First tritium endpoint measurement with Cyclotron Radiation Emission Spectroscopy (CRES)*

Project 8 aims to determine the absolute neutrino mass scale from the shape of the tritium beta decay spectrum near its endpoint. For this purpose, the collaboration developed a precise frequency-based technique for measuring electron energies: Cyclotron Radiation Emission Spectroscopy (CRES). This talk presents Project 8's analysis of the first tritium spectrum obtained with CRES. The spectrum contains 3770 events collected in 82 live days. No events were observed above the endpoint, demonstrating CRES's ultra-low background capability. We calibrate the detector response and study systematic errors using ^83mKr conversion electron data. With ^83mKr data taken in a shallower magnetic trap than used for tritium runs, eV-scale energy resolution was obtained—an order of magnitude from the sub-eV resolution needed for Project 8's target neutrino mass sensitivity of 40 meV. We perform Bayesian and frequentist analyses of the tritium beta spectrum, producing Project 8's first tritium endpoint measurement and neutrino mass limit.