To the standard model and beyond, A theoretical and experimental study of Leptoquark phenomenology and its traces in LFV $B \rightarrow \tau\mu$ decays in theory and with LHCb

Abstract

In recent years a number of experimental measurements in the field of flavour physics have showed hints towards a source of physics beyond the Standard Model. Theoretical studies have been performed to understand how new physics could account for these measured deviations from the SM. One of the proposed solutions is the existence of leptoquarks, particles coupling quarks and leptons directly. These leptoquarks will inevitably lead to new phenomenology such as lepton flavour violating decays. In this thesis leptoquarks are studied from a theoretical and experimental point of view. In the theoretical studies a high energy theory producing vector leptoquarks at low energy, the 4321-model, is studied in detail and its contributions to the lepton flavour violating $B_{(s)}\rightarrow \tau \mu$ decay are considered. This is followed by the study of a minimal BSM explanation of the flavour anomalies. A $\chi^2$-fit of the parameters of this minimal explanation to the flavour anomalies is performed and it is shown that this minimal model describes the experimental data better than the Standard Model, with a $\Delta \chi^2 = 68.88$ with respect to the Standard Model. Experimentally, the lepton flavour violating decay of $B_{(s)}\rightarrow(\tau\rightarrow \pi\pi\pi\nu_{\tau})\mu$ is studied with the LHCb experiment. First an HLT2 trigger line for this decay is written, to be used in the next data taking run of LHCb. Next a start was made with the search for the $B_{(s)}\rightarrow\tau\mu$ decay in Run 2 data. Potential $\Lambda_b$-baryon backgrounds were studied with RapidSim, an analytic reconstruction of the missing neutrino momentum was performed and a fit was performed on the invariant mass distribution of the $B^{0}\rightarrow D^{-} (\rightarrow K^+\pi^-\pi^-)\pi^+$ normalisation channel. This work shows how in theory leptoquark models can account for the flavour anomalies and give contributions to lepton flavour violating $B$-meson decays. Experimentally this work forms a basis for both the search for $B_{(s)}\rightarrow(\tau\rightarrow \pi\pi\pi\nu_{\tau})\mu$ in Run 2 data as well as for the search in future data runs like Run 3 of the LHCb experiment.