Abstract
Earth's rotation about the Sun produces an annual modulation in the expected scattering rate at direct dark matter detection experiments. The annual modulation as a function of the recoil energy ER imparted by the dark matter particle to a target nucleus is expected to vary depending on the detector material. However, for most interactions a change of variables from ER to vmin, the minimum speed a dark matter particle must have to impart a fixed ER to a target nucleus, produces an annual modulation independent of the target element. We recently showed that if the dark matter-nucleus cross section contains a non-factorizable target and dark matter velocity dependence, the annual modulation as a function of vmin can be target dependent. Here we examine more extensively the necessary conditions for target-dependent modulation, its observability in present-day experiments, and the extent to which putative signals could identify a dark matter-nucleus differential cross section with a non-factorizable dependence on the dark matter velocity.
Highlights
Despite being the dominant form of matter in the Universe, the exact nature of the dark matter (DM) is still unknown
One of the most well-motivated candidates for DM is a particle with few GeV to hundreds of TeV mass and weak-scale interactions, referred to as a weakly interacting massive particle (WIMP)
This implies that inelastic magnetic DM scattering with these elements will always lead to an observation of tmax between late May and early June, and the annual modulation will be consistent with inelastic scattering through differential cross sections that are independent of velocity
Summary
Despite being the dominant form of matter in the Universe, the exact nature of the dark matter (DM) is still unknown. For most interactions, some observables associated with the annual modulation like the modulation fraction or the time of maximum and minimum signal, tmax and tmin, do not depend on the target nuclide when expressed as functions of vmin. This is the minimum speed a DM particle must have in Earth’s frame to impart a recoil energy ER on a target nucleus. We pointed out in [17] that when the DM-nucleus differential cross section has a nonfactorizable velocity dependence, as for DM interacting through a magnetic dipole or an anapole moment, tmax and tmin are no longer target-independent functions of vmin.
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