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A<sc>bstract</sc> The Japan Proton Accelerator Research Complex (J-PARC) currently delivers a 1 MW, 3 GeV proton beam to the Materials and Life Science Experimental Facility (MLF). Power is expected to increase to 1.3 MW, driven by the needs of Hyper-Kamiokande. As a result, the MLF presently provides the highest neutron yield of any spallation source, while potentially holding the best current and foreseeable conditions for Coherent Elastic Neutrino-Nucleus Scattering (CEνNS) experimentation. We explore this potential, using as examples detector technologies presently funded for construction and under development. We quantify their sensitivity to a rich variety of particle physics scenarios, finding that very-high-statistics CEνNS measurements with significant sensitivity to relevant scenarios are feasible at this facility within the next few years. 

We demonstrate the feasibility of probing the charged lepton-flavor-violating decay mu+ -> e+ X0 for the presence of a slow-moving neutral boson X0 capable of undergoing gravitational binding to large structures and, as such, able to participate in some cosmological scenarios. A short exposure to surface antimuons from beam line M20 at TRIUMF generates a branching ratio limit of <1E-5. This is comparable to or better than previous searches for this channel, although in a thus- far-unexplored region of X0 phase space very close to the kinematic limit of the decay, where m_X0 approaches m_mu+ . The future improved sensitivity of the method using a customized p-type point-contact germanium detector is described.