Reverse Direct Detection: Cosmic Ray Scattering With Light Dark Matter (1810.07705v3)

Abstract: Sub-GeV dark matter candidates are of increasing interest, because long-favored candidates such as GeV-scale WIMPs have not been detected. For low-mass dark matter, model-independent constraints are weak or nonexistent. We show that for such candidates, because the number density is high, cosmic ray propagation can be affected by elastic scattering with dark matter. We call this type of search `reverse direct detection,' because dark matter is the target and Standard Model particles are the beam. Using a simple propagation model for galactic cosmic rays, we calculate how dark matter affects cosmic ray spectra at Earth, and set new limits on the dark matter-proton and dark matter-electron cross sections. For protons, our limit is competitive with cosmological constraints, but is independent. For electrons, our limit covers masses not yet probed, and improves on cosmological constraints by one to two orders of magnitude. We comment on how future work can significantly improve the sensitivity of cosmic-ray probes of dark matter interactions.

Reverse Direct Detection: Cosmic Ray Scattering With Light Dark Matter

The paper "Reverse Direct Detection: Cosmic Ray Scattering With Light Dark Matter" by Cappiello, Ng, and Beacom explores an innovative approach to probing dark matter (DM) interactions, particularly for sub-GeV dark matter masses, through cosmic ray (CR) interactions. The paper addresses the constraints on DM-proton and DM-electron scattering cross sections, highlighting the potential of cosmic ray data in complementing traditional dark matter searches, which have so far largely focused on GeV-scale Weakly Interacting Massive Particles (WIMPs) without success.

Theoretical Framework and Methodology

The authors introduce a novel model-independent method termed "reverse direct detection." Unlike conventional direct detection experiments, where dark matter is considered the incoming particle and Standard Model (SM) matter as the target, this approach considers cosmic rays as the incident particles that scatter off a relatively stationary dark matter target. The primary premise is that DM interactions can alter the energy spectra of cosmic rays, which can be measured at Earth.

To quantify this effect, the authors use a simple galactic cosmic ray propagation model, accounting for potential energy loss of CRs due to elastic scattering with dark matter. The focus is on determining the cross-section limits for various interactions by examining changes in CR spectra, utilizing data from AMS and CREAM experiments among others. For DM-proton interactions, the constraints derived are competitive with cosmological limits, whereas for DM-electron scattering, they significantly surpass existing constraints.

Numerical Results and Key Insights

The analysis yields new limits for DM interactions. For DM-proton scattering, the derived cross-section limits are noteworthy for being competitive with current leading constraints, especially at lower DM mass scales. The paper reports a limit of $\sigma_{\chi p} \lesssim 10^{-27}$ cm$^2$ for DM masses below 1 MeV. In comparison, for DM-electron scattering, their method probes a previously unexplored parameter space, improving limits by one to two orders of magnitude compared to cosmological constraints. The authors emphasize the constraints' dependence on energy, showcasing how their method operates at higher CR energies where conventional searches lose sensitivity.

Implications and Future Prospects

The findings have significant implications for dark matter detection strategies, particularly in the sub-GeV mass range. By providing limits on DM interactions that complement and, in some instances, outperform other methods, the paper highlights the importance of integrating astrophysical observations with terrestrial experiments and cosmological tests. The reverse direct detection framework opens potential new avenues for understanding non-WIMP dark matter models, as it can be sensitive to DM candidates that interact with SM particles through mechanisms not easily detectable by existing technologies.

Looking ahead, further work can refine these constraints through improved cosmic ray modeling and additional observational data. The potential inclusion of CR anisotropies and exploring different CR species could offer more granularity and possibly uncover new interaction signatures. Advanced simulations of galactic CR propagation and a better understanding of DM distribution could enhance model fidelity and further tighten interaction constraints.

The paper's insights and methodologies mark a compelling advance in dark matter research through cosmic-ray physics. By addressing the limitations of existing detection methods and presenting a robust alternative framework, it paves the way for fresh investigative directions and theoretical explorations in the quest to uncover the elusive nature of dark matter.