2021-10-27 Seminar Report

Among the varieties of dark matter (DM) candidates, the so-called WIMP (an abbreviation of the weakly interacting massive particle) is famous for its beautiful mechanism to achieve the current DM density. In the early Universe, WIMP was in the thermal bath of the Standard Model (SM) particles. In this stage, (i) DM+DM->SM+SM, (ii)SM+SM->DM+DM interaction as well as (iii)DM+SM-> DM+SM interaction occur frequently enough. The strategy searching for each process corresponds to indirect, collider, and direct detection experiments. The cross-section (i.e., the interaction rate) of the above processes correlates with each other. The dropping off of the interaction rate of the process (i) below to the Hubble expansion rate of the Universe fixes the number density of DM particles. We need only two parameters in this WIMP freeze-out scenario: mass and the interaction cross-section. Nowadays the constraints from direct detection experiments are so severe that WIMP in the low mass range of m

However, such constraints could be evaded by considering a different mechanism for fixing the particle number density. If DM interacts as (i)' DM+DM->DM+SM, which is a process referred to as co-annihilation, the cross-section of the process (iii) could be suppressed in a velocity-dependent way. A concrete example introduced in this talk is the one considering the SM particle in the process (i)' as the neutrino. From the momentum conservation, DM accumulated in the Sun annihilates to produce boosted DM and neutrino, hence we expect a double-peak spectrum in large-volume neutrino experiments for this case. The solution to the core-cusp problems of usual WIMP and the origin of the neutrino mass, which is another important problem in the Standard Model, are also within the focus of this story.

The world of DM is not closed on its own. It should be a key to understanding nature and obtaining a picture of our Universe!

Reported by Nagisa Hiroshima

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