Nuclear structure studies and source calibration tests relevant for 136Xe neutrinoless double beta decay

Abstract

The fact that neutrinos have small non-zero masses provides one of the best motivations to search for new physics, beyond the current Standard Model (SM) of elementary particles. Consequently, many experimental programs are being put at the forefront to probe the absolute scale of neutrino masses and to determine whether the neutrinos are Majorana fermions, i.e the same as their own antiparticles. One of the most practical ways to address this is to search for a rare, yet-to-be observed nuclear decay mode, called neutrinoless double beta decay (0νββ). The observation of 0νββ would demonstrate the Majorana nature of neutrinos. More importantly, its measured decay rate will allow a means to determine the absolute neutrino mass-scale, provided the nuclear matrix element (NME) for the decay is well known. However, achieving the above has proven to be very challenging. From an experimental point of view, searching for such a rare decay requires large ultrasensitive detectors and high levels of background rejection. On the other hand, the large uncertainties in calculated NME values significantly affect current extracted limits on the effective Majorana neutrino mass. In this work, I explore calibration techniques that aim to study the detector response for a next-generation ton-scale 136Xe double beta decay experiment (nEXO), with a liquid xenon time projection chamber (TPC). Additionally, I use a high-resolution spectroscopic study of lowlying states in 136Cs, produced via a 137Ba(d,3He) single-nucleon transfer reaction.