NuTeV anomaly, neutrino mixing, and a heavy Higgs boson

Recent results from the NuTeV experiment at Fermilab and the deviation of the Z invisible width, measured at CERN LEP and the SLAC Linear Collider, from its standard model (SM) prediction suggest the suppression of neutrino-Z couplings. Such suppressions occur naturally in models which mix the neutrinos with heavy gauge singlet states. We postulate a universal suppression of the $Z\nu \nu$ couplings by a factor of $\left(1\mathrm{}-\varepsilon \right)$ and perform a fit to the Z-pole and NuTeV observables with $\varepsilon$ and the oblique correction parameters S and T. Compared to a fit with S and T only, the inclusion of $\varepsilon$ leads to a dramatic improvement in the quality of the fit. The values of S and T preferred by the fit can be obtained within the SM by a simple increase in the Higgs boson mass. However, if the W mass is also included in the fit, a non-zero U parameter becomes necessary which cannot be supplied within the SM. The preferred value of $\varepsilon$ suggests that the seesaw mechanism may not be the reason why neutrinos are so light.