High Energy / Nuclear Theory / RIKEN seminars

[HET seminar] "Roller Coaster Dark Matter"

by Brian Batell




Ultra-light weakly coupled scalar bosons are a well-motivated and widely studied class of dark matter candidates.  Such bosonic dark matter is generically produced in the early universe through the misalignment mechanism.  The conventional misalignment mechanism depends on the initial scalar field value, which controls the amplitude of oscillations around the true minimum and resulting dark matter abundance at late times.  We propose a `roller coaster' mechanism that dynamically generates the correct initial misalignment angle starting from generic initial conditions after inflation.  The ride begins at high temperatures, when an ultra-weak coupling of dark matter to a fermion in thermal equilibrium causes the minimum of the effective potential to be shifted from its zero-temperature value. The scalar field is lifted towards its potential minimum at large field values, generating misalignment.  As the universe cools, the fermion number density becomes suppressed, the effective scalar mass becomes comparable to Hubble, and the scalar is dropped from its generated misalignment value and begins to oscillate, ultimately behaving as dark matter. We describe the mechanism in detail in a generic toy model and then discuss the phenomenology of a realistic scenario in which the scalar dark matter is coupled to the muon.