Type III Neutrino Seesaw, Freeze-In Long-Lived Dark Matter, and the $W$ Mass Shift

TL;DR

This paper explores a neutrino mass model involving heavy fermion triplets, a light fermion singlet, and a scalar triplet, which leads to a novel dark matter production mechanism and a slight upward shift in the $W$ mass, aligning with recent measurements.

Contribution

It introduces a new neutrino mass framework with additional particles and symmetries, enabling freeze-in dark matter and explaining the $W$ mass shift.

Findings

Long-lived dark matter produced via Higgs decay.

Slight upward shift in the $W$ mass consistent with measurements.

New particle interactions under a softly broken $Z_2$ symmetry.

Abstract

In the framework of seesaw neutrino masses from heavy fermion triplets $(\Sigma^+,\Sigma^0,\Sigma^-)$, the addition of a light fermion singlet $N$ and a heavy scalar triplet $(\rho^+,\rho^0,\rho^-)$ has some important consequences. The new particles are assumed to be odd under a new $Z_2$ symmetry which is only broken softly, both explicitly and spontaneously. With $N-\Sigma^0$ 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.