Primordial nucleosynthesis constraints on Z ' properties

Abstract

In models involving new TeV-scale Z(') gauge bosons, the new U(1)(') symmetry often prevents the generation of Majorana masses needed for a conventional neutrino seesaw mechanism, leading to three superweakly interacting "right-handed" neutrinos nu(R), the Dirac partners of the ordinary neutrinos. These can be produced prior to big bang nucleosynthesis by the Z(') interactions, leading to a faster expansion rate and too much He-4. We quantify the constraints on the Z(') properties from nucleosynthesis for Z(') couplings motivated by a class of E-6 models parametrized by an angle theta(E6). The rate for the annihilation of three approximately massless right-handed neutrinos into other particle pairs through the Z(') channel is calculated. The decoupling temperature, which is higher than that of ordinary left-handed neutrinos due to the large Z(') mass, is evaluated, and the equivalent number of new doublet neutrinos DeltaN(nu) is obtained numerically as a function of the Z(') mass and couplings for a variety of assumptions concerning the Z-Z(') mixing angle and the quark-hadron transition temperature T-c. Except near the values of theta(E6) for which the Z(') decouples from the right-handed neutrinos, the Z(') 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 T-c=150 MeV with the theoretically favored range of Z-Z(') mixings, DeltaN(nu)less than or similar to0.3 for M(Z)(')greater than or similar to4.3 TeV for any value of theta(E6). Larger mixing or larger T-c often lead to unacceptably large DeltaN(nu) except near the nu(R) decoupling limit.