Assessment of molecular effects on neutrino mass measurements from tritium beta decay

TL;DR

This paper reviews the molecular effects on neutrino mass measurements from tritium beta decay, emphasizing the importance of accurate theoretical models and experimental validation for upcoming high-precision experiments like KATRIN.

Contribution

It re-evaluates the molecular final-state spectrum theory of tritium decay and discusses its impact on neutrino mass measurements, highlighting discrepancies and validation efforts.

Findings

Modern calculations align with spectroscopic data.

Re-evaluation of historical neutrino mass data shows consistency with zero.

Discrepancies exist between calculated and experimental dissociation rates.

Abstract

The beta decay of molecular tritium currently provides the highest sensitivity in laboratory-based neutrino mass measurements. The upcoming Karlsruhe Tritium Neutrino (KATRIN) experiment will improve the sensitivity to 0.2 eV, making a percent-level quantitative understanding of molecular effects essential. The modern theoretical calculations available for neutrino-mass experiments agree with spectroscopic data. Moreover, when neutrino-mass experiments performed in the 1980s with gaseous tritium are re-evaluated using these modern calculations, the extracted neutrino mass-squared values are consistent with zero instead of being significantly negative. On the other hand, the calculated molecular final-state branching ratios are in tension with dissociation experiments performed in the 1950s. We re-examine the theory of the final-state spectrum of molecular tritium decay and its effect on the determination of the neutrino mass, with an emphasis on the role of the vibrational- and rotational-state distribution in the ground electronic state. General features can be reproduced quantitatively from considerations of kinematics and zero-point motion. We summarize the status of validation efforts and suggest means for resolving the apparent discrepancy in dissociation rates.