Tritium Beta Spectrum Measurement and Neutrino Mass Limit from Cyclotron Radiation Emission Spectroscopy

The absolute scale of the neutrino mass plays a critical role in physics at every scale, from the subatomic to the cosmological. Measurements of the tritium end-point spectrum have provided the most precise direct limit on the neutrino mass scale. In this Letter, we present advances by Project 8 to the cyclotron radiation emission spectroscopy (CRES) technique culminating in the first frequency-based neutrino mass limit. With only a ${\mathrm{cm}}^3$-scale physical detection volume, a limit of $m_{\beta }<155\mathrm{eV}/c^2$ ($152\mathrm{eV}/c^2$) is extracted from the background-free measurement of the continuous tritium beta spectrum in a Bayesian (frequentist) analysis. Using ${}^{83\mathrm{m}}\mathrm{Kr}$ calibration data, a resolution of $1.66\pm 0.19\mathrm{eV}$ (FWHM) is measured, the detector response model is validated, and the efficiency is characterized over the multi-keV tritium analysis window. These measurements establish the potential of CRES for a high-sensitivity next-generation direct neutrino mass experiment featuring low background and high resolution.

Figure 1

Cutaway of the cryogenic CRES cell, where electrons are produced in radioactive decay and magnetically trapped. The cell waveguide has a cold interior diameter of 10.03 mm and length of 132 mm (distance between rf windows). Cyclotron radiation travels axially up the waveguide (left in rotated view), toward the amplifiers and readout electronics.

Figure 2

Spectrogram of the first CRES event detected from a tritium beta decay electron. Raw time-series data are Fourier transformed in time bins of $40.96\mathrm{\mu }\mathrm{s}$, yielding 24.41-kHz frequency bins.

Figure 3

Data and fits of the $17.8\mathrm{keV}$ ${}^{83\mathrm{m}}\mathrm{Kr}$ conversion electron K line, as measured in the shallow (high-resolution) and the deep (high-statistics) electron trapping configurations. The shallow trap exhibits an instrumental resolution of $1.66\pm 0.19\mathrm{eV}$ (FWHM), while the deep trap provides direct calibration of the tritium data-taking conditions.

Figure 4

The $17.8\mathrm{keV}$ ${}^{83\mathrm{m}}\mathrm{Kr}$ conversion electron line recorded in the deep trap with varying magnetic background fields (red to blue). The gray curve shows the efficiency’s response to frequency variation, extrapolated from single trap data. The green curve is corrected for energy dependence of emitted cyclotron power and shows the relative efficiency predicted for tritium data.

Figure 5

Measured tritium end-point spectrum with Bayesian and frequentist fits. Inset: frequentist neutrino mass and end-point contours.