Measurements on the production of a Z boson in association with a photon in proton-proton collisions and a search for anomalous neutral triple gauge couplings at the CERN Large Hadron Collider

Abstract

Within the framework of the Standard Model of particle physics, p-p collisions produce the final state of a photon and missing momentum from a Z boson solely through quark exchange in the t and u channels, since direct couplings between the neutral electroweak bosons Z and γ are not permitted at the tree level. However, the presence of new physics can induce the couplings Z Z γ and Z γ γ, and enable the s channel, resulting in an enhancement in the rate of production. This enhancement would be particularly pronounced at the high-energy tail of the photon transverse momentum distribution. In this thesis, I present a search for these anomalous neutral triple gauge couplings (aNTGCs) under an effective vertex model and a measurement of the crosssection of the process pp → Z(→ νν) + γ. The analysis is based on data collected by the CMS detector at the LHC between 2016 and 2018 at a center of mass energy of 13 TeV, corresponding to an integrated luminosity of 138 fb−1 . Moreover, two machinelearning techniques are introduced to enhance the sensitivity of the search. The first is an identification method designed specifically for photons with transverse momentum exceeding 200 GeV. The second is a tagger that suppresses background photons at high pseudorapidities of the detector that originate from Bremsstrahlung radiation emitted by muons within a diffuse halo produced by interactions of the proton beams with the beampipe material. The photon identification method improves the signal selection efficiency by 12% compared to a cut-based approach, while the beam halo tagger extends the pseudorapidity coverage of the photons in the search, allowing for the inclusion of photons in the CMS endcaps that were previously dominated by beam halo background. From the analysis of the distribution of the photon transverse momentum, limits on the strengths of the aNTGCs have been measured.