Sneutrino cold dark matter with lepton-number violation

The tau sneutrino is proposed as a candidate for galactic halo dark matter, and as the cold dark matter (CDM) component of the universe. A lepton-number-violating sneutrino mass, ν̃ν̃, splits the tau sneutrino into two mass eigenstates: ν̃→ν̃±. The absence of a Zν̃−ν̃− coupling implies that the lighter mass eigenstate, ν̃−, does not annihilate via the s-channel Z-exchange to a low cosmological abundance, and furthermore, halo sneutrinos do not scatter excessively in Ge detectors. For the majority of the relevant parameter space, the event rate in Ge detectors is ≥10−2 events/kg/day. The lepton number violation required for sneutrino CDM implies that the tau neutrino mass is mντ≳5 MeV, large enough to be excluded by B factory experiments. Events of the form l+l−E/ or jjE/, with low mll or mjj, may be observed at LEP2. A seesaw mechanism is investigated as the origin for the lepton number violation, and several other cosmological and particle physics consequences of sneutrino CDM are discussed.