Improved precision calculation of the 0νββ contact term within chiral effective field theory

Abstract

Neutrinoless double-beta (0νββ) decay is an as-yet unobserved nuclear process, which stands to provide crucial insights for model building beyond the Standard Model of particle physics. Its detection would simultaneously confirm the hypothesis that neutrinos are Majorana fermions, thus violating lepton-number conservation, and provide the first measurement of the absolute neutrino mass scale. This work aims to improve the estimation within chiral effective field theory of the so-called “contact term” for 0νββ decay, a short-range two-nucleon effect that is unaccounted for in traditional nuclear approaches to the process. We conduct a thorough review of the justifications for this contact term and the most precise computation of its size to date [gνNN=1.3(6) at renormalization point μ=mπ], whose precision is limited by a truncation to elastic intermediate hadronic states. We then perform an extension of this analysis to a subleading class of inelastic intermediate states that we characterize, delivering an updated figure for the contact coefficient [gνNN=1.4(3) at μ=mπ] with uncertainty reduced by half. Such nuclear results, especially with enhanced precision, show promise for the resolution of disagreements between estimates of 0ννββ from different many-body methods. Published by the American Physical Society 2025