Masses of a Fourth Generation with Two Higgs Doublets

TL;DR

This paper investigates the mass constraints of a hypothetical fourth fermion generation within two Higgs doublet models, considering different Higgs masses and recent collider data, revealing potential scenarios where the fourth generation could exist.

Contribution

It provides the first detailed analysis of fourth-generation fermion masses in two Higgs doublet models, accounting for recent Higgs boson mass hints and electroweak precision constraints.

Findings

Inverted mass hierarchies are possible for light Higgs but large quark mass splittings are unlikely.

Heavy Higgs (600 GeV) imposes different constraints on fermion masses.

Two Higgs doublet models can potentially accommodate a fourth generation despite collider hints.

Abstract

We use sampling techniques to find robust constraints on the masses of a possible fourth sequential fermion generation from electroweak oblique variables. We find that in the case of a light (115 GeV) Higgs from a single electroweak symmetry breaking doublet, inverted mass hierarchies are possible for both quarks and leptons, but a mass splitting more than M(W) in the quark sector is unlikely. We also find constraints in the case of a heavy (600 GeV) Higgs in a single doublet model. As recent data from the Large Hadron Collider hints at the existence of a resonance at 124.5 GeV and a single Higgs doublet at that mass is inconsistent with a fourth fermion generation, we examine a type II two Higgs doublet model. In this model, there are ranges of parameter space where the Higgs sector can potentially counteract the effects of the fourth generation. Even so, we find that such scenarios produce qualitatively similar fermion mass distributions.