A Majorana mass term for the tau neutrino would induce neutrino-antine
utrino mixing and thereby a process which violates fermion number by t
wo units. We study the possibility of distinguishing between a massive
Majorana and a Dirac tau neutrino, by measuring fermion number violat
ing 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 lepto
ns 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 Maj
orana and Dirac tau neutrinos.