UC Berkeley

With the Higgs boson discovery at the Large Hadron Collider, the Standard Model continues to prove its success in particle physics and cosmology. Despite its remarkable achievements, the Standard Model remains incomplete owing to theoretical difficulties such as the hierarchy problem, the strong charge-parity (CP) problem, and the lack of dark matter candidates. Supersymmetry has been proposed to solve the hierarchy problem by protecting the Higgs mass from radiative quantum corrections. In supersymmetry, the lightest supersymmetric particles are well-motivated candidates for dark matter. When the Peccei-Quinn symmetry is evoked to solve the strong CP problem, the axion becomes another viable candidate.

One can attempt to simultaneously solve the aforementioned problems with supersymmetric axion theories. Nonetheless, the scalar superpartner of the axion, the saxion, significantly changes the conventional picture of cosmology and dark matter production. During inflation, a potential for saxions is induced and displaces the saxion field value away from the minimum of the potential today. This results in a saxion condensate that dominates the energy density of the Universe -- a non-standard matter-dominated epoch. The saxion subsequently decays to the particles in the thermal bath, generating a large amount of entropy and diluting the dark matter abundance. The saxion hence plays a crucial role in the cosmological evolution of dark matter. We focus on the cases where dark matter is the axion from the misalignment mechanism, or the axino/gravitino lightest supersymmetric particle that is populated from the thermal scattering and freeze-in processes. The former case allows the Peccei-Quinn symmetry and grand unification to be of the same origin, whereas the latter case allows a high reheat temperature after inflation, solving the axino/gravitino problems. We will also discuss interesting phenomenology for this class of theories such as dark radiation and displaced vertices at colliders.