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

TL;DR

This paper reports the first neutrino mass limit using Cyclotron Radiation Emission Spectroscopy (CRES) on tritium beta decay, demonstrating the technique's potential for high-sensitivity neutrino mass measurements with low background.

Contribution

The study introduces the first frequency-based neutrino mass limit using CRES, showcasing its high resolution and low background capabilities for future neutrino mass experiments.

Findings

Neutrino mass limit of <155 eV from CRES measurement

Achieved energy resolution of 1.66 eV (FWHM)

Validated detector response and efficiency with calibration data

Abstract

The absolute scale of the neutrino mass plays a critical role in physics at every scale, from the particle to the cosmological. Measurements of the tritium endpoint 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 cm$^3$-scale physical detection volume, a limit of $m_\beta{<}$155 eV ($152$ eV) is extracted from the background-free measurement of the continuous tritium beta spectrum in a Bayesian (frequentist) analysis. Using $^{83{\rm m}}$Kr calibration data, an improved resolution of 1.66${\pm}$0.19 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.