Search for Dilepton Signatures Containing Tau Leptons in Top Events at CDF

We present a measurement of the branching ratio of the top quark decaying exclusively into third generation particles: into a b quark, a tau lepton and a tau neutrino. The analysis is performed on the data collected by CDF during the Tevatron run II. We use top pair events in which, each top decaying into Wb, one W produces an electron or a muon, and the other W gives a tau decaying into hadrons and a neutrino. The result constitutes a first test of the lepton universality in the realm of the top physics.