Sealing the fate of a fourth generation of fermions

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 Letter, 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 γγ and 4ℓ± search channels corresponding to a Higgs boson with a mass MH≈125 GeV, cannot rule out the fourth generation possibility if the H→γγ decay rate is evaluated when naively implementing the leading O(GFmf′2) electroweak corrections. Including the exact next-to-leading order electroweak corrections leads to a strong suppression of the H→γγ rate and makes this channel unobservable with present data. Finally, we point out that the observation by the Tevatron Collaborations of a ≳2σ excess in the mass range MH=115-135 GeV in the channel qq¯→WH→Wbb¯ 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.