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A<sc>bstract</sc> We investigate two-neutrino double beta decay (2νββ) in chiral effective field theory. We find contributions from weak magnetism and double-weak pion-exchange at next-to-leading-order in the chiral power counting. We discuss the impact of the chiral corrections on the electron spectra and find that they should be included in analyses of 2νββdecay that aim to uncover new physics signatures in the electron spectrum. We illustrate this point by revisiting the effect of sterile neutrinos and non-standard charged interactions. We also find that the pion-exchange contributions involve nuclear matrix elements that are related to those appearing in neutrinoless double beta decay (0νββ). We investigate whether the 0νββnuclear matrix elements can be obtained from detailed measurements of the energy spectrum of the outgoing electrons in 2νββtransitions. 

The accuracy of $V_{ud}$determinations from superallowed $\beta$decays critically hinges on control over radiative corrections. Recently, substantial progress has been made on the single-nucleon, universal corrections, while nucleus-dependent effects, typically parametrized by a quantity ${\delta }_{\mathrm{NS}}$, are much less well constrained. Here, we lay out a program to evaluate this correction from effective field theory (EFT), highlighting the dominant terms as predicted by the EFT power counting. Moreover, we compare the results to a dispersive representation of ${\delta }_{\mathrm{NS}}$and show that the expected momentum scaling applies even in the case of low-lying intermediate states. Our EFT framework paves the way toward calculations of ${\delta }_{\mathrm{NS}}$and thereby addresses the dominant uncertainty in $V_{ud}$. Published by the American Physical Society2024 

A<sc>bstract</sc> We presentνDoBe, a Python tool for the computation of neutrinoless double beta decay (0νββ) rates in terms of lepton-number-violating operators in the Standard Model Effective Field Theory (SMEFT). The tool can be used for automated calculations of 0νββrates, electron spectra and angular correlations for all isotopes of experimental interest, for lepton-number-violating operators up to and including dimension 9. The tool takes care of renormalization-group running to lower energies and provides the matching to the low-energy effective field theory and, at lower scales, to a chiral effective field theory description of 0νββrates. The user can specify different sets of nuclear matrix elements from various many-body methods and hadronic low-energy constants. The tool can be used to quickly generate analytical and numerical expressions for 0νββrates and to generate a large variety of plots. In this work, we provide examples of possible use along with a detailed code documentation. The code can be accessed through: GitHub:https://github.com/OScholer/nudobe Online User-Interface:https://oscholer-nudobe-streamlit-4foz22.streamlit.app/