Impact of a Higgs boson at a mass of 126 GeV on the standard model with three and four fermion generations

TL;DR

This paper analyzes the impact of a 126 GeV Higgs boson on the Standard Model with three and four fermion generations, using latest experimental data to evaluate model fits and exclusions.

Contribution

It provides a comprehensive statistical analysis of the Standard Model with three and four generations, incorporating recent Higgs search results and electroweak measurements to assess model viability.

Findings

Best-fit Higgs mass is 126.0 GeV.

SM4 with a fourth fermion generation is excluded at 5.3 sigma.

Indirect top mass prediction aligns with direct measurements.

Abstract

We perform a comprehensive statistical analysis of the standard model (SM) with three and four generations using the latest Higgs search results from LHC and Tevatron, the electroweak precision observables (EWPOs) measured at LEP and SLD and the latest determinations of M_W, m_t and alpha_s. For the three-generation case we analyse the tensions in the electroweak fit by removing individual observables from the fit and comparing their predicted values with the measured ones. In particular, we discuss the impact of the Higgs search results on the deviations of the EWPOs from their best-fit values. Our indirect prediction of the top mass is m_t=175.7(+3.0)(-2.2) GeV at 68.3% CL, in good agreement with the direct measurement. We also plot the preferred area in the M_W - m_t plane. The best-fit Higgs mass is 126.0 GeV. For the case of the SM with a perturbative sequential fourth fermion generation (SM4) we discuss the deviations of the Higgs signal strengths from their best-fit values. The H -> gamma gamma signal strength now disagrees with its best-fit SM4 value at more than 4 sigma. We perform a likelihood-ratio test to compare the SM and SM4 and show that the SM4 is excluded at 5.3 sigma. Without the Tevatron data on H -> b bbar the significance drops to 4.8 sigma.