Abstract
Dark matter can scatter and excite a nucleus to a low-lying excitation in a direct detection experiment. This signature is distinct from the canonical elastic scattering signal because the inelastic signal also contains the energy deposited from the subsequent prompt de-excitation of the nucleus. A measurement of the elastic and inelastic signal will allow a single experiment to distinguish between a spin-independent and spin-dependent interaction. For the first time, we characterise the inelastic signal for two-phase xenon detectors in which dark matter inelastically scatters off the 129Xe or 131Xe isotope. We do this by implementing a realistic simulation of a typical tonne-scale two-phase xenon detector and by carefully estimating the relevant background signals. With our detector simulation, we explore whether the inelastic signal from the axial-vector interaction is detectable with upcoming tonne-scale detectors. We find that two-phase detectors allow for some discrimination between signal and background so that it is possible to detect dark matter that inelastically scatters off either the 129Xe or 131Xe isotope for dark matter particles that are heavier than approximately 010 GeV . If, after two years of data, the XENON1T search for elastic scattering nuclei finds no evidence for dark matter, the possibility of ever detecting an inelastic signal from the axial-vector interaction will be almost entirely excluded.
Highlights
Background ratesOur ultimate aim is to assess the discovery potential of the inelastic signal
The canonical search for dark matter with direct detection experiments is for an elastic scattering process where the dark matter causes the nucleus to recoil
The inelastic scattering rate does not have the nucleon-number-squared enhancement (∼ 104) found with elastic spin-independent interactions so the inelastic signal will only be measurable for spin-dependent interactions, whose elastic scattering rate does not have the nucleon-number-squared enhancement
Summary
We first review the usual formalism for elastic scattering of dark matter with a xenon nucleus in terms of the recoil energy of the nucleus. We show that this is extended to the case of inelastic scattering. Xenon detectors do not directly measure the energy but rather the scintillation light. We describe our modelling of the generation and detection of the scintillation light, which is based on the NEST formalism [33,34,35,36]. We describe the properties of present and upcoming tonne-scale direct detection experiments and discuss the observable signals and their rate
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