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 π → eνγ, 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 neutronto-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 < ∼ 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.

The status of tests of the standard electroweak model and of searches for new physics in allowed nuclear β decay and neutron decay is reviewed including both theoretical and experimental developments. The sensitivity and complementarity of recent and ongoing experiments are discussed with emphasis on their potential to look for new physics. Measurements are interpreted using a model-independent effective field theory approach enabling to recast the outcome of the analysis in many specific new physics models. Special attention is given to the connection that this approach establishes with high-energy physics. A new global fit of available β-decay data is performed incorporating, for the first time in a consistent way, superallowed 0 + → 0 + transitions, neutron decay and nuclear decays. The constraints on exotic scalar and tensor couplings involving left-or right-handed neutrinos are determined while a constraint on the pseudoscalar coupling from neutron decay data is obtained for the first time as well. The values of the vector and axial-vector couplings, which are associated within the standard model to V ud and g A respectively, are also updated. The ratio between the axial and vector couplings obtained from the fit under standard model assumptions is C A /C V = −1.27510(66). The relevance of the various experimental inputs and error sources is critically discussed and the impact of ongoing measurements is studied. The complementarity of the obtained bounds with other low-and high-energy probes is presented including ongoing searches at the Large Hadron Collider.

We describe non-standard contributions to semileptonic processes in a model independent way in terms of an SU (2) L × U (1) Y invariant effective lagrangian at the weak scale, from which we derive the low-energy effective lagrangian governing muon and beta decays. We find that the deviation from Cabibbo universality, ∆ CKM ≡ |V ud | 2 + |V us | 2 + |V ub | 2 − 1, receives contributions from four effective operators. The phenomenological bound ∆ CKM = (−1 ± 6) × 10 −4 provides strong constraints on all four operators, corresponding to an effective scale Λ > 11 TeV (90% CL). Depending on the operator, this constraint is at the same level or better then the Z pole observables. Conversely, precision electroweak constraints alone would allow universality violations as large as ∆ CKM = −0.01 (90% CL). An observed ∆ CKM = 0 at this level could be explained in terms of a single four-fermion operator which is relatively poorly constrained by electroweak precision measurements.

We discuss low-energy and collider constraints on the effective couplings characterizing non-standard charged current interactions. A direct comparison of low-energy and LHC probes can be performed within an effective theory framework, when the new physics mediating these interactions originates in the multi-TeV scale. We find that for the effective couplings involving right-handed neutrinos the LHC bounds from pp → e + M ET + X are at the (sub)percent level, already stronger than those from β decays. For operators involving left-handed neutrinos, the (axial-)vector and pseudo-scalar effective couplings are best probed at low energy, while scalar and tensor couplings are currently probed at the same level by beta decays and the LHC channels pp → e + M ET + X and, by using SU(2) gauge invariance, pp → e + e − + X. Future beta decay experiments at the 0.1% level or better will compete in sensitivity with higher statistics and higher energy data from the LHC.