Exploring new physics with O(keV) electron recoils in direct detection experiments

Abstract

Motivated by the recent XENON1T results, we explore various new physics models that can be discovered through searches for electron recoils in $$ \mathcal{O} $$ O (keV)-threshold direct-detection experiments. First, we consider the absorption of axion-like particles, dark photons, and scalars, either as dark matter relics or being produced directly in the Sun. In the latter case, we find that keV mass bosons produced in the Sun provide an adequate fit to the data but are excluded by stellar cooling constraints. We address this tension by introducing a novel Chameleon-like axion model, which can explain the excess while evading the stellar bounds. We find that absorption of bosonic dark matter provides a viable explanation for the excess only if the dark matter is a dark photon or an axion. In the latter case, photophobic axion couplings are necessary to avoid X-ray constraints. Second, we analyze models of dark matter-electron scattering to determine which models might explain the excess. Standard scattering of dark matter with electrons is generically in conflict with data from lower-threshold experiments. Momentum-dependent interactions with a heavy mediator can fit the data with dark matter mass heavier than a GeV but are generically in tension with collider constraints. Next, we consider dark matter consisting of two (or more) states that have a small mass splitting. The exothermic (down)scattering of the heavier state to the lighter state can fit the data for keV mass splittings. Finally, we consider a subcomponent of dark matter that is accelerated by scattering off cosmic rays, finding that dark matter interacting though an $$ \mathcal{O} $$ O (100 keV)-mass mediator can fit the data. The cross sections required in this scenario are, however, typically challenged by complementary probes of the light mediator. Throughout our study, we implement an unbinned Monte Carlo analysis and use an improved energy reconstruction of the XENON1T events.

Ultralight scalars in leptonic observables

“…In particular, its mass and couplings are assumed to be the same as in vacuum. Some mechanisms have been recently proposed [46,47] (see also previous work in [48]) that would make this assumption invalid. These works are mainly motivated by the recent XENON1T results, which include a 3.5 σ excess of low-energy electron recoil events [49].…”

Sun heated MeV-scale dark matter and the XENON1T electron recoil excess

“…These operators all originate from a heavy mediator whose mass is much heavier than the typical momentum transfer. The latter two q-dependent form factors can be derived, for example, from dimension six operators with scalar-pseudoscalar interactionχγ 5 χēe and pseudoscalar-pseudoscalar interactionχγ 5 χēγ 5 e, respectively [24].…”

“…For a given UV-completed model to realize the form factors, one requires the cutoff scale to be large enough to avoid the constraints from collider results [36]. For F = q αme , a dimension five operator φ * φēγ 5 e representing the interaction between scalar DM and electron spin, is discussed in [24] for m DM 100 GeV where the cutoff scale can be hundreds of GeV that is consistent with both collider and electron Electric Dipole Moment (EDM) constraints. In our case, the lower dark matter mass can lead to an even larger cutoff around 10 6 GeV [24].…”

“…For F = q αme , a dimension five operator φ * φēγ 5 e representing the interaction between scalar DM and electron spin, is discussed in [24] for m DM 100 GeV where the cutoff scale can be hundreds of GeV that is consistent with both collider and electron Electric Dipole Moment (EDM) constraints. In our case, the lower dark matter mass can lead to an even larger cutoff around 10 6 GeV [24]. On the other hand, for F = ( q αme ) 2 , if one takes the dimension six operator χγ 5 χēγ 5 e, the cutoff scale is always lower than 1 GeV for the parameters in eq.…”

Exothermic dark mesons in light of electron recoil excess at XENON1T

“…Pseudo-Dirac fermion dark matter [6] or complex scalar dark matter [7] with a dark U(1) symmetry have been proposed to explain the XENON1T excess. In these scenarios for exothermic dark matter, it is common to take dark matter and mediator particles to be light below GeV scale [8][9][10][11][12], belonging to the category that has been of growing interest in recent years as alternatives to WIMP dark matter.…”