A Disappearance Search for Sterile Neutrinos with the CAPTAIN-Mills Detector at the Los Alamos Neutron Science Center

The LSND and MiniBooNE short baseline neutrino oscillation experiments have shown evidence for sterile neutrinos at $\Delta m^2 \sim 1$ eV$^2$. Both experiments used pure muon neutrino beams to search for electron neutrino appearance, i.e., $\nu _\mu \rightarrow \nu _e$, yet corresponding disappearance experiments have shown no anomalies. We will deploy the CAPTAIN-Mills detector, a 7-ton fiducial volume, single-phase, liquid argon scintillation detector, and use the coherent elastic neutrino-nucleus scattering (CE$\nu$NS) process to measure muon neutrino disappearance at the Lujan Facility at the Los Alamos Neutron Science Center. Using CE$\nu$NS greatly enhances the event rate compared to other oscillation experiments. Lujan is a 100-kW stopped pion source that nominally delivers a 250-ns wide, 800-MeV proton beam onto a tungsten target at 30 Hz, but the beam width can be significantly narrowed to 30 ns. Lujan's fast pulsing is advantageous for isolating the prompt 30-MeV muon neutrino from the delayed muon-decay neutrinos and neutron backgrounds. In this talk, I will describe the CAPTAIN-Mills detector, the Lujan neutrino source, the expected sensitivities for sterile neutrinos, and show results from our neutron background survey.