Baryon-number violation in chiral effective field theory
Baryon number (B) is a quantum number assigned to baryons (B = 1), with the proton and the neutron being the lightest baryons, and to their antiparticles (B = −1). In all physical processes observed so far B has been found to be conserved. It is however not associated to a gauge symmetry in the Standard Model (SM) and it can therefore be violated by beyond-the-SM (BSM) physics. B violation can effectively and model-independent be described by extending the SM with higher-dimensional operators composed of SM fields and satisfying the SM gauge symmetry.
In this thesis we use these higher-dimensional operators as a starting point for describing B violation in chiral effective field theory (χEFT), the low-energy effective field theory of quantum chromodynamics with pions and nucleons as effective degrees of freedom. We construct a χEFT description for |∆B| = 2 interactions. We calculate the neutron-antineutron oscillation time as function of the pion mass and unknown constants that depend on hadronic matrix elements and BSM physics. Furthermore, we determine the ∆B = 2 deuteron decay rate and we relate it to the neutron-antineutron oscillation time. For describing ∆B = 1 nucleon and deuteron decay we introduce imaginary parts for low-energy constants accompanying B-conserving operators, which generate inclusive decay rates. We show the ∆B = 1 deuteron decay rate is dominated by the sum of the free proton and neutron decay rate. The first nuclear corrections we expect at the few-percent level.
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