Abstract
We address the impact of a modified $W \ell \nu$ coupling on a wide range of observables, such as $\tau$ leptonic and mesonic decays, leptonic decays of pseudoscalar mesons, as well as semileptonic meson decays. In particular, we concentrate on deviations from lepton flavour universality, focusing on the ratios $R_{P} = \Gamma (P \to \ell \nu) / \Gamma (P \to \ell' \nu)$, with $P=K, \pi, D, D_s$, $R(D)={\Gamma (B^+ \to D \tau^+ \nu)}/{\Gamma (B^+ \to D\ell^+ \nu)}$, $R \tau={\Gamma (\tau\to \mu\nu\nu)}/{\Gamma (\tau\to e\nu\nu)}$, $R^{\ell \tau}_P=\Gamma(\tau\to P\nu)/\Gamma(P\to \ell \nu)$, and $\text{BR}(B \to \tau \nu)$. We further consider leptonic gauge boson decays, such as $W\to \ell \nu $ and $Z \to \nu \nu$. For all the above observables, we provide the corresponding complete analytical expressions, derived for the case of massive neutrinos. Working in the framework of the Standard Model extended by additional sterile fermions, which mix with the active (left-handed) neutrinos, we numerically study the impact of active-sterile mixings on the above mentioned observables.
Highlights
Generations, U corresponds to the unitary PMNS matrix, UPMNS
As an illustrative example, we consider the case of the Inverse Seesaw [13] to discuss the potential of a model with sterile neutrinos regarding tree-level contributions to leptonic and semileptonic meson decays
In this work we have tried to reconcile theory and experiment in leptonic and semileptonic decays, under the hypothesis of New Physics contributions associated with the lepton
Summary
One of the simplest extensions of the SM allowing to accommodate massive neutrinos consists in the introduction of right-handed states νR, singlets under the SM gauge group. The SM mass Lagrangian is enlarged with a Dirac mass term mDνRνL, and should lepton number violation be allowed, with a Majorana mass term mM νRc νR Within this class of models, the standard type-I seesaw [14,15,16,17] is an appealing framework, where a natural explanation for the smallness of neutrino masses can be found by assuming that the Majorana masses of the right-handed neutrinos are large, leading to a suppression of mν ∼ m2D/mM. Low-scale seesaw models, in which the new singlet fermions are lighter, with masses around the electroweak scale, are more attractive from a phenomenological point of view In this case, the new states can be produced in collider or low-energy experiments and their contributions to physical processes can be sizable. In our work we will consider this type of models.
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