Joint analysis of Higgs decays and electroweak precision observables in the Standard Model with a sequential fourth generation

TL;DR

This study evaluates the compatibility of a Standard Model with a sequential fourth generation using Higgs decay data and electroweak precision observables, finding the three-generation model fits the data better.

Contribution

It provides a combined analysis of Higgs and electroweak data to assess the viability of the SM4, including fermion mass estimates and the impact of quark mixing.

Findings

Standard Model with four generations fits data worse than three generations.

Best-fit masses for fourth-generation fermions are around 632 GeV for t' and 58 GeV for neutrino.

Increased lower bounds on fourth-generation lepton mass affect model viability.

Abstract

We analyse the impact of LHC and Tevatron Higgs data on the viability of the Standard Model with a sequential fourth generation (SM4), assuming Dirac neutrinos and a Higgs mass of 125 GeV. To this end we perform a combined fit to the signal cross sections of pp -> H -> gamma gamma,ZZ*,WW* at the LHC, to p pbar -> VH -> V b bbar (V = W, Z) at the Tevatron and to the electroweak precision observables. Fixing the mass of the fourth generation down-type quark b' to 600 GeV we find best-fit values of m_t' = 632 GeV, m_l4 = 113.6 GeV and m_nu4 = 58.0 GeV for the other fourth-generation fermion masses. We compare the chi-square values and pulls of the different observables in the three and four-generation case and show that the data is better described by the three-generation Standard Model. We also investigate the effects of mixing between the third and fourth-generation quarks and of a future increased lower bound on the fourth-generation charged lepton mass of 250 GeV.