Search for axion-like particles in ultraperipheral PbPb collisions at the LHCb experiment

The Standard Model (SM) of particle physics is currently the most accepted theory for describing the fundamental interactions among elementary particles. However, the model alone fails to explain several phenomena, from neutrino oscillations to dark matter. One current puzzle of the SM is the so-called strong CP problem. Although Charge-Parity (CP) symmetry is known to be violated by the weak interactions, it is conserved by the strong interaction to a high precision. Nevertheless, the SM Lagrangian has a term that could allow strong-induced CP violation, and only an extreme fine-tuning would prevent this effect. The solution could come with the introduction of a new particle, the axion. This idea was later generalized into a broader class of particles known as axion-like particles (ALPs). Despite extensive searches, no evidence of ALPs has been observed so far, with only upper limits on their coupling constants being reported. This dissertation describes the search for ALPs decaying into a pair of photons in ultraperipheral PbPb collisions at a center-of-mass energy of 5.02 TeV. The data corresponds to an integrated luminosity of 0.2 nb(-1) , collected by the LHCb experiment. The forward configuration of the LHCb detector is well-suited for investigating low-mass ALPs, a challenging range for other detectors. Photon candidates are reconstructed and identified using information from the electromagnetic calorimeter. Our analysis covers an ALP mass range from 3 to 10 GeV and ALP-photon couplings from 10(-1) to 1 TeV(-1) . Although our exclusion region is small and overlaps with recent analyses, this study demonstrates the potential, in run 3 and beyond, for LHCb to contribute to ALP searches in ultraperipheral collisions.