Primordial nucleosynthesis constraints on [Formula Presented] properties

In models involving new TeV-scale [Formula Presented] gauge bosons, the new [Formula Presented] symmetry often prevents the generation of Majorana masses needed for a conventional neutrino seesaw mechanism, leading to three superweakly interacting “right-handed” neutrinos [Formula Presented] the Dirac partners of the ordinary neutrinos. These can be produced prior to big bang nucleosynthesis by the [Formula Presented] interactions, leading to a faster expansion rate and too much [Formula Presented] We quantify the constraints on the [Formula Presented] properties from nucleosynthesis for [Formula Presented] couplings motivated by a class of [Formula Presented] models parametrized by an angle [Formula Presented] The rate for the annihilation of three approximately massless right-handed neutrinos into other particle pairs through the [Formula Presented] channel is calculated. The decoupling temperature, which is higher than that of ordinary left-handed neutrinos due to the large [Formula Presented] mass, is evaluated, and the equivalent number of new doublet neutrinos [Formula Presented] is obtained numerically as a function of the [Formula Presented] mass and couplings for a variety of assumptions concerning the [Formula Presented] mixing angle and the quark-hadron transition temperature [Formula Presented] Except near the values of [Formula Presented] for which the [Formula Presented] decouples from the right-handed neutrinos, the [Formula Presented] mass and mixing constraints from nucleosynthesis are much more stringent than the existing laboratory limits from searches for direct production or from precision electroweak data, and are comparable to the ranges that may ultimately be probed at proposed colliders. For the case [Formula Presented] with the theoretically favored range of [Formula Presented] mixings, [Formula Presented] for [Formula Presented] for any value of [Formula Presented] Larger mixing or larger [Formula Presented] often lead to unacceptably large [Formula Presented] except near the [Formula Presented] decoupling limit.