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.
Highlights
The existence of three generations of fermions has been firmly established experimentally [1]
The possibility of a fourth generation of fermions has not been excluded, it is strongly constrained by precision measurements of electroweak observables
These observables are mainly influenced by the mass differences between the fourth-generation leptons or quarks
Summary
The existence of three generations of fermions has been firmly established experimentally [1]. The possibility of a fourth generation of fermions has not been excluded, it is strongly constrained by precision measurements of electroweak observables. These observables are mainly influenced by the mass differences between the fourth-generation leptons or quarks. CCCCCA: The complex phases are not shown for clarity Within this model, mixing is allowed only between the third and the fourth generations. These decay chains imply that two jets from b quarks and one to four W bosons are expected in the final state for fourth-generation quarks produced both singly and in pairs. The different production processes are classified according to the number of observed W bosons
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