Combined search for the quarks of a sequential fourth generation

by Bi-Weekly IIHE Internal Seminar: Gerrit Van Onsem

Thursday 11 Oct 2012, 10:00 → 11:00 Europe/Brussels

IIHE(Large Seminary Room) (IIHE)

A simple extension of the standard model is a model with a sequential fourth generation of fermions. Many searches for these fermions have found no evidence for their existence and resulted in stringent limits on the parameter space of this model. We have developed a novel search strategy for a new generation of quarks of the up- and down-type that reduces the parameter space even further. In this talk, I present a search for final states with fourth-generation quarks produced singly or in pairs in a dataset corresponding to an integrated luminosity of 5/fb recorded by the CMS experiment at the LHC in 2011. The combined search is performed in the decay channels with at least one isolated muon or electron. We derive model-dependent limits on the mass of the fourth-generation quarks and the relevant CKM matrix elements. 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 are the world's highest limits on fourth generation quark masses, and significantly reduce the allowed parameter space for a fourth generation of fermions.