Cyclotron Radiation Emission Spectroscopy of Electrons from Tritium Beta Decay and \(^{83\rm m}\)Kr Internal Conversion

Project 8 has developed a novel technique, Cyclotron Radiation Emission Spectroscopy (CRES), for direct neutrino mass measurements. A CRES-based experiment on the beta spectrum of tritium has been carried out in a small-volume apparatus. We provide a detailed account of the experiment, focusing on systematic effects and analysis techniques. In a Bayesian (frequentist) analysis, we measure the tritium endpoint as \(18553^{+18}_{-19}\) (\(18548^{+19}_{-19}\)) eV and set upper limits of 155 (152) eV (90% C.L.) on the neutrino mass. No background events are observed beyond the endpoint in 82 days of running. We also demonstrate an energy resolution of \(1.66\pm0.19\) eV in a resolution-optimized magnetic trap configuration by measuring \(^{83\rm m}\)Kr 17.8-keV internal-conversion electrons. These measurements establish CRES as a low-background, high-resolution technique with the potential to advance neutrino mass sensitivity.