Scattering

Dark photon

Momentum transfer

Physics

Electron

Dark matter

Optics

Inelastic scattering

Recoil

Atomic physics

Particle physics

Electron scattering

Photon

Nuclear physics

Physics Letters B

Detection of electron recoils by dark matter (DM) may reveal the structure of the dark sector. We consider a scenario where a heavier DM particle inelastically scatters off an electron and is converted into a lighter DM particle. A small mass difference between the two DM particles is transferred into electron recoil energy. We investigate the DM-electron interaction mediated by a massive dark photon and evaluate the inelastic DM scattering rate, taking account of the atomic structure. It is found that the scattering rate is significantly enhanced because of the small mass splitting, which allows for a small momentum transfer matched with the size of the electron wave function. We show that there exists a viable parameter space which explains the excess of electron recoil events around 2-3 keV recently reported by the XENON1T experiment.

Inelastic dark matter electron scattering and the XENON1T excess