Improving electro-weak fits with TeV-scale sterile neutrinos

Abstract

We study the impact of TeV-scale sterile neutrinos on electro-weak precision observables and lepton number and flavour violating decays in the framework of a type-I see-saw extension of the Standard Model. At tree level sterile neutrinos manifest themselves via non-unitarity of the PMNS matrix and at one-loop level they modify the oblique radiative corrections. We derive explicit formulae for the S, T, U parameters in terms of the neutrino masses and mixings and perform a numerical fit to the electro-weak observables. We find regions of parameter space with a sizable active-sterile mixing which provide a better over-all fit compared to the case where the mixing is negligible. Specifically we find improvements of the invisible Z-decay width, the charged-to-neutralcurrent ratio for neutrino scattering experiments and of the deviation of the W boson mass from the theoretical expectation.

Sterile neutrinos in leptonic and semileptonic decays

“…In [23], an effective approach was used, and thus these constraints will only be considered in cases with multi-TeV singlet states. Our numerical results, which will be presented in section 4, are in agreement with the results of [24,25].…”

“…For Y ν ∼ few × 10 −2 , large active-sterile mixings can occur, possibly leading to a decrease in the Z boson decay width (in agreement with [24,25]). This can be confirmed on the right panel of figure 3, where we display Γ(Z → νν) versus the corresponding largest entry of the neutrino Yukawa couplings, max(Y ν ij ).…”

“…In the presence of singlet neutrinos, electroweak precision constraints were first addressed in [23] and recently studied in [24,25]. In [23], an effective approach was used, and thus these constraints will only be considered in cases with multi-TeV singlet states.…”

“…The apparent difference between the SM line (corresponding to a full computation of this observable) and our SM-limit (obtained for the regime of very smallη, for which the PMNS becomes unitary) in figure 3 is due to the fact that the latter is based on a tree-level computation; should higher order corrections be taken into account, all points would be shifted towards the SM theoretical prediction. Clearly, and even though the non-unitarity of the PMNS only indirectly affects Z → νν decays (via the small sterile component in the active light neutrino eigenstates) this EW precision observable is a crucial consistency check of any model with extra sterile fermions, as the invisible decay width cannot exceed the SM prediction [24,25]. As can be seen in the left panel of figure 3, a sizable reduction of the invisible decay width could indeed occur for a regime of largeη.…”

Impact of sterile neutrinos on nuclear-assisted cLFV processes

“…We take into account the impact of sterile neutrinos on the invisible Z-decay width (which has been addressed in [55][56][57][58]), requiring compatibility with LEP results on Γ(Z → νν) [59]; in addition, we further require that potential new contributions to the cLFV Z decay width do not exceed the present uncertainty on the total Z width [59]:…”