Abstract
Traditional direct searches for dark matter, looking for nuclear recoils in deep underground detectors, are challenged by an almost complete loss of sensitivity for light dark matter particles. Consequently, there is a significant effort in the community to devise new methods and experiments to overcome these difficulties, constantly pushing the limits of the lowest dark matter mass that can be probed this way. From a model-building perspective, the scattering of sub-GeV dark matter on nucleons essentially must proceed via new light mediator particles, given that collider searches place extremely stringent bounds on contact-type interactions. Here we present an updated compilation of relevant limits for the case of a scalar mediator, including a new estimate of the near-future sensitivity of the NA62 experiment as well as a detailed evaluation of the model-specific limits from Big Bang nucleosynthesis. We also derive updated and more general limits on DM particles upscattered by cosmic rays, applicable to arbitrary energy- and momentum dependences of the scattering cross section. Finally we stress that dark matter self-interactions, when evaluated beyond the common s-wave approximation, place stringent limits independently of the dark matter production mechanism. These are, for the relevant parameter space, generically comparable to those that apply in the commonly studied freeze-out case. We conclude that the combination of existing (or expected) constraints from accelerators and astrophysics, combined with cosmological requirements, puts robust limits on the maximally possible nuclear scattering rate. In most regions of parameter space these are at least competitive with the best projected limits from currently planned direct detection experiments.
Highlights
In this work we have considered the prospects of future direct detection experiments to test uncharted parameter space for light dark matter
It is natural in this context to expect additional light particles mediating the interactions between dark matter and the target nuclei in order to achieve a sufficiently large scattering cross section
To alleviate the strong cosmological bounds from the Cosmic Microwave Background (CMB) we have concentrated on a scenario in which dark matter couples via a scalar mediator such that dark matter annihilations proceed via p-wave and are strongly suppressed at the time of the CMB
Summary
The existence of dark matter (DM) is one of the main arguments to expect physics beyond the SM In this case theoretical considerations seem to point to the electroweak scale [23], independently of the arguments mentioned above, but direct searches for DM in the form of weakly interacting massive particles (WIMPs) have started to place ever more stringent constraints on this possibility [24, 25]. We evaluate in detail the thermal evolution of the dark sector to compute the DM abundance and updated bounds from Big Bang Nucleosynthesis (BBN) — and demonstrate that DM self-interactions lead to stringent bounds that cannot be evaded even if DM is not thermally produced via the common freeze-out mechanism We combine these new results with various updated accelerator constraints and projections, and present them in a form directly usable by experimentalists probing the sub-GeV range. Two interesting and often studied options are vector mediators kinetically mixed with the SM hypercharge gauge boson or scalar mediators with Higgs mixing
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