De Iorio, Agostino (2021) Search for new physics contributions entering the tWb electroweak vertex at 13 TeV with the CMS experiment. [Tesi di dottorato]


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Item Type: Tesi di dottorato
Resource language: English
Title: Search for new physics contributions entering the tWb electroweak vertex at 13 TeV with the CMS experiment
De Iorio,
Date: 15 April 2021
Number of Pages: 217
Institution: Università degli Studi di Napoli Federico II
Department: Fisica
Dottorato: Fisica
Ciclo di dottorato: 33
Coordinatore del Corso di dottorato:
Iorio, Alberto Orso MariaUNSPECIFIED
Date: 15 April 2021
Number of Pages: 217
Keywords: Electroweak physics, top quark physics, GEM detectors, BSM searches, CKM matrix, heavy resonances, single top processes
Settori scientifico-disciplinari del MIUR: Area 02 - Scienze fisiche > FIS/01 - Fisica sperimentale
Date Deposited: 16 Apr 2021 05:26
Last Modified: 07 Jun 2023 10:36

Collection description

The Standard Model (SM) of Particles Physics is the current framework in which the electromagnetic, weak, and strong interactions successfully find an explanation. The SM is verified with high precision for most processes it describes, and all particles it predicted have been now been observed, after the discoveries of the Higgs boson at the LHC in 2012 and of the top quark at the TeVatron in 1994. Since then, intense studies have been performed to measure the features and the properties of such particles and their couplings. The top quark in particular holds a special place in the Standard Model, as it is the most massive elementary particle ever discovered. As a consequence of its mass, it is also characterised by a very short lifetime: the decay occurs before it can hadronise, implying that most of the top quark properties can be directly inferred from the its decay products, as if it was a free particle. Another notable feature of top quark physics is that a distinct hierarchy can be identified in top quark decays: it almost exclusively decays in a W boson and a b quark. Decay modes in a W boson and a d or s quarks are allowed, but they are strongly suppressed, so much that up to now they have never been studied directly. This feature stems from the fact that there is a distinct preference of top quarks for couplings with b quarks via electroweak charged current interactions. The mixing among families is regulated in the SM by a matrix of fundamental parameters named the Cabibbo-Kobayashi-Maskawa (CKM) matrix. Through the study of top quark decays and electroweak couplings it is possible to extract the magnitude of the CKM matrix elements related to the third row, which, according to constraints from low energy measurements, should exhibit a remarkable hierarchical structure. Such hierarchical structure might also be an indication of a new more fundamental underlying physics model. In order to precisely study this sector of the SM, the best tool currently available is the Large Hadron Collider (LHC). The LHC is a circular accelerator designed to provide proton-proton collisions with a luminosity of 10$^{34}$ cm$^{−2}$s$^{−1}$ and a centre-of-mass energy of sqrt(s) = 14 TeV. The LHC is equipped with four main experiments: ALICE, ATLAS, CMS, and LHCb. The aim of LHC is to provide further proof on the validity of the SM and to give clues of new physics at the TeV scale. The first goal of the present thesis work is the first direct measurement of the CKM matrix elements |Vtb|,|Vts| and |Vtd| in events where single top quarks are produced via electroweak charged-current interactions. The main mechanisms for electroweak charged current production of single top quarks is the t−channel. The data analysed correspond to an integrated luminosity of 35.9 fb$^{−1}$ of proton-proton collisions at a centre-of-mass energy of sqrt(s) = 13 TeV collected with the CMS experiment at the LHC during 2016. The performed analysis explicitly probes in great detail the left-handed only interactions at the tWb vertex at energy regimes around the electroweak scale. New physics might rise in right-handed couplings, or manifest at higher energy regimes, both indirectly or directly via new resonances. There is experimental evidence, both in particle physics and in astrophysics observations, suggesting that the SM in not the ultimate fundamental theory. Many physics models have been proposed to extend it in a more general picture, so to provide an explanation to such phenomena, and several of them predict new particles that could have a mass larger than the one of the top quark. The top quark could therefore have a privileged relationship with new physics particles and play a crucial role in their discovery. The LHC is the perfect tool to perform direct searches for new particles thanks to the high-energy collisions and the large number of top quark produced. In the present work a search for a beyond the SM W′ boson decaying in tb quarks in leptonic final states is presented. Data from proton-proton collisions with a centre-of-mass energy of sqrt(s) = 13 TeV, corresponding to 137.2 fb$^{−1}$, collected by the CMS experiment at the LHC from 2016 to 2018 are analysed. An upgrade of the LHC accelerator complex in the next years will allow to significantly increase the collision rate, further improving the results obtained so far and effectively extending the physics reach of the machine. The CMS experiment foresees a series of detector upgrades to cope with the new challenging conditions. Part of this work documents the testing and validation of the Gas Electron Multiplier chambers that are part of the Muon System upgrade.


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