Chatrchyan, S.Işıldak, Bora2014-11-172014-11-172012-12-011550-2368http://hdl.handle.net/10679/590https://doi.org/10.1103/PhysRevD.86.112003Results 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 fb(-1) 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.enginfo:eu-repo/semantics/restrictedAccessArticle861100031229450000110.1103/PhysRevD.86.112003PP CollisionsRoot-S=7 TeVFinal-stateHeavy2-s2.0-84871203513