Long Lived Fourth Generation and the Higgs

TL;DR

This paper explores a fourth generation of particles with long-lived neutrinos affecting Higgs decay modes, potentially contributing to dark matter and detectable at colliders and in dark matter experiments.

Contribution

It introduces a scenario with long-lived fourth generation neutrinos with both Dirac and Majorana masses, analyzing their impact on Higgs phenomenology and dark matter.

Findings

Fourth generation neutrinos can be as light as 40 GeV.

Higgs can predominantly decay into neutrino pairs below W-boson threshold.

The relic density of the neutrino dark matter can be up to 20% of the universe's dark matter.

Abstract

A chiral fourth generation is a simple and well motivated extension of the standard model, and has important consequences for Higgs phenomenology. Here we consider a scenario where the fourth generation neutrinos are long lived and have both a Dirac and Majorana mass term. Such neutrinos can be as light as 40 GeV and can be the dominant decay mode of the Higgs boson for Higgs masses below the W-boson threshold. We study the effect of the Majorana mass term on the Higgs branching fractions and reevaluate the Tevatron constraints on the Higgs mass. We discuss the prospects for the LHC to detect the semi-invisible Higgs decays into fourth generation neutrino pairs. Under the assumption that the lightest fourth generation neutrino is stable, it's thermal relic density can be up to 20% of the observed dark matter density in the universe. This is in agreement with current constraints on the spin dependent neutrino-neutron cross section, but can be probed by the next generation of dark matter direct detection experiments.