THE TAU-NEUTRINO AS A MAJORANA PARTICLE

A Majorana mass term for the tau neutrino would induce neutrino-antineutrino mixing and thereby a process which violates fermion number by two units. We study the possibility of distinguishing between a massive Majorana and a Dirac tau neutrino, by measuring fermion number violating processes in a deep inelastic scattering experiment nup --> tauX. We show that, if the neutrino beam is obtained from the decay of high energetic pions, the probability of obtaining ''wrong sign'' tau leptons is suppressed by a factor O(m(nutau)2theta2/m(mu)2) instead of the naively expected suppression factor theta2m(nutau)2/E(nu)2, where E(nu) is the tau neutrino energy, m(nutau) and m(mu) are the tau neutrino and muon masses, respectively, and theta is the nu(mu)-nu(tau) Mixing angle. If m(nutau) is of the order of 10 MeV and theta is of the order of 0.01-0.04 (the present bounds are (m(nutau) < 35 MeV, theta < 0.04) the next round of experiments may be able to distinguish between Majorana and Dirac tau neutrinos.