Search for exotic physics in double-$\beta$ decays with GERDA Phase II

TL;DR

This paper reports on GERDA Phase II's search for exotic physics in double-beta decays of $^{76}$Ge, setting new limits on Majorons, Lorentz violation, and light exotic fermions, advancing understanding of physics beyond the Standard Model.

Contribution

First limits on Lorentz invariance violation effects and exotic fermions in double-beta decay using $^{76}$Ge with GERDA Phase II data.

Findings

Improved half-life limits for Majoron-involving decays (~10^23 yr)

First constraints on Lorentz violation effects in double-beta decay (~10^{-6} GeV)

First experimental limits on light exotic fermions including sterile neutrinos

Abstract

A search for Beyond the Standard Model double-$\beta$ decay modes of $^{76}$Ge has been performed with data collected during the Phase II of the GERmanium Detector Array (GERDA) experiment, located at Laboratori Nazionali del Gran Sasso of INFN (Italy). Improved limits on the decays involving Majorons have been obtained, compared to previous experiments with $^{76}$Ge, with half-life values on the order of 10$^{23}$ yr. For the first time with $^{76}$Ge, limits on Lorentz invariance violation effects in double-$\beta$ decay have been obtained. The isotropic coefficient $\mathring{a}_\text{of}^{(3)}$, which embeds Lorentz violation in double-$\beta$ decay, has been constrained at the order of $10^{-6}$ GeV. We also set the first experimental limits on the search for light exotic fermions in double-$\beta$ decay, including sterile neutrinos.