Search for single top production in e+e− collisions at s up to 209 GeV

Abstract Single top production via the flavour changing neutral current reactions e + e − → t c , t u is searched for within the 214 pb −1 of data collected by ALEPH at centre-of-mass energies between 204 and 209 GeV. No deviation from the Standard Model expectation is observed and upper limits on the single top production cross sections are derived. The combination with data collected at lower centre-of-mass energies yields an upper limit on the branching ratio BR(t→Zc)+BR(t→Zu) m t =174 GeV/ c 2 .