Bounding Effective Operators at the One-Loop Level: The Case of Four-Fermion Neutrino Interactions

The contributions of non-standard four-neutrino contact interactions to electroweak observables are considered at the one-loop level by using the effective quantum field theory. The analysis is done in terms of three unknown parameters: the strength of the non-standard neutrino interactions, $\tilde{F}$, an additional derivative coupling needed to renormalize the divergent contributions that appear when the four-neutrino interactions are used at the loop level and a non-standard non-derivative $Z$-${\bar\nu} \nu$ coupling. Then, the precise measurements of the invisible width of the $Z$-boson at LEP and the data on the neutrino deep-inelastic scattering yield the result $\tilde{F} = (-100 \pm 140) G_F$. Assuming that there are no unnatural cancellations between the contributions of the three effective couplings a much stronger bound is obtained: $\abs{\tilde{F}}\ltap~ 2 G_F$, which is a factor 200 better than the one obtained in previous analyses based on tree level calculations.