Status of the fourth fermion generation before ICHEP2012: Higgs data and electroweak precision observables

TL;DR

This paper assesses the viability of a fourth fermion generation in the Standard Model using Higgs and electroweak data, finding strong exclusion of the model at 3.1 sigma for certain parameters.

Contribution

It introduces a new method for likelihood ratio tests to compare non-nested models like SM3 and SM4 using current collider data.

Findings

SM4 is excluded at 3.1 sigma for Higgs mass 126.5 GeV and quark masses above 600 GeV.

The study provides a comprehensive global fit of SM4 parameters to collider and precision data.

It demonstrates the application of a novel statistical approach for model comparison in high-energy physics.

Abstract

We perform a global fit of the parameters of the Standard Model with a sequential fourth generation (SM4) to LHC and Tevatron Higgs data and electroweak precision data. Using several likelihood ratio tests we compare the performance of the SM4 and SM3 at describing the measured data. Since the SM3 and SM4 are not nested (i.e. the SM3 can not be considered as a special case of the SM4 with some parameters fixed) the usual analytical formulae for p-values in likelihood ratio tests do not hold. We thus apply a new method to compute these p-values. For a Higgs mass of 126.5 GeV and fourth-generation quark masses above 600 GeV we find that the SM4 is excluded at 3.1 sigma.