Abstract
We consider the generic possibility that the Universe's energy budget includes some form of relativistic or semi-relativistic dark radiation (DR) with non-gravitational interactions with Standard Model (SM) particles. Such dark radiation may consist of SM singlets or a non-thermal, energetic component of neutrinos. If such DR is created at a relatively recent epoch, it can carry sufficient energy to leave a detectable imprint in experiments designed to search for very weakly interacting particles: dark matter and underground neutrino experiments. We analyze this possibility in some generality, assuming that the interactive dark radiation is sourced by late decays of an unstable particle, potentially a component of dark matter, and considering a variety of possible interactions between the dark radiation and SM particles. Concentrating on the sub-GeV energy region, we derive constraints on different forms of DR using the results of the most sensitive neutrino and dark matter direct detection experiments. In particular, for interacting dark radiation carrying a typical momentum of $\sim30$~MeV$/c$, both types of experiments provide competitive constraints. This study also demonstrates that non-standard sources of neutrino emission (e.g. via dark matter decay) are capable of creating a "neutrino floor" for dark matter direct detection that is closer to current bounds than is expected from standard neutrino sources.
Highlights
The dominance of dark matter (DM) and dark energy (DE) in the total energy balance of the Universe is a widely acknowledged astounding fact
In this paper we address several questions related to dark radiation (DR), weakly interacting massive particles (WIMPs) direct detection experiments, and neutrino physics
We have considered a hypothetical possibility that along with nonrelativistic DM, somerelativistic particles form a cosmic dark radiation (DR) background that may have a noticeable interaction rate with the standard model (SM) particles
Summary
The dominance of dark matter (DM) and dark energy (DE) in the total energy balance of the Universe is a widely acknowledged astounding fact. We require that the amount of dark radiation does not exceed 10% of the dark matter energy density, in accordance with recently updated constraints [27] Such set of inequalities leaves, a lot of freedom for what DR can be, but restricts a number of possibilities for how the nonthermal DR can be created. Neutrinos provide a small—but in future important— background for the searches of weakly interacting massive particles (WIMPs) in direct detection experiments [31,32,33].1. Given the huge amount of efforts devoted to the scaling-up of the WIMP direct detection experiments, one should investigate possible signals from additional hypothetical components of the neutrino flux, and from DR in general.
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