Sealing the fate of a fourth generation of fermions

TL;DR

This paper examines the implications of a hypothetical fourth generation of fermions on Higgs boson detection, showing that current experimental results strongly disfavor such scenarios, especially when electroweak corrections are properly included.

Contribution

It provides a detailed analysis of how electroweak corrections affect Higgs decay rates in fourth-generation models, challenging previous interpretations of collider data.

Findings

Fourth generation models are incompatible with observed Higgs signals when electroweak corrections are included.

Tevatron excesses cannot be explained by a fourth generation fermion scenario.

Perturbative fourth generation fermions are strongly disfavored by current collider data.

Abstract

The search for the effects of heavy fermions in the extension of the Standard Model with a fourth generation is part of the experimental program of the Tevatron and LHC experiments. Besides being directly produced, these states affect drastically the production and decay properties of the Higgs boson. In this note, we first reemphasize the known fact that in the case of a light and long-lived fourth neutrino, the present collider searches do not permit to exclude a Higgs boson with a mass below the WW threshold. In a second step, we show that the recent results from the ATLAS and CMS collaborations which observe an excess in the $\gamma \gamma$ and $4\ell^\pm$ search channels corresponding to a Higgs boson with a mass $M_H \approx 125$ GeV, cannot rule out the fourth generation possibility if the $H \to \gamma \gamma$ decay rate is evaluated when naively implementing the leading ${\cal O}(G_F m_{f'}^2)$ electroweak corrections. Including the exact next-to-leading order electroweak corrections leads to a strong suppression of the $H \to \gamma \gamma$ rate and makes this channel unobservable with present data. Finally, we point out that the observation by the Tevatron collaborations of a $\gsim 2\sigma$ excess in the mass range $M_H = 115$-135 GeV in the channel $q\bar q \to WH \to Wb\bar b$ can definitely not be accommodated by the fourth generation fermion scenario. All in all, if the excesses observed at the LHC and the Tevatron are indeed due to a Higgs boson, they unambiguously exclude the perturbative fermionic fourth generation case. In passing, we also point out that the Tevatron excess definitely rules out the fermiophobic Higgs scenario as well as scenarios in which the Higgs couplings to gauge bosons and bottom quarks are significantly reduced.