Upper bounds on neutrino masses

Using a recent experimental bound on τ decay into three charged leptons and a weak assumption concerning a general “see-saw” mechanism for neutrino masses, we show that both v_μ and v_τ must be lighter than 65 eV. If the “see-saw” is driven by a right-handed W boson or by a “horizontal” gauge boson, they must be heavier than 50 PeV.