Search for single top production in e+e− collisions at –202 GeV

Single top production via flavour changing neutral currentsin the reactions is searched for in approximately 411 pb−1 of data collected by ALEPHat centre-of-mass energies in the range between 189 and 202 GeV. In total, 58 events are selected in the data to be compared with 50.3 expected from Standard Modelbackgrounds. No deviation from the Standard Modelexpectation is observed. Upper limits at 95% CL on single top production cross sections at –202 GeV are derived. A model-dependent limit on the sum of branching ratios BR(t→Zc)+BR(t→Zu)<17% is obtained.