Baryogenesis

Explaining the origin of the slight excess of matter over antimatter in the universe remains one of the important, unsolved problems in particle physics, nuclear physics and cosmology. The so-called baryon asymmetry of the universe (BAU) is characterized by the ratio of the baryon number density to the photon entropy density, YB. We know from studies of the cosmic microwave background and big bang nucleosynthesis that YB is of order 10-10. Although this is a tiny number, it is hard to understand why YB differs from zero if the universe possessed equal amounts of matter and antimatter at its birth. Sakharov noted that if the universe began with symmetric initial conditions, then the particle physics interactions that took place as the universe evolved would have to contain three ingredients: (1) baryon number violation, (2) violation of C- and CP-symmetry, and (3) a departure from thermal equilibrium (the last criterion can be evaded if CPT symmetry is violated).

Although the electroweak interactions of the Standard Model (SM) satisfy all three of the Sakharov criteria, the electroweak phase transition is not sufficiently strongly first-order and the CP-violating effects are not sufficiently pronounced to account for as large a BAU as we observe. Consequently, there must have been additional physics beyond the SM to produce it. This physics could have been operative anywhere between the weak scale and the GUT scale. A variety of scenarios have been proposed, with each corresponding to baryogenesis occurring at a different scale. At present, however, we have no convincing evidence for any particular scenario.

From the standpoint of testability, weak scale baryogenesis is a particularly attractive possibility. During the next several years, experiments at the Large Hadron Collider (LHC) at CERN will probe for new physics at this scale. Concurrently, a new generation of searches for the permanent electric dipole moments (EDMs) of leptons, nucleons, and atoms will probe for CP-violating effects that could be associated with weak scale baryogenesis. If theoretically robust calculations of YB can be performed, then the LHC and EDM experiments can be used to determine whether or not electroweak baryogenesis is a viable alternative for explaining the BAU. If not, then we will have to pursue more speculative possibilities, such as GUT scale baryogenesis or leptogenesis.

Theoretically, my collaborators* and I have been developing refined computations of YB using the techniques of non-equilibrium quantum field theory (NEQFT). A key element in these computations is deriving the equations that govern the quantum transport of various particle densities during the electroweak phase transition. The solutions to these equations depend on a subtle competition between CP-conserving and CP-violating terms. Our group has been the first to compute both the CP-conserving terms as well as the CP-violating sources using NEQFT - a project that is partially completed and on going. We have also shown how the presence of a hierarchy of physical scales in weak scale baryogenesis allows one to derive the transport equations using a systematic expansion of the Schwinger-Dyson equations in powers of scale ratios. This observation allows us to estimate the theoretical uncertainties associated with the computation of YB - an important aspect of testing electroweak baryogenesis with experiment. To be concrete, we have used the Minimal Supersymmetric Standard Model (MSSM) as an illustrative case. However, our methods are general and we plan to apply them to other scenarios for new weak scale physics.