Number of neutrinos fromW,Zhadroproduction: Last count

Abstract

We determine the number of light neutrinos directly from data on (i) the production of W and Z bosons in pp¯ collisions and (ii) deep-inelastic scattering of leptons on hydrogen and deuterium. Ambiguities related to the use of structure functions are avoided. We conclude that at 95% confidence level, $N_{\nu }$≤3 and m(top)≤60 GeV. $N_{\nu }$=4 is allowed if the fourth neutrino is accompanied by a heavy charged lepton lighter than $M_W$. .AE