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

Single top production via the flavour changing neutral current reactions e +e− → t̄c, t̄u 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 ( → Zc) + BR(t → Zu) < 14%, for BR(t → γ c) + BR(t → γ u) = 0 and mt = 174 GeV/c2.  2002 Elsevier Science B.V. All rights reserved. 1 Also at CERN, 1211 Geneva 23, Switzerland. 2 Now at Fermilab, PO Box 500, MS 352, Batavia, IL 60510, USA. 3 Also at Dipartimento di Fisica di Catania and INFN Sezione di Catania, 95129 Catania, Italy. 4 Now at University of Florida, Department of Physics, Gainesville, Florida 32611-8440, USA. 5 Also Istituto di Cosmo-Geofisica del C.N.R., Torino, Italy. 6 Now at Institut für Experimentelle Kernphysik, Universität Karlsruhe, 76128 Karlsruhe, Germany. 7 Supported by CICYT, Spain. 8 Supported by the National Science Foundation of China. 9 Supported by the Danish Natural Science Research Council. 10 Supported by the UK Particle Physics and Astronomy Research Council. 11 Supported by the US Department of Energy, grant DE-FG0295-ER40896. 12 Now at Departement de Physique Corpusculaire, Université de Genève, 1211 Genève 4, Switzerland. 13 Supported by the Commission of the European Communities, contract ERBFMBICT982874. 14 Supported by the Leverhulme Trust. 15 Permanent address: Universitat de Barcelona, 08208 Barcelona, Spain. 16 Supported by Bundesministerium für Bildung und Forschung, Germany. 17 Supported by the Direction des Sciences de la Matière, C.E.A. 18 Supported by the Austrian Ministry for Science and Transport. 19 Now at SAP AG, 69185 Walldorf, Germany. 20 Now at Groupe d’Astroparticules de Montpellier, Université de Montpellier II, 34095 Montpellier, France. 21 Now at BNP Paribas, 60325 Frankfurt am Mainz, Germany. 22 Supported by the US Department of Energy, grant DE-FG03-92ER40689. 23 Now at Institut Inter-universitaire des hautes Energies (IIHE), CP 230, Université Libre de Bruxelles, 1050 Bruxelles, Belgique. 24 Also at Dipartimento di Fisica e Tecnologie Relative, Università di Palermo, Palermo, Italy. 25 Now at McKinsey and Compagny, Avenue Louis Casal 18, 1203 Geneva, Switzerland. 26 Now at Honeywell, Phoenix AZ, USA. 27 Now at INFN Sezione di Roma II, Dipartimento di Fisica, Università di Roma Tor Vergata, 00133 Roma, Italy. 28 Now at Centre de Physique des Particules de Marseille, Univ. Méditerranée, F-13288 Marseille, France. 29 Also at Department of Physics, Tsinghua University, Beijing, People’s Republic of China. 30 Now at SLAC, Stanford, CA 94309, USA. 31 Deceased. 32 Also at Groupe d’Astroparticules de Montpellier, Université de Montpellier II, 34095 Montpellier, France. 178 Physics Letters B 543 (2002) 173–182