<em>C</em> and <em>CP</em> Violation in Light-Meson Decays

Abstract

This dissertation delves into the systematic parameterization of a simultaneous violation of C and CP in the light-meson sector, i.e., a source of CP violation that has been mostly disregarded over the past six decades. In order to uniquely identify these signals for new physics, which may, for instance, contribute to the matter–antimatter asymmetry, we focus on decays of the η meson. Throughout, we intuitively consider the Standard Model as an effective low-energy approximation of an unknown underlying theory at some high-energy scale Λ and rely on model-independent approaches of effective field theories and dispersion theory in analogy to their well-established applications in strong interactions, however, without making any assumption on possible new fundamental forces. <br /> We start our analysis by revisiting fundamental neutrinoless quark-level operators in the spirit of Standard Model effective field theories up to and including mass dimension 8, which violate C and CP and conserve flavor, lepton-, and baryon-number. Providing the first complete set of these operators, we find that chirality-violating and -conserving ones are suppressed by v/Λ⁴ or 1/Λ⁴, with Higgs vev v, respectively. Subsequently, we carefully translate these quark-level operators to chiral perturbation theory, relying on well-established spurion techniques, in order to access interactions in the light-meson sector and apply the thereby obtained new effective quantum field theory to more than 20 decays in total. As our key results, we present respective observables in explicit dependence on Λ and identify hierarchies in the chiral power counting as well as possible correlations. <br /> We complement these findings with a dispersion-theoretical analysis of the C- and CP-violating decays that appear at lowest order in the chiral expansion and their correlations. The most promising candidates to find evidence for C and CP violation are the hadronic three-body decays η^(′) → π⁺π^-π⁰ and η′→ ηπ⁺π^-. Taking final-state interactions non-perturbatively into account, we study the asymmetries in the momentum distribution of the charged pions and determine the isoscalar and isotensor transitions in η^(′) → π⁺π^-π⁰ and the isovector one in η′→ ηπ⁺π^-, whose leading-order coupling constants are matched to the previously derived effective field theory. The remaining decays found to contribute at lowest chiral order are η → π⁰ℓ⁺ℓ^- and η′ → ηℓ⁺ℓ^-. We approach these with dispersion theory, by using the hadronic three-body decays addressed above to predict hadronic contributions to the C- and CP-odd η → π⁰γ* and η′ → ηγ* transition form factors. This strategy allows us to correlate C and CP violation in different decays in a non-perturbative manner. A combination of both of these measurements allows us to tighten the constraints on C and CP violation in η → π⁺π^-π⁰. <br /> With a variety of possible extensions, this thesis provides a dedicated theoretical framework to interpret hypothetical future findings, cf. the JEF or REDTOP collaborations, properly.