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, \tilde{\nu}\tilde{\nu}, splits the tau sneutrino into two mass eigenstates: \tilde{\nu} \rightarrow \tilde{\nu}_\pm. The absence of a Z \tilde{\nu}_- \tilde{\nu}_- coupling implies that the lighter mass eignestate, \tilde{\nu}_-, 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 \geq 10^{-2} events/kg/day. The lepton number violation required for sneutrino CDM implies that the tau neutrino mass is m_{\nu_\tau} \gtrsim 5 MeV, large enough to be excluded by B factory experiments. Events of the form l^+ l^- \not E or jj \not E , with low m_{ll} or m_{jj}, 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.