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A heavy unstable fourth family neutrino and the τ-lifetime discrepancy


The existence of the fourth family heavy unstable neutrino ν4 with mν4 > mτ and the dominant mixing angle ∥Uτ4∥2=0.042-0.10 is postulated from the present data on the lifetime and the leptonic branching ratios of the τ-. The fast decay of a heavy ν4 makes it undetectable in present experiments, while the future detection at Z0 factories is shown to be very promising, provided mν4≤46 GeV. The contributions of this massive ν4 and other fourth family fermions to the total decay widths of Z0 and W± are considered. If the 90% CL of the present UA2's data on λz/λw is valid, either the t-quark or ν4 or both should be seen (mt≤76 GeV or mν4≤46 GeV in the near future. Neutrino counting via e+e- →γνν is predicted to yield Nν=3. The mass of the fourth family charged lepton is predicted to be mL≤43 GeV if the prototype relation between the mixing angle and the mass eigenvalues is valid. The present UA1's bound on mL, mL≥41 GeV, is shown to be invalid, while the correct lower bound is the 25 GeV of TRISTAN, implying the likely possibility of detecting the fourth charged lepton L± in the present and forthcoming e+e- colliders. These predictions can be tested in the near future at the Z0 factories at SLC and LEP. © 1988.

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