A study of the relativistic corrections to polarized tritium $\beta$-decay

TL;DR

This paper analyzes relativistic corrections to polarized tritium beta decay spectra, focusing on their impact on neutrino mass measurements and the potential for experimental detection of anisotropies.

Contribution

It provides the first systematic derivation of relativistic corrections to the tritium beta decay spectrum, including effects of nuclear polarization and their implications for neutrino mass experiments.

Findings

Relativistic corrections are small but measurable in polarized decay spectra.

Polarization induces angular anisotropy without improving neutrino mass constraints.

The derived formulas aid in interpreting future high-precision beta decay experiments.

Abstract

Forthcoming experiments like Project8 and Ptolemy aim at investigating with high precision the end-point of the tritium $\beta$-decay spectrum sensitive to the neutrino mass. In light of this, using the standard parametrization in terms of nuclear polar form factors, we analyze the complete relativistic expression for the spectrum of the $\beta$-electron emitted by a tritium nucleus. Given the small parameters in the problem, we systematically discuss the approximations that can be made, and present the first two corrections to the standard lowest order formula. We particularly discuss the case of an initially polarized target, and the consequences on the spectrum as a function of the neutrino mass. We show that, while it induces an angular anisotropy that can be measured by future experiments, such anisotropy cannot be used as an additional handle to constrain the neutrino mass.