Millicharged Cosmic Rays and Low Recoil Detectors

Abstract

We consider the production of a "fast flux" of hypothetical millicharged particles (mCPs) in the interstellar medium (ISM). We consider two possible sources induced by cosmic rays: (a) pp →(meson)→(mCP) which adds to atmospheric production of mCPs, and (b) cosmic-ray upscattering on a millicharged component of dark matter. We notice that the galactic magnetic fields retain mCPs for a long time, leading to an enhancement of the fast flux by many orders of magnitude. In both scenarios, we calculate the expected signal for direct dark matter detection aimed at electron recoil. We observe that in Scenario (a) neutrino detectors (ArgoNeuT and Super-Kamiokande) still provide superior sensitivity compared to dark matter detectors (XENON1T). However, in scenarios with a boosted dark matter component, the dark matter detectors perform better, given the enhancement of the upscattered flux at low velocities. Given the uncertainties, both in the flux generation model and in the actual atomic physics leading to electron recoil, it is still possible that the XENON1T-reported excess may come from a fast mCP flux, which will be decisively tested with future experiments.

Millicharged Particles from the Heavens: Single- and Multiple-Scattering Signatures

“…Various effects of millicharged particles as dark matter, leading to stringent constraints in the MeV-GeV mass range, are discussed in Refs. [31,32,[39][40][41][42][43]. If a millicharged particle is discovered via the atmospheric-production and scattering-detection approach we propose, then it is at most a very small fraction of the relic dark matter in the universe.…”

“…[30], as have limits from particles produced in the interstellar medium and from Earth relics recently discussed in Refs. [31] and [32], respectively.…”

“…In that context, our signal rates predicted in XENON1T will be overpredicted by a factor of several -see, for instance, Refs. [31,74] for further discussion of this effect. Given the scaling of our signal rate with ε 4 and the fact that XENON1T is not the most powerful experiment considered in this work, we disregard this effect in estimating our constraints.…”

“…We note here, as in the main text, that our XENON1T analysis and signal-event prediction rate from Eq. ( 9) do not include the fact that the electrons in the XENON1T detector are bound [31,74]. This is relevant because their binding energies are non-negligible compared to the recoil energy observed in the detector, and this implies that our signal rate predictions are optimistic.…”

“…In tandem with accelerator-based searches, atmospheric-based production of MCP has been proposed [30] (see also Refs. [31,32] which considered even more distant production mechanisms that can contribute to these searches), with large-volume neutrino detectors like Super-Kamiokande serving as the best candidates to search for these particles.…”