Higgs Properties and Fourth Generation Leptons

TL;DR

This paper explores how a hypothetical fourth generation of leptons affects Higgs boson decay rates and mass predictions, showing that heavy leptons suppress certain decay channels and influence the Higgs mass range.

Contribution

It introduces a model with a new vector-like lepton generation and analyzes its effects on Higgs decay branching ratios and mass constraints, extending previous standard-model predictions.

Findings

Heavy leptons reduce h -> gamma gamma branching ratio to about 30% of SM value.

Higgs production rate remains close to SM if new quarks are very heavy.

Higgs mass is constrained around 175 GeV for certain cutoff scales.

Abstract

It is possible that there are additional vector-like generations where the quarks have mass terms that do not originate from weak symmetry breaking, but the leptons only get mass through weak symmetry breaking. We discuss the impact that the new leptons have on Higgs boson decay branching ratios and on the range of allowed Higgs masses in such a model (with a single new vector-like generation). We find that if the fourth generation leptons are too heavy to be produced in Higgs decay, then the new leptons reduce the branching ratio for h -> gamma gamma to about 30% of its standard-model value. The dependence of this branching ratio on the new charged lepton masses is weak. Furthermore the expected Higgs production rate at the LHC is very near its standard-model value if the new quarks are much heavier than the weak scale. If the new quarks have masses near the cutoff for the theory then for cutoffs greater than 10^15 GeV, the new lepton masses cannot be much heavier than about 100 GeV and the Higgs mass must have a value around 175 GeV.