Exotic Contributions to Double Beta Decay
Published in University College London, 2020
Searching for exotic contributions to double beta (\(\beta\beta\)) decay is paramount for probing the nature of neutrino masses and beyond the Standard Model physics. In this study, in addition to considering Majorana neutrinos, we introduce a hypothetical Majoron particle (\(\phi\)) which couples only to neutrinos via both right and left-handed currents. This particle is assumed massive and scalar. After a brief review of neutrino physics, we discuss the theory and experiments behind \(\beta\beta\) decay, including the key concepts of nuclear physics and experimental constraints which rule such reactions. We also discuss corrections and realistic approximations needed to be made throughout the calculation of the theoretical amplitudes. We then investigate three of the most promising \(\beta\beta\) decay modes: the standard neutrinoless double beta \(0\nu\beta\beta\) decay, the simplest Majoron emitting neutrinoless double beta \(0\nu\beta\beta\phi\) decay, and as an extension to the known literature, we mainly investigate the two peculiar modes of the simplest Majoron induced double beta \(2\nu_{\phi}\beta\beta\) decay. We name the first mode s-Channel. It is an extension to \(0\nu\beta\beta\phi\) decay, in which the Majoron is virtual and decays into a pair of neutrinos. The second mode is called t+u-channel. It is a very distinct process, similar to \(2\nu\beta\beta\) decay, in which the two emitted neutrinos are scattered via a Majoron exchange. We show that the neutrino emitting processes favour emitting electrons with total energy less than half of the decay’s kinetic energy release. Both \(0\nu\beta\beta\phi\) and \(2\nu_{\phi}\beta\beta\) are demonstrated to be heavily suppressed by the Majoron mass \(m_{\phi}\). Also, for \(2\nu_{\phi}\beta\beta\) decay, we show that increasing \(m_{\phi}\) shifts the electron energy distribution to a higher share of the kinetic energy release. This behaviour makes the normalised energy distribution of both \(2\nu_{\phi}\beta\beta\) and the standard double beta \(2\nu\beta\beta\) decays overlap and become almost indistinguishable for a large enough \(m_{\phi}\). Our results motivate further theoretical dedication and experimental searches for such exotic \(\beta\beta\) decay modes.