Closing in on a perturbative fourth generation

TL;DR

This paper reevaluates the constraints on a perturbative fourth generation of fermions, proposing viable scenarios with different Higgs masses and specific decay channels, to guide LHC search strategies.

Contribution

It provides updated bounds on a perturbative fourth generation, identifies viable parameter space scenarios, and suggests targeted LHC search strategies including pair and single production modes.

Findings

Certain fourth generation scenarios remain viable within current constraints.

Proposed benchmark points for LHC searches include specific decay channels.

Search strategies should incorporate both pair and single production signatures.

Abstract

A perturbative new family of fermions is now severely constrained, though not excluded yet. We reconsider the current bounds (i.e., direct and from Higgs searches, R_b, oblique parameters) on the fourth generation parameter space assuming the case of a small CKM mixing with the third generation. We identify viable scenarios featuring either a light or a heavy Higgs boson. A set of representative benchmark points targeted for LHC searches is proposed with a normal (inverted) quark mass hierarchy where t' -> b'W (b' -> tW) decays are sizable. In the case where the fourth generation couplings to the lighter quark families are small, we suggest that search strategies at the LHC should include both pair (strong) and single (weak) production with bb+nW (n=2,...,6) final state signatures.