Description
I will discuss new opportunities to search for weakly-coupled long-lived particles (LLP) produced in pion, muon, and kaon decays at rest. We estimate the sensitivity to the visible decays of these LLPs in neutrino detectors at spallation sources. At higher energies, I will also discuss complementary studies of leptophilic axion-like particles produced in tau decays, which yield strong constraints and promising future reach from fixed-target and collider experiments. Together, these approaches map a broad and experimentally accessible landscape for discovering a wide range of weakly-coupled particles.