Prying Open the Dark Sector Window with SBND Off-Target Mode

Abstract

Accelerator-based neutrino experiments with high-intensity proton beams and advanced detector technologies provide a powerful and complementary approach to probing physics beyond the Standard Model. The MiniBooNE experiment at Fermilab pioneered a dedicated Booster Neutrino Beam (BNB) off-target (beam-dump) run, setting leading constraints on sub-GeV dark matter. In this work, we explore the physics opportunities enabled by operating the Short-Baseline Near Detector (SBND) at Fermilab in a future BNB off-target configuration, as well as in a dedicated beam-dump configuration. By redirecting the proton beam away from the nominal beryllium target, or by employing a dedicated beam-dump, neutrino-induced backgrounds are substantially suppressed, thereby enhancing SBND's sensitivity to many new physics scenarios. We demonstrate that such running modes significantly extend the reach for new physics. As representative examples, we present projected sensitivities to light dark matter, axion-like particles, heavy neutral leptons, and meson-portal scenarios.

Figures (7)

• Figure 1: Sensitivity to scalar DM at the 90% confidence level for a vector mediator model with m$_{A^{'}}$$=3m$$_\chi$ and $\alpha_D=0.5$. The lines represent the projected sensitivities for different beam configurations at SBND. The DM is produced via meson decays and proton bremsstrahlung, with detection through DM-electron elastic scattering, DM-proton elastic scattering, and inelastic nucleon interactions. Existing constraints from BaBar, LSND, E137, COHERENT CsI, MiniBooNE, NA64, and direct detection experiments are included in the shaded gray region called "Excluded Region", and the black line indicates the parameters satisfying the thermal relic abundance.
• Figure 2: SBND sensitivity in Off-target beam-dump mode (blue contours) and in target neutrino mode running (orange contours) to axion-like particles under several benchmark model assumptions; top: gluon dominance, middle: photon-dominance, bottom: electron-dominance.
• Figure 3: ALP-HNL sensitivity for all three scenarios, including background for the decay into $\nu \gamma$ for the neutrino mode.
• Figure 4: Sensitivity to the tau flavor HNL mixing $|U_{\tau4}|^2$ via the $B-L$ assisted HNL production, taking $m_{Z^\prime}/m_N = 2.1$ (left) and $m_{Z^\prime}/m_N = 5$ (right). The sensitivity curve on the right plot terminates due to a limitation imposed on $m_{Z^\prime}$ in the calculation of proton bremsstrahlung; see ref. Capozzi:2024pmh for details.
• Figure 5: Parameter space limits for the meson portal vector $X$ for $m_X = 10$ MeV are shown over the vector neutral pion coupling ($g_{\pi^0}$) and charged pion coupling ($g_{\pi^\pm}$) are shown for the SBND off-target projections (blue lines) and the target mode running with $10^{21}$ POT (orange). The pink contours show the MiniBooNE best-fit regions ($1\sigma, 2\sigma$) as derived in ref. CCM:2023itc. Existing constraints are shown from stopped-pion experiments KARMEN, CCM, and LSND which are dominantly sensitive to the neutral pion decays $\pi^0 \to \gamma X$.