Observing a light dark matter beam with neutrino experiments

TL;DR

The paper investigates fixed-target neutrino experiments as probes of light MeV-scale dark matter interacting through a light vector mediator with kinetic mixing κ. It models a boosted dark matter beam produced in π^0/η decays and analyzes NC-like elastic scattering signals in LSND and MiniBooNE, deriving event-rate predictions for eχ and χN scattering and translating them into exclusion regions in (m_χ, m_V, κ). The results show that substantial portions of the natural MeV-scale parameter space, particularly for 1–5 MeV DM with 2m_χ < m_V < m_η and κ down to ≈10^−5, are excluded, placing strong constraints on DM explanations of the galactic 511 keV line. The work highlights the complementarity of fixed-target probes to meson-decay and astrophysical constraints and discusses future prospects for extending sensitivity with additional facilities and channels.

Abstract

We consider the sensitivity of fixed-target neutrino experiments at the luminosity frontier to light stable states, such as those present in models of MeV-scale dark matter. To ensure the correct thermal relic abundance, such states must annihilate via light mediators, which in turn provide an access portal for direct production in colliders or fixed targets. Indeed, this framework endows the neutrino beams produced at fixed-target facilities with a companion `dark matter beam', which may be detected via an excess of elastic scattering events off electrons or nuclei in the (near-)detector. We study the high luminosity proton fixed-target experiments at LSND and MiniBooNE, and determine that the ensuing sensitivity to light dark matter generally surpasses that of other direct probes. For scenarios with a kinetically-mixed U(1)' vector mediator of mass m_V, we find that a large volume of parameter space is excluded for m_DM ~ 1-5 MeV, covering vector masses 2 m_DM < m_V < m_eta and a range of kinetic mixing parameters reaching as low as kappa ~ 10^{-5}. The corresponding MeV-scale dark matter scenarios motivated by an explanation of the galactic 511 keV line are thus strongly constrained.

Figures (5)

• Figure 1: Tree-level annihilation (left) and scattering (right) of scalar dark matter in the U(1)$'$ hidden sector.
• Figure 2: Expected number of elastic scattering events of dark matter off electrons at the LSND detector for $m_\chi=1$ MeV. The regions show greater than 10 (light), 1000 (medium) and $10^6$ (dark) expected events. The area below the black line corresponds to $\alpha'>4 \pi$.
• Figure 3: Expected number of neutral current-like dark matter electron scattering events at the MiniBooNE detector for $m_\chi=1$ MeV. The regions show greater than 10 (light), 1000 (medium) and $10^6$ (dark) expected events. The plot on the left shows dark matter resulting from $\pi^0$ decays, while the plot on the right combines dark matter from both $\pi^0$ and $\eta$ decays. The area below the black line corresponds to $\alpha'>4 \pi$.
• Figure 4: Expected number of neutral current-like dark matter nucleon scattering events at the MiniBooNE detector. The regions show greater than 10 (light), 1000 (medium) and $10^6$ (dark) expected events. The plot on the left is for $m_\chi=1$ MeV, while the plot on the right is for $m_\chi=50$ MeV. The area below the black line corresponds to $\alpha'>4 \pi$, while the dashed curve indicates the total number of (background) neutrino events observed.
• Figure 5: As in Fig. 2, we show the sensitivity of LSND's elastic scattering analysis lsnd_elastic to scattering of dark matter off electrons, when $m_\chi< m_V/2$. The dashed LSND contour corresponds to the 90% confidence limit as discussed in the text, based on LSND's comparison of the observed data to the Standard Model neutrino background lsnd_elastic. The confidence limit is shown for two dark matter masses, 1 MeV (left) and 10 MeV (right). The solid black line indicates the parameters required to ensure saturation of the dark matter relic density. The darker shaded regions show exclusions due to loop corrections from the vector to $g-2$ of the muon and electron, while the dark band is the preferred region to shift the muon $g-2$ value into line with experiment tests.