[Paper Review] Cosmic-ray dark matter confronted by constraints on new light mediators

Nicole F. Bell, Jayden L. Newstead+1 more|arXiv (Cornell University)|Sep 20, 2023

Dark Matter and Cosmic PhenomenaPhysics and Astronomy72 references3 citations

TL;DR

This paper investigates cosmic-ray dark matter (CRDM) upscattering as a probe for sub-GeV dark matter via light mediators coupling to nucleons. Using constraints from Borexino, XENON1T, LZ, and Super-K, it finds that existing limits on the mediators—particularly from meson decays and stellar cooling—already rule out the parameter space where CRDM signals would be detectable, rendering CRDM constraints model-dependent and largely ineffective for probing dark matter.

ABSTRACT

The detectability of light dark matter in direct detection experiments is limited by the small kinetic energy of the recoiling targets. Thus, scenarios where dark matter is boosted to relativistic velocities provide a useful tactic to constrain sub-GeV dark matter particles. Of the possible dark matter boosting mechanisms, cosmic-ray upscattering is an appealing paradigm as it doesn't require any additional assumptions beyond dark matter coupling to nucleons or electrons. However, detectable signals are obtained only with relatively large cross sections which, in turn, can only be realized with large couplings, light mediators or composite dark matter. In this work we consider a general set of light mediators that couple dark matter to hadrons. Using data from Borexino, XENON1T, LZ and Super-K, we show that existing constraints on such mediators preclude appreciable cosmic-ray dark matter upscattering. This finding highlights the limited applicability of cosmic-ray upscattering constraints and suggests they only be used in model-dependent studies.

Motivation & Objective

• To assess the viability of cosmic-ray dark matter (CRDM) upscattering as a probe for sub-GeV dark matter in direct detection experiments.
• To evaluate whether CRDM signals can evade existing constraints on the light mediators mediating DM-nucleon interactions.
• To determine whether current and future direct detection and neutrino experiments can probe parameter space not already excluded by mediator constraints.
• To quantify the impact of mediator constraints—especially from meson decays, stellar cooling, and SN1987A—on CRDM detection prospects.
• To highlight the model dependence of CRDM constraints and the limitations of using CRDM as a general-purpose probe for light dark matter.

Proposed method

• Formulates a general class of simplified models with scalar, vector, axial-vector, and pseudoscalar mediators coupling dark matter to nucleons.
• Computes the CRDM flux using momentum-dependent cross sections, accounting for kinematics of cosmic-ray upscattering on nucleons.
• Applies constraints from meson decays (e.g., $K \to \pi \pi$), stellar cooling (HB and RG stars), and SN1987A neutrino emission on the mediator couplings.
• Evaluates direct detection sensitivity using data from XENON1T, LZ, Borexino, and Super-K, focusing on nuclear recoil thresholds and exposure scaling.
• Compares CRDM detection bounds with existing mediator constraints across the parameter space of mediator mass and coupling.
• Considers attenuation effects from elastic scattering in Earth's overburden, though focuses on the dominant constraint from mediator physics rather than propagation.

Experimental results

Research questions

• RQ1 Can cosmic-ray upscattering produce a detectable flux of relativistic dark matter in direct detection experiments for sub-GeV dark matter?
• RQ2 To what extent are CRDM detection signals constrained by existing limits on the light mediators mediating DM-nucleon interactions?
• RQ3 Are there any regions of the parameter space—particularly for light mediators—where CRDM detection would be viable despite existing mediator constraints?
• RQ4 How do constraints from meson decays, stellar cooling, and SN1987A compare with those from direct detection experiments in ruling out CRDM signals?
• RQ5 What improvements in detector sensitivity (e.g., JUNO, DUNE, Hyper-K) are needed to probe the remaining unconstrained parameter space for CRDM?

Key findings

• For all considered mediator types—scalar, vector, axial-vector, and pseudoscalar—existing constraints on the mediators already exclude the parameter space where CRDM signals would be detectable in current experiments.
• The strongest constraints on mediator couplings come from meson decays (e.g., $K \to \pi \pi$) and stellar cooling, particularly for scalar and pseudoscalar mediators.
• The vector and axial-vector mediator constraints are dominated by meson decay and SN1987A limits, respectively, with the axial coupling likely subject to even stronger astrophysical bounds.
• In the SI models, LZ’s xenon target with low threshold and nuclear cross-section enhancement yields the strongest CRDM bounds across the parameter space.
• For SD models, Borexino provides the strongest constraint due to its sensitivity to electron recoils and low energy threshold.
• Future detectors like JUNO, DUNE, and Hyper-K would improve CRDM limits only slowly, scaling with the fourth root of exposure, and cannot access regions already excluded by mediator constraints.