Abstract
We study in detail sub-GeV dark matter scattering off electrons in xenon, including the expected electron recoil spectra and annual modulation spectra. We derive improved constraints using low-energy XENON10 and XENON100 ionization-only data. For XENON10, in addition to including electron-recoil data corresponding to about $1-3$ electrons, we include for the first time events with $\gtrsim 4$ electrons. Assuming the scattering is momentum independent, this strengthens a previous cross-section bound by almost an order of magnitude for dark matter masses above 50 MeV. The available XENON100 data corresponds to events with $\gtrsim 4$ electrons, and leads to a constraint that is comparable to the XENON10 bound above 50 MeV. We demonstrate that a search for an annual modulation signal in upcoming xenon experiments (XENON1T, XENONnT, LZ) could substantially improve the above bounds even in the presence of large backgrounds. We also emphasize that in simple benchmark models of sub-GeV dark matter, the dark matter-electron scattering rate can be as high as one event every ten (two) seconds in the XENON1T (XENONnT or LZ) experiments, without being in conflict with any other known experimental bounds. While there are several sources of backgrounds that can produce single- or few-electron events, a large event rate can be consistent with a dark matter signal and should not be simply written off as purely a detector curiosity. This fact motivates a detailed analysis of the ionization-only ("S2-only") data, taking into account the expected annual modulation spectrum of the signal rate, as well as the DM-induced electron-recoil spectra, which are another powerful discriminant between signal and background.
Highlights
Direct-detection experiments play a crucial role in our quest to identify the nature of dark matter (DM), and the last few years have seen intense interest and significant progress in expanding their sensitivity to particles below ∼1 GeV
We study in detail sub-GeV dark matter scattering off electrons in xenon, including the expected electron recoil spectra and annual modulation spectra
The available XENON100 data corresponds to events with about 4–50 electrons, and leads to a constraint that is comparable to the XENON10 bound above 50 MeV for FDM 1⁄4 1
Summary
Direct-detection experiments play a crucial role in our quest to identify the nature of dark matter (DM), and the last few years have seen intense interest and significant progress in expanding their sensitivity to particles below ∼1 GeV. The main factor limiting the sensitivity of XENON10 is the large number of observed S2-only events and the absence of a background model ESSIG, VOLANSKY, and YU events are conservatively assumed to originate from DM) Plausible origins of these events include the photodissociation of negatively charged impurities; field emission from the cathode; and, especially, electrons that are initially created by highly ionizing background events, but become trapped in the liquid-gas interface and spontaneously released at a later time [2,22,23,24]. Dramatic improvements in sensitivity in the near future are likely with SuperCDMS [5,26], SENSEI [27], and possibly other experiments These experiments will initially have target masses of only Oð1 kgÞ, far less than current and future xenon experiments (Table I). The large exposures will allow for an annual modulation analysis [28], which can significantly improve upon the current limit even if the background rates are high
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