NuTeV anomaly, lepton universality, and nonuniversal neutrino-gauge couplings

In previous studies we found that models with flavor-universal suppression of the neutrino-gauge couplings are compatible with NuTeV and $Z$-pole data. In this paper we expand our analysis to obtain constraints on flavor-dependent coupling suppression by including lepton universality data from $W$, $\tau$, $\pi$ and $K$ decays in fits to model parameters. We find that the data are consistent with a variety of patterns of coupling suppression. In particular, in scenarios in which the suppression arises from the mixing of light neutrinos with heavy gauge singlet states (neutrissimos), we find patterns of flavor-dependent coupling suppression which are also consistent with constraints from $\mu \to e\gamma$.

Figure 1

The limits on ${\Delta }_{e\tau }$ and ${\Delta }_{\mu \tau }$ from (a) $W$-decay, (b) $\tau$-decay, (c) $\pi$ and $K$-decay, and (d) all decays combined. The $1\sigma$ bands are shown for each coupling constant ratio ignoring correlations. The shaded areas represent the $68\%$ (dark gray) and $90\%$ (light gray) confidence contours including correlations.

Figure 2

The 68% and 90% confidence contours projected onto various planes for the five-paremeter fit with $S$, $T$, $U$, ${\epsilon }_e$ and ${\epsilon }_{\mu }$. The bands associated with each observable show the $1\sigma$ limits in the respective planes. The origin is the reference SM with $M_{\mathrm{t}\mathrm{o}\mathrm{p}}=178.0\mathrm{G}\mathrm{e}\mathrm{V}$ and $M_{\mathrm{H}\mathrm{i}\mathrm{g}\mathrm{g}\mathrm{s}}=115\mathrm{G}\mathrm{e}\mathrm{V}$. The dashed contour in 2(f) is the 90% lepton universality bound.