Right-handed Neutrinos from Leptoquark Decays: the Monolepton Channel at the Large Hadron Collider

Abstract

The Standard Model of particle physics explains the particle content of matter and their interactions at the fundamental level. While the theory predicts and explains most subatomic events quite accurately, it has some limitations; also, some anomalies have been observed. Consequently, various theories beyond the standard model have been developed. Leptoquarks (LQs) and Right-handed Neutrinos (RHNs) are two hypothetical particles appearing in many of these new theories. They are well-known for their potential to address multiple issues. Experiments at the Large Hadron Collider (LHC) have put bounds on their parameters (if they exist) with certain assumptions. However, no direct experimental constraints exist on Leptoquark (LQ) couplings with quarks and right-handed neutrinos (RHNs) yet. If the RHNs are lighter than the LQs, they can be produced from the LQ decays. This simultaneous probe for RHNs and the LQs that dominantly couple to RHNs has never been searched for in experiments. In this thesis, we investigate the signals arising from the decays of Right-Handed Neutrinos (RHNs) coming from the decays of a scalar Leptoquark (LQ) that dominantly couples to RHNs. Our investigation covers both pair and single productions of the LQ and also RHN pair production through t-channel LQ exchange. These typically lead to two RHNs which can subsequently decay to multiple charged leptons and jets. While the dilepton final state is the most promising, our analysis centres on the more challenging monolepton final state to compute the discovery and exclusion reach of LQs and RHNs at the LHC. Here, we consider a scalar Leptoquark with an electromagnetic charge of either 1/3 or 2/3 (referred to as ϕ1 and ϕ2 respectively), primarily coupling to second-generation quarks and RHNs. We discuss the Standard Model and some Beyond Standard Model theories in chapter 1. We discuss particle accelerators and their simulation software in chapter 2. We analyse the monolepton final state to estimate the discovery reach at the high-luminosity LHC in chapter 3. In chapter 4, we visualise the distributions of various parameters and then look at them collectively to maximise the discovery reach. We present the results of the work in chapter 5. In chapter 6, we digress from the main topic and study a computation technique that can be used to automatically compute the bounds on LQ parameters from the current LHC data. We discuss an alternative methodology to compute the test on a contingency table where the table elements involve couplings as variables. These individual elements are summed up to form polynomials, and the polynomial form of χ2is computed. This allows for using computational techniques (like gradient descent) that cannot be utilised if the value of χ2 is computed using the regular tabular method at discrete values of the couplings.