Jan Stefan Heisig, University of Virginia

"From coannihilation to conversion-driven freeze-out: Dark matter genesis beyond the WIMP paradigm" 

Abstract: The nature of dark matter poses one of the most pressing questions in fundamental physics today. Thermal freeze-out of a weakly interacting massive particle (WIMP) has proved to be a successful framework for explaining the measured dark matter abundance in the Universe. However, the sizeable couplings of dark matter to the standard model particles required in its simplest realizations have been put under severe pressure by experimental null-results at colliders, direct and indirect detection experiments. Hence, fulfilling the relic density constraint often requires the exploration of ‘exceptional’ regions, e.g., the region where coannihilation effects increase the effective annihilation rate. In this talk, we revisit the assumptions commonly made within the coannihilation scenario and discuss a new variant of dark matter freeze-out, dubbed conversion-driven freeze-out (or coscattering). In this scenario, the relic abundance is set by the freeze-out of conversion processes requiring significantly smaller couplings of dark matter to the standard model. While this parameter region is largely immune to conventional WIMP searches, it predicts the signature of long-lived particles at colliders, making it a prime target for upcoming LHC searches. Conversion-driven freeze-out also provides an interesting link to neutrino physics. Considering sterile neutrino dark matter, we discuss its implications for the active neutrino masses and Yukawa couplings.