Type III neutrino seesaw, freeze-in long-lived dark matter, and the W mass shift

Abstract

In the framework of seesaw neutrino masses from heavy fermion triplets (Σ+,Σ0,Σ−), the addition of a light fermion singlet N and a heavy scalar triplet (ρ+,ρ0,ρ−) has some important consequences. The new particles are assumed to be odd under a new Z2 symmetry which is only broken softly, both explicitly and spontaneously. With N−Σ0 mixing, freeze-in long-lived dark matter through Higgs decay becomes possible. At the same time, the W mass is shifted slightly upward, as suggested by a recent precision measurement.