Global Fit to Modified Neutrino Couplings and the Cabibbo-Angle Anomaly

Abstract

Recently, discrepancies of up to 4σ between the different determinations of the Cabibbo angle were observed. In this context, we point out that this "Cabibbo-angle anomaly" can be explained by lepton flavor universality violating new physics in the neutrino sector. However, modified neutrino couplings to standard model gauge bosons also affect many other observables sensitive to lepton flavor universality violation, which have to be taken into account in order to assess the viability of this explanation. Therefore, we perform a model-independent global analysis in a Bayesian approach and find that the tension in the Cabibbo angle is significantly reduced, while the agreement with other data is also mostly improved. In fact, nonzero modifications of electron and muon neutrino couplings are preferred at more than 99.99% C.L. (corresponding to more than 4σ). Still, since constructive effects in the muon sector are necessary, simple models with right-handed neutrinos (whose global fit we update as a by-product) cannot fully explain data, pointing towards more sophisticated new physics models.

ν electroweak baryogenesis

“…Notice that the CP invariant is suppressed by the sixth power of θ, hence the importance of ensuring large mixing and the reason to consider low scale seesaw realizations in this context that decouple its size from the smallness of neutrino masses. Nevertheless, constraints from precision electroweak and flavour observables exist [72][73][74][75][76][77][78][79][80] on the combination θθ † . Indeed, θθ † represents the coefficient of the only dimension 6 operator obtained at tree level 3 when integrating out the heavy neutrino degrees of freedom [82].…”

“…This choice for m dα implies that the neutrino Yukawa couplings to the second and third heavy states have the same magnitude. Additionally, it makes the coupling to the muon to vanish, for which the bounds on θθ † are the most stringent [78][79][80]. Therefore, the matrix m D now depends on a single parameter, m dτ .…”

Hadronic Vacuum Polarization:(g−2)μversus Global Electroweak Fits

“…However, low-energy precision experiments have accumulated intriguing hints for the violation of lepton flavor universality within recent years [see, e.g., Refs. [8][9][10][11][12][13][14][15][16][17][18] for b → cτν, b → sl þ l − , and RðV us Þ]. In particular, the Brookhaven measurement of the anomalous magnetic moment of the muon ðg − 2Þ μ [19] shows a tension of about 3.7σ with the SM prediction [for the electron, an analogous 2.5σ difference (but with opposite sign) between the SM prediction [44,45] based on the Cs measurement of the fine-structure constant α [46] and the direct measurement of ðg − 2Þ e [47] has emerged [48,49] ].…”

β Decays as Sensitive Probes of Lepton Flavor Universality

“…Here, there is the longstanding discrepancy in ðg − 2Þ μ of about 3.7σ and a recently emerging deviation in the electron case of 2.5σ, interestingly, with the opposite sign [67][68][69][70][71][72][73]. Furthermore, it has been shown that LFU violating neutrino interactions with SM gauge bosons give an excellent fit of electroweak data, including LFU tests [74].…”

“…In this context, it seems natural to consider the discrepancies between the different determinations of V ud (or, equivalently, V us under the assumption of CKM unitarity), in particular the direct determination from β decays, not as a sign of right-handed currents or as a violation of CKM unitarity, but rather as a sign of LFUV [74]. In fact, the most precise determination of V ud from K → μν involves muons, while β decays can only have electrons in the final states.…”