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 seeming violation of universality in the τ lepton coupling to the W boson suggested by LEP II data is studied using an Effective Field Theory (EFT) approach. Within this framework we explore how this feature fits into the current constraints from electroweak precision observables using different assumptions about the flavor structure of New Physics, namely [U (2) × U (1)] 5 and U (2) 5 . We show the importance of leptonic and semileptonic tau decay measurements, giving 3 − 4 TeV bounds on the New Physics effective scale at 90% C.L. We conclude under very general assumptions that it is not possible to accommodate this deviation from universality in the EFT framework, and thus such a signal could only be explained by the introduction of light degrees of freedom or New Physics strongly coupled at the electroweak scale. 2 The code can be freely downloaded at the web page http://ific.uv.es/lhcpheno/.

A Higgsless electroweak theory may be populated by spin-1 resonances around E ∼ 1 TeV as a consequence of a new strong interacting sector, frequently proposed as a tool to smear the high-energy behaviour of scattering amplitudes, for instance, elastic gauge boson scattering. Information on those resonances, if they exist, must be contained in the low-energy couplings of the electroweak chiral effective theory. Using the facts that: i) the scattering of longitudinal gauge bosons, WL, ZL, can be well described in the high-energy region (E MW ) by the scattering of the corresponding Goldstone bosons (equivalence theorem) and ii) the zeros of the scattering amplitude carry the information on the heavier spectrum that has been integrated out; we employ the O(p 4 ) electroweak chiral Lagrangian to identify the parameter space region of the low-energy couplings where vector resonances may arise. An estimate of their masses is also provided by our method.

We analyse the violations of lepton-flavour universality in the ratios B(P → ℓν)/B(P → ℓ ′ ν) using a general effective theory approach, discussing various flavour-symmetry breaking patterns of physics beyond the SM. We find that in models with Minimal Lepton Flavour Violation the effects are too small to be observed in the next generations of experiments in all relevant meson systems (P = π, K, B). In a Grand Unified framework with a minimal breaking of the flavour symmetry, the effects remain small in π and K decays while large violations of lepton-flavour universality are possible in B → ℓν decays.

The existence of a new strong interacting sector around E ∼ 1 TeV is a common feature of Higgsless electroweak theories but also of theories with a light Higgs, for instance, when this is not elementary. In those schemes, this new interaction could be at the origin of an extended spectra with, in particular, spin-1 resonances that could be hinted in elastic gauge boson scattering. Information on those resonances, if they exist, must be contained in the low-energy couplings of the electroweak chiral effective theory. Using the facts that: i) the scattering of longitudinal gauge bosons, W L , Z L , can be well described in the high-energy region (E M W ) by the scattering of the corresponding Goldstone bosons (equivalence theorem) and that ii) the zeros of the scattering amplitude carry the information on the heavier spectrum that has been integrated out; we employ the O(p 4 ) electroweak chiral Lagrangian, with or without a light Higgs state to identify the parameter space region of the low-energy couplings where vector resonances may arise.