Abstract: Hadronic tau decays are studied as probe of new physics. We determine the dependence of several inclusive and exclusive tau observables on the Wilson coefficients of the low-energy effective theory describing charged-current interactions between light quarks and leptons. The analysis includes both strange and non-strange decay channels. The main result is the likelihood function for the Wilson coefficients in the tau sector, based on the up-to-date experimental measurements and state-of-the-art theoretical techniques. The likelihood can be readily combined with inputs from other low-energy precision observables. We discuss a combination with nuclear beta, baryon, pion, and kaon decay data. In particular, we provide a comprehensive and model-independent description of the new physics hints in the combined dataset, which are known under the name of the Cabibbo anomaly.

Abstract: Scalar and tensor interactions were once competitors to the now well-established V – A structure of the standard model weak interactions. We revisit these interactions and survey constraints from low-energy probes (neutron, nuclear, and pion decays) as well as collider searches. Currently, the most stringent limit on scalar and tensor interactions arise from 0(+) -> 0(+) nuclear decays and the radiative pion decay pi -> e nu gamma, respectively. For the future, we find that upcoming neutron beta decay and LHC measurements will compete in setting the most stringent bounds. For neutron beta decay, we demonstrate the importance of lattice computations of the neutron-to-proton matrix elements to setting limits on these interactions, and provide the first lattice estimate of the scalar charge and a new average of existing results for the tensor charge. Data taken at the LHC is currently probing these interactions at the 10(-2) level (relative to the standard weak interactions), with the potential to reach the less than or similar to 10(-3) level. We show that, with some theoretical assumptions, the discovery of a charged spin-0 resonance decaying to an electron and missing energy implies a lower limit on the strength of scalar interactions probed at low energy.

Abstract: This note reviews the role of superallowed transitions in determining the strength of the weak interaction among the lightest quarks and in searching for new physics beyond the standard electroweak model. The two sets of superallowed decays in nuclei considered here are pure Fermi and mirror transitions. The first have been scrutinized for more than 50 years. The most relevant results are presented and the role of the nucleus-dependent radiative correction and nucleus-independent inner radiative correction are reviewed. In this context, the systematic study of mirror transitions started about 15 years ago. Despite the significant progress made since then, the data is still limited by experimental uncertainties. Combining the results from all superallowed transitions, which are fully consistent, provides a test of unitarity of the first row of the Cabibbo-Kobayashi-Maskawa matrix, which displays a 2 sigma tension with the standardmodel.Superallowed transitions in beta decay are considered to be the “cleanest” ones in terms of hadronic contributions arising from the nuclear medium. These transitions have been identified since the early days in the study of beta decay and have played a crucial role in determining the strength of weak processes involving the lightest u and d quarks. They offer today a sensitive window to search for NP through high precision measurements. This paper reviews the contributions of pure Fermi and mirror superallowed transitions, to determine parameters within the SMor to constrain NP. It relies in particular on the results of four recent reviews and global analyses which can be found in Refs. [1-4]. Although neutron decay is the simplest mirror transition, the recent progress in neutron decay is not covered here besides mentioning the most relevant results.