Combined search for the quarks of a sequential fourth generation

TL;DR

This paper reports a search for a hypothetical fourth generation of quarks at the LHC, setting new mass limits and constraining the parameter space for such particles using a novel combined search strategy.

Contribution

It introduces a new combined search method for fourth-generation quarks and establishes the most stringent mass limits to date within a simple extension of the standard model.

Findings

Mass-degenerate fourth-generation quarks below 685 GeV are excluded at 95% CL.

The mass limit shifts by about +/- 20 GeV with a 25 GeV mass difference.

The results significantly restrict the parameter space for a fourth fermion generation.

Abstract

Results are presented from a search for a fourth generation of quarks produced singly or in pairs in a data set corresponding to an integrated luminosity of 5 inverse femtobarns recorded by the CMS experiment at the LHC in 2011. A novel strategy has been developed for a combined search for quarks of the up and down type in decay channels with at least one isolated muon or electron. Limits on the mass of the fourth-generation quarks and the relevant Cabibbo-Kobayashi-Maskawa matrix elements are derived in the context of a simple extension of the standard model with a sequential fourth generation of fermions. The existence of mass-degenerate fourth-generation quarks with masses below 685 GeV is excluded at 95% confidence level for minimal off-diagonal mixing between the third- and the fourth-generation quarks. With a mass difference of 25 GeV between the quark masses, the obtained limit on the masses of the fourth-generation quarks shifts by about +/- 20 GeV. These results significantly reduce the allowed parameter space for a fourth generation of fermions.