Speaker
Description
Proton beam facilities offer unique sensitivity to sub-GeV dark sector particles produced in meson and muon decays at rest. The Spallation Neutron Source (SNS), operating at $1.3~GeV$ with multi-gigawatt beam power, is therefore well suited to searches for dark matter and other feebly interacting particles in the $10–100~MeV$ mass range. Although recent studies have demonstrated strong discovery potential at the SNS, the ultimate reach of existing experiments is often constrained by background.
We present a program that exploits the PROSPECT segmented, hydrocarbon-based liquid scintillator detector to search for new physics arising from $\mu^+$ decays at rest in the SNS target. Originally developed for precision reactor antineutrino measurements, PROSPECT features optical segmentation, $^6$Li-doped scintillator, pulse shape discrimination, and double-ended light readout, enabling powerful rejection of cosmogenic and radiogenic backgrounds. We further examine how detector design and analysis choices impact background suppression, highlighting the complementary roles of target segmentation and pulse shape discrimination in a high–light-yield scintillator detector.
Using signals delayed in time relative to the SNS proton beam, we assess the sensitivity to heavy neutral leptons and axion-like particles with visible decay products, concentrating on electron–positron final states. Backgrounds are constrained using existing PROSPECT data, along with validated simulations of cosmic neutrons and muons, enabling a realistic estimate of achievable background levels for a refurbished and optimized detector configuration. Under these conditions, the best-performing detector configuration enables near-background-free searches, achieving world-leading sensitivities in selected regions of parameter space.