Search for beyond Standard Model processes in the muon + missing transverse momentum final state with LHC Run 2 data recorded by the CMS experiment

<!--HTML-->The curiosity that characterises humankind has driven us to a collective effort to understand Nature, from the exploration of the Universe to the study of the inner structure of matter. Our knowledge regarding the most fundamental pieces of matter has been gathered in a theoretical model, the Standard Model (SM), accurately describing the constituents of matter: the particles and their interactions. Despite having an, experimentally proven, predictive power, the SM does not provide a description for all the phenomena observed in Nature. Physicists firmly believe,driven both by theoretical calculations and experimental facts, that a more complete theory must lie beyond our current SM. The SM would thus be an effective theory valid at energies currently under tests. How far in energy reach does the SM validity extends is something not yet clear. In this context, this doctoral thesis presents a work aimed to search for processes Beyond the Standard Model (BSM), predicted in several theoretical models, in an effort to complete the SM.<br>The LHC is the biggest and most energetic particle accelerator ever built, and therefore, one of the most promising places to search for new particle physics processes. This work is based on the analysis of proton-proton collision data at the center-of-mass energy of 13 TeV, collected with the CMS detector during 2016, 2017and 2018 (so-called Run 2 period) at the LHC, corresponding with a total integrated luminosity of 137 fb$^{-1}$. The final state with an energetic muon and apparent transverse momentum imbalance, is chosen since it is a general final state in which many hypothetical new signals could end up, thus offering many possibilities and interpretations of the experimental results. The theoretical context and the BSM scenarios considered are presented in Chapter 1, while the experimental setup, the LHC collider and the CMS experiment is explained in Chapter 2.<br>Muons, and in particular energetic ones, are a very clean experimental signature,relatively easy to be detected and reconstructed, and with a high accuracy and efficiency. Thus final states containing muons are among the best ones to carry out inclusive searches. In this work the search is oriented towards new massive particles, foreseen in many theoretical models of BSM. These new particles are assumed to decay in a light lepton plus an undetected particle, making the high momentum muons one of the most important ingredients of this work. The detailed study of high momentum muons is presented in Chapters 3 and 4.<br>The strategy followed in this work consists in looking for deviations in the data with respect to the prediction of the different SM processes contributing to the signal definition.<br>The deviation might show up as a resonance in the reconstructed mass of the muon and the missing transverse momentum, if the collider provides sufficient energy to produce it directly. Or, in case the new particle is above the kinematic limit, the deviation could still be visible indirectly, in the interference region between the SM particles masses and the new massive ones. The analysis strategy and scientific results and their interpretations are presented in Chapters 5 and 6.<br>Among the different models that predict the existence of a resonance we explore the following: a heavy analogue of the SM W boson, a W’ as predicted by the Sequential Standard Model (SSM), a model including additional spatial dimensions and SM partners, W$_{KK}^{(2)}$ boson as predicted by the Split Universal Extra Dimensions model, or a new supersymmetric particle as predicted by R–Parity Violation Supersymmetry models.<br>In a more general frame, by studying any possible deviation in the high mass region we measure the $W$ and $Y$ oblique electroweak parameters, indicators of any BSM effect. Finally, if the Higgs boson is a composite particle, it would also manifest as a resonance or a deviation in this final state. Therefore, the compositeness of the Higgs boson is also explored.<br>All of them have been explored during this doctoral work, with the aim of contributing to the exhaustive task to increase our knowledge of Nature.