Readout strategies for directional dark matter detection beyond the neutrino background

Abstract

The search for weakly interacting massive particles (WIMPs) by direct detection faces an encroaching background due to coherent neutrino-nucleus scattering. As the sensitivity of these experiments improves, the question of how to best distinguish a dark matter signal from neutrinos will become increasingly important. A proposed method of overcoming this so-called "neutrino floor" is to utilize the directional signature that both neutrino and dark matter induced recoils possess. We show that directional experiments can indeed probe WIMP-nucleon cross-sections below the neutrino floor with little loss in sensitivity due to the neutrino background. In particular we find at low WIMP masses (around 6 GeV) the discovery limits for directional detectors penetrate below the non-directional limit by several orders of magnitude. For high WIMP masses (around 100 GeV), the non-directional limit is overcome by a factor of a few. Furthermore we show that even for directional detectors which can only measure 1-or 2-dimensional projections of the 3-dimensional recoil track, the discovery potential is only reduced by a factor of 3 at most. We also demonstrate that while the experimental limitations of directional detectors, such as sense recognition and finite angular resolution, have a detrimental effect on the discovery limits, it is still possible to overcome the ultimate neutrino background faced by non-directional detectors.PACS numbers: 95.35.+d; 95.85.Pw

Dark matter astrophysical uncertainties and the neutrino floor

“…For spin-dependent in-teractions as well as non-relativistic effective field theory operators, complementarity between target materials will be a powerful and relatively easy method for discriminating neutrinos [19,20]. Independent of the WIMP-nucleus interaction however, directional detection, if experimentally feasible, will prove the most powerful scheme for distinguishing WIMPs and neutrinos [17,18]. The angular signature of WIMP and neutrino recoils are entirely distinct and this is true for any relationship set of astrophysical inputs or WIMP-nucleus interactions.…”

“…Firstly by using Xenon it allows us to make direct comparisons with previous work on the neutrino floor e.g., Refs. [5,15,[17][18][19] whilst also providing us with the simplicity of a single target nucleus. Using a very low threshold also allows us to capture the low WIMP mass neutrino floor without having to change the target nucleus or use targets with multiple different nuclei.…”

“…The approach taken by Refs. [15,18,19] uses a profile likelihood ratio test statistic to calculate so-called "discovery limits" which are defined as the minimum cross section for which a 3σ discovery of a WIMP is possible in 90% of hypothetical experiments. The profile likelihood ratio comprises a hypothesis test between the null hypothesis H 0 (neutrino only) and the alternative hypothesis H 1 which includes both neutrinos and a WIMP signal whilst incorporating systematic uncertainties, in this case on the flux of each neutrino component Φ and the astrophysical parameters Θ.…”

“…As initially shown by Ruppin et al [19], the differences in the tails of the recoil energy distributions of WIMPs and neutrinos allow the "floor" to be overcome albeit with very large numbers of events (> O(1000)). Furthermore other studies have shown that the neutrino background can be mitigated with the use of annual modulation effects [16], direction dependence [17,18] or complementarity between multiple target nuclei [19]. A recent work by Dent et al [20] made use of the non-relativistic effective field theory formalism which posits additional operators to describe the nuclear response to a WIMP interaction.…”

Effective field theory treatment of the neutrino background in direct dark matter detection experiments

“…These time variations generate a phase difference between the Solar neutrino and the WIMP signal. Grothaus [19] and O'Hare et al [20] discussed the prospects for exploiting the difference in the direction of the nuclear recoil energy induced by the WIMPs and Solar neutrinos. For these timing and directional-based techniques, an exposure on the scale of 100 tonne years is required to distinguish the Solar neutrino background from a WIMP signal.…”