Counting neutrino species in high-energy proton-antiproton collisions

The ratio of the cross sections $\frac{\sigma (p\overline{p}\to ZX)}{\sigma (p\overline{p}\to WX)}$ is calculated in the framework of QCD. Although the absolute value of the cross section has large uncertainties, most of them are found to cancel in the ratio. Remaining uncertainties are studied in detail. An upper bound on the number of low-mass neutrinos is obtained. The effect of $t$-quark mass and fourth charged lepton is discussed.