
The contributions of nonstandard fourneutrino contact interactions to electroweak observables are considered at the oneloop level by using the effective quantum field theory language. The analysis is done in terms of three unknown parameters: the strength of the nonstandard neutrino interactions, F , an additional derivative coupling needed to renormalize the divergent contributions that appear when the fourneutrino interactions are used at the loop level and a nonstandard nonderivative Zv¯v coupling. Then, the precise measurements of the invisible width of the Zboson at LEP and the data on the neutrino deepinelastic scattering yield the result F = (−100±140)GF. Assuming that there are no unnatural cancellations between the contributions of the three effective couplings a much stronger bound is obtained: F  ≲ 2GF, which is a factor 200 better than the one obtained in previous analyses based on tree level calculations.
