Phenomenology of TeV-scale leptoquarks at the large hadron collider: Current status and future prospects

Abstract

Despite its successful predictions, the Standard Model (SM) is not a complete theory. There are theoretical issues and experimental observations that it cannot explain. Over the years, experimental measurements of several observables have shown deviations from their SM-predicted values. For instance, the latest measurement of the anomalous magnetic moment of the muon (g − 2)µ by Fermilab shows a deviation of 4.2σ. There is a discrepancy of 7σ between the SM value and the experimentally observed value of the W boson mass as reported by the CDF collaboration. The SM shows lepton-flavour universality (LFU), i.e., the couplings of leptons to the gauge bosons are flavour independent in the SM. However, the measurements of the leptonic decays of B-mesons at BaBar, Belle, and LHCb indicate some LFU violations. These deviations are popularly referred to as B-anomalies—those seen in the RD(∗) and RK(∗) observables are some of the well-known examples. Until recently, the experimental values of RD and RD∗ exceeded their SM predictions by 1.4σ and 2.5σ, respectively. On the other hand, the deviations in the RK and RK∗ measurements were smaller than the theoretical predictions by about 3.1σ. The current measurement of RD still stands 2σ away from the SM, putting the global average of RD(∗) (combined) at 3.2σ. The deviations in the measurement of (g −2)µ, W boson mass anomaly and the B-anomalies led to numerous beyond the Standard Model (BSM) explanations in the literature. Among these, the most popular ones involve a class of hypothetical particles known as the Leptoquarks (LQs). LQs are scalar (sLQ) or vector (vLQ) bosons that are electromagnetically charged. They are colour triplets with nonzero lepton numbers. They can form various weak representations—singlet, doublet or triplet. They appear in theories such as R-parity violating Supersymmetry, SU(5) Grand Unified Theories, Pati-Salam models, Technicolour models, etc. The LHC has an active LQ-search program. Their connections to the lepton and colour sector make them ideal candidates for resolving the above anomalies. In this thesis, we study the phenomenology of TeV-scale LQs, especially of the models proposed to address the anomalies and investigate their current status. We obtain the latest bounds from low-energy observables and the latest LHC data. Many of these LQ models involve sizable cross-generational LQ-lepton-quark couplings, leading to exotic decay modes of LQs. We estimate the discovery/exclusion prospects of all possible LQs in some interesting or new channels at the high-luminosity LHC (HL-LHC)