New light neutral particles in meson decays

Abstract

The elementary fermions that so far have been discovered can be divide into two subgroups: leptons and quarks. Quarks,unlike leptons , can have strong interactions. As a result, they cannot be found in nature as free particles but they are always confined in bound states called hadrons. The two well-established groups of hadrons are mesons ,which are composed of valence quark and anti-quark and baryons which are composed of of three valence quarks.In recent years, some exotic hadrons consisting of four valence quarks (tetraquarks) and five valence quarks (pentaquarks) have been registered and the study of their properties is under progress. In this thesis, we focus on mesons, especially on pions, Kaons and B-mesons. We enumerate different species of mesons, categorized based on their quark content (i.e., strangeness, beauty and charm) and their spin. We discuss their well established properties such as lifetime and decay modes within the standard model. We then review CP-violation in neutral Kaon and B-meson systems and how the parameters of the CKM quark mixing can be derived from experiments. We then discuss the recent observations in the B-meson decay modes that deviate from the standard model predictions. These deviations are known as B-anomalies. We review some classes of beyond standard model explanations for these anomalies. We follow with a short discussion of the recently reported KOTO anomaly and its possible explanations.In the end, we show how studying the different decay modes of Kaons and pions can help us to search for new physics beyond the standard model of the elementary particles. In particular we show that if there are new neutral particles with a mass of less than 50 MeV coupled to the neutrinos, a deviation from the standard model prediction for the energy spectrum of the final particles in charged meson decays is expected. We study how such a coupling can be constrained by studying meson decay modes