Session D04: Few-Nucleon Aspects of BSM Searches

Nuclear Ab Initio Calculations of Beta Decay Spectra

The recent decade has seen a surge in experimental efforts to measure β-decay observables in various nuclei, in search of beyond the standard model (BSM) signatures. These measurements dictate a need in highly accurate standard model theoretical predictions, to be compared with.

Motivated by these, we present a theoretical formalism for the analysis of these experiments, within and beyond the Standard Model, with controlled accuracy, based on a perturbative analysis of the theoretical observables related to the phenomena, including, e.g., high order nuclear recoil and shape corrections. The accuracy is analyzed by identifying a hierarchy of small parameters, related to the low momentum transfer characterizing β-decays. We focus on calculations of the correlations between the beta and neutrino particles, and the spectrum of allowed and forbidden decays. In the latter, we show increased sensitivity to highlight BSM signatures.

Concentrating on nuclei relevant to the ongoing experiments, we combine our theory with state-of-the-art nuclear many body calculations, and predict the β-neutrino correlation coefficients and β-energy spectrum for 6He and 23Ne. We compare these to existing, as well as novel measurements (the latter are conducted at SARAF accelerator, Israel), and achieve significant constraints on the standard model.   

Chiral EFTs for Neutrinoless Double Beta Decay

Neutrinoless double beta decay (NLDBD) is the most sensitive probe of lepton-number violation. Its discovery would be a clear signal of physics beyond the Standard Model, confirm the Majorana nature of neutrinos, and provide insight into scenarios of baryogenesis through leptogenesis. Whenever lepton-number violation arises at a scale well above the electroweak scale, it can be described by effective interactions in an effective-field theory (EFT) framework. In this talk, I will outline the necessary steps to assess the impact of these effective interactions on NLDBD half lives, focussing on the matching onto Chiral EFT at low energies. In particular, I will show why consistent renormalization of Chiral EFT requires the inclusion of a contact interaction at leading order and discuss ways to estimate the impact of this contact term. Finally, I will illustrate the constraints on the lepton-number violating interactions that can be set in this EFT framework.