4D Seminar — Kevin Langhoff (UC Berkeley/LBNL) “The Irreducible Axion Background”

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Constraints on decaying dark matter are extremely strong. However, if one relaxes the condition that a given species of particle makes up all of dark matter, existing searches for decaying dark matter can be converted into searches for other BSM particles which make up only a fraction of the dark matter relic abundance. For many light BSM particles there exists a freeze-in relic abundance which is irreducible under any modification to standard cosmology before BBN. This irreducible abundance allows one to set very strong constraints which are robust to modifications in standard cosmology before BBN. In this talk, I will apply this idea to axions.