Supersymmetry, from baryogenesis at the electroweak phase transition to low-energy precision experiments

Abstract

Electroweak-scale supersymmetry is one of the most popular extensions of the Standard Model and has many important implications for nuclear physics, particle physics, and cosmology. First, supersymmetric electroweak baryogenesis may explain the origin of the matter-antimatter asymmetry in the universe. In this scenario, electroweak symmetry is broken in the early universe by a first-order phase transition, when bubbles of broken phase nucleate and expand, eventually consuming the unbroken phase. Charge density is generated within the expanding bubble wall and diffuses into the unbroken phase. Through inelastic collisions in the plasma, this charge is partially converted into left-handed quark and lepton charge, which in turn leads to the production of baryon number through weak sphaleron transitions. In this work, we study these charge transport dynamics, from its generation within the bubble wall, to the final baryon asymmetry. We evaluate which collisions are important for baryogenesis, and what is their impact upon the final baryon asymmetry. Our main result is that bottom and tau Yukawa interactions, previously neglected, can play a crucial role, affecting the magnitude and sign of baryon asymmetry. We investigate how this works in detail in the Minimal Supersymmetric Standard Model (MSSM); we suggest that these interactions may be even more important in gauge-singlet extensions of the MSSM. Second, low-energy precision measurements of weak decays may provide interesting signals of supersymmetry. We study in detail the supersymmetric radiative corrections to (i) leptonic pion decay branching ratios, and (ii) muon and beta decay coefficients. A deviation from the Standard Model predictions would imply strong departures from the minimal, commonly-assumed, theoretical assumptions about supersymmetry breaking.