Search for Soft Unclustered Energy Patterns at the Large Hadron Collider and studies of radiation damage in plastic scintillators

Abstract

Data collected by the Compact Muon Solenoid experiment at the Large Hadron Collider, which collides protons at a center-of-mass energy of approximately 14 TeV, enable tests of the Standard Model of Particle Physics and searches for physics beyond the Standard Model. This thesis presents studies of radiation damage to plastic scintillators, used to produce signals in the CMS calorimeter system, and a search for a beyond-the-Standard-Model particle, the Soft Unclustered Energy Pattern (SUEP). The effects of ionizing radiation on the light output and the optical properties of plastic scintillators are assessed for a variety of materials, dopant concentrations, fluors, antioxidant concentrations, sample thicknesses, doses, and dose rates. Depending on the dose rate, the samples exhibit internal regions with different refractive indices separated by a visible boundary. Measurements of the boundary depth indicate compatibility with the expected oxygen penetration depth during irradiation. The refractive indices of the internal regions are higher than those of the outer regions, which match the indices of unirradiated samples. Dark sectors with a large t'Hooft coupling can give rise to SUEP, which are final states characterized by large charged particle multiplicities. A search for SUEP producing muons in the final state is also presented. The expected 95% CL upper limits for the cross sections indicate that, once unblinded, the search will be able to set upper limits about 3-4 orders of magnitude lower than the theory predictions, meaning that it is capable of either excluding the theory or discovering new physics.