CONSTRAINTS ON LEFT–RIGHT SYMMETRIC AND E6 SUPERSTRING MODELS FROM THE NUMBER OF LIGHT NEUTRINO SPECIES

Using the experimental value for the rate $R^{\rm LEP}_{\rm exp}=\Gamma_{\rm inv}/\Gamma_{l\bar l}=5.942\pm 0.016$, we derive constraints on the number of light neutrino species with the invisible width method in the framework of a left–right symmetric model (LRSM) and a E6 superstring model as a function of their respective mixing angles for the neutral vector gauge bosons. Using the above LEP result we get the constraints -1.6×10-3≤ ϕ ≤ 1.1 × 10-3 and -1.3 × 10-2 ≤ ϕE6 ≤ 0.4 × 10-2, which are stronger than those obtained in previous studies of these models.