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) $pp\rightarrow$(meson)$\rightarrow$(mCP) which adds to atmospheric production of mCPs, and (b) cosmic-ray up-scattering 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.
Highlights
Cosmic rays exhibit a rich phenomenology in Earth’s local neighborhood
As an illustration of the various components of the flux arriving at a terrestrial detector, we show the differential flux in Fig. 2 for the benchmark point where one part per million of dark matter is millicharged fχ 1⁄4 10−6, ε 1⁄4 3 × 10−3, and mχ 1⁄4 200 MeV
Of fundamental importance is the realization that, for mχ ≫ me, the electron recoil threshold sets a limit on the velocity of millicharged particles (mCPs)
Summary
Cosmic rays exhibit a rich phenomenology in Earth’s local neighborhood. Highly boosted charged particles strike protons in the interstellar medium and in the upper atmosphere, to produce showers of more numerous (and less energetic) charged particles. Even if collective damping effects are important for coherent mDM motion, the RBDM component will be incoherent, having been generated by random and isotropic collisions throughout the volume of the cosmic-ray diffusion zone In both scenarios, our results rely crucially on the diffusive motion of mCPs (see Fig. 1), which leads to a residency time in our local Galaxy that is 4 orders of magnitude larger than would be expected if the mCPs were to exhibit ballistic motion. Given enough uncertainty in the current treatment of atomic physics, we find that the claimed few-keV-scale excess of recoil electrons may still plausibly come from the fast flux of millicharged particles, but this subject requires a more detailed investigation Departing from these two relatively conservative scenarios, we explore the sensitivity to a fast mCP flux of more exotic origin, such as, e.g., dark sector decay to mCP, finding that in this case the sensitivity may extend to very small values for the millicharge. VI, we summarize our results and comment on the future impact of low recoil detectors for mCP and mDM searches
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