On the generality of the Cohen and Glashow constraints on the neutrino velocity

We discuss the kinematic limits for the process u_mu rightarrow u_mu + e^+ + e^- in the assumption that neutrinos are superluminal. We derive our results by assuming that: i) it exists one reference frame in which energy and momentum are conserved; ii) the Hamilton-Jacobi equation v=dE/dp is valid; iii) the present experimental information on the neutrino velocity at different energies are correct. We show that the considered process cannot be avoided unless very peculiar neutrino dispersion laws are assumed.