of rapidity and mass of the di-pion system. In this exclusive process, only one or two new particles are produced while the colliding particles remain intact, allowing for a fully determined final state. The clean signature of CEP, characterized by the centrally produced system being separated from the intact outgoing ions by rapidity gaps, makes it particularly interesting. The low number of collisions per bunch crossing inside the LHCb detector creates an ideal environment for detecting these events. Additionally, LHCb’s excellent track reconstruction and particle identification systems, combined with its triggers, allow for the solation of low multiplicity events. In the process studied in this thesis, the lead ion scatters with a quasi-real photon, producing a vector meson. This meson is placed on-mass-shell by exchanging vacuum quantum numbers via the pomeron trajectory. The mass of the produced vector meson sets the energy scale, providing insight into the transition between soft and hard QCD regimes. By fitting the di-pion invariant mass distribution, we determine the mass and width of the ρ meson, showing broad consistency with S¨oding’s model at low masses. At higher masses, a clear resonance structure appears at 1.7 GeV. The analysis measures hadronic and photoproduction cross-sections across twelve bins of rapidity ranging from 2.05 to 4.9, with an average cross-section of 495 ± 35sysmb. The nuclear suppression values are found to be S+IA = 0.010 ± 0.10sys and S− IA = 0.45 ± 0.06sys using the impulse approximation, and S+ Glauber = 0.060 ± 0.40sys and S−Glauber = 0.85 ± 0.05sys using the Glauber model.