Probing sub-GeV leptophilic dark matter at Belle II and NA64

Abstract

We study the Belle II constraints on the sub-GeV dark matter that interacts with charged leptons. Two kinds of models with different interaction mechanisms between dark matter and charged leptons are considered in the analysis: the EFT operators and the light mediator models. The Belle II monophoton constraints on the EFT operators are found to be of similar size as the LEP constraints. However, for the light mediator models, the Belle II mono-photon constraints can be much stronger than the LEP limits. For both EFT operators and the light mediator models, the Belle II limits can be several orders of magnitude stronger than the current dark matter direct detection limits, as well as the white dwarf limit. The light mediator can also be searched for in the leptonic decay final states, which is complementary to the mono-photon channel and can even become the better discovery channel for certain parameter space.