We present a dark matter model to explain the excess events in the electron recoil data recently reported by the Xenon1T experiment. In our model, dark matter annihilates into a pair of on-shell particles , which subsequently decay into the final state; interacts with electrons to generate the observed excess events. Because of the mass hierarchy, the velocity of can be rather large and can have an extended distribution, providing a good fit to the electron recoil energy spectrum. We estimate the flux of from dark matter annihilations in the galaxy and further determine the interaction cross section, which is sizable but sufficiently small to allow to penetrate the rocks to reach the underground labs.

We propose to search for millicharged particles in electron colliders operated with the center-of-mass energies at Oð1-10Þ GeV, which include Belle II, BESIII, BABAR, and also the proposed experiment Super Tau Charm Factory (STCF). We use the monophoton final state at electron colliders to probe the parameter space of millicharged particles, that is spanned by millicharge ϵ and mass m. We find that electron colliders have sensitivity to the previously unexplored parameter space for millicharged particles with MeV-GeV mass: ϵ ≲ Oð10 −1 Þ for 0.5 GeV ≲ m ≲ 3.5 GeV in BABAR, ϵ ≲ Oð10 −3 Þ for 0.1 GeV ≲ m ≲ 1.5 GeV in BESIII, ϵ ≲ 10 −3 − 10 −2 for 0.1 GeV ≲ m ≲ 4 GeV in Belle II, and ϵ ≲ Oð10 −4 Þ for 1 MeV ≲ m ≲ 1 GeV in STCF.

An analysis is given of the Belle II sensitivities and NA64 constraints on the sub-GeV Dirac dark matter that interacts with charged leptons. We consider two different types of interactions between sub-GeV Dirac dark matter and the charged leptons: the EFT operators and the light vector mediators. We compute the Belle II mono-photon sensitivities on sub-GeV dark matter with 50 ab−1 data which are expected to be accumulated in the full Belle II runs. Although the Belle II mono-photon sensitivities on the EFT operators are of similar size as the LEP constraints, Belle II can probe new parameter space of the light vector mediator models that are unexplored by LEP. For both the EFT operators and the light vector mediator models, the Belle II mono-photon sensitivities can be several orders of magnitude stronger than the current dark matter direct detection limits, as well as the white dwarf limits. The light vector mediator can also be directly searched for by reconstructing the invariant mass of its di-lepton decay final states at Belle II, which is found to be complementary to the mono-photon channel. We compute the NA64 constraints on the sub-GeV Dirac dark matter and provide analytic expressions of the dark matter cross section in the Weizsäcker-Williams approximation, for the EFT operators, and for the light vector mediator models. We find that the current NA64 data (with 2.84 × 1011 electron-on-target events) provide strong constraints on sub-GeV dark matter. Although the NA64 constraints are found to be about one order of magnitude smaller than the Belle II sensitivities for the EFT operators, NA64 can probe some regions of the parameter space in the light vector mediator models that are beyond the reach of Belle II. We also find that Belle II and NA64 can probe the canonical dark matter annihilation cross section in thermal freeze-out in a significant portion of the parameter space of the models considered.