Measurement of the Michel parameters and the helicity in lepton decays

A measurement of the Michel parameters and the average ντ helicity in τ lepton decays is described. The data was collected with the ALEPH detector at LEP during the years 1991 to 1995. A total integrated luminosity of 155 pb−1 is analysed. The Michel parameters ρl, ξl, (ξδ)l (l = e, μ), and ημ are determined for the leptonic decays, and the chirality parameters ξπ, ξρ, and ξa1 for the hadronic final states. Under the assumptions of e-μ universality and ξπ = ξρ = ξa1 , the values ρl = 0.742 ± 0.016, ηl = 0.012 ± 0.026, (ξδ)l = 0.776 ± 0.051, ξl = 0.986 ± 0.074, and ξh = 0.992 ± 0.011 are obtained. No significant deviation is observed from the Standard Model assumption of the V −A structure of the charged weak interaction. To be submitted to The European Physical Journal C ∗See next pages for the list of authors. The ALEPH Collaboration A. Heister, S. Schael Physikalisches Institut das RWTH-Aachen, D-52056 Aachen, Germany R. Barate, I. De Bonis, D. Decamp, C. Goy, J.-P. Lees, E. Merle, M.-N. Minard, B. Pietrzyk Laboratoire de Physique des Particules (LAPP), INP-CNRS, F-74019 Annecy-le-Vieux Cedex, France S. Bravo, M.P. Casado, M. Chmeissani, J.M. Crespo, E. Fernandez, M. Fernandez-Bosman, Ll. Garrido, E. Graugés, M. Martinez, G. Merino, R. Miquel, Ll.M. Mir, A. Pacheco, H. Ruiz Institut de Física d’Altes Energies, Universitat Autònoma de Barcelona, E-08193 Bellaterra (Barcelona), Spain A. Colaleo, D. Creanza, M. de Palma, G. Iaselli, G. Maggi, M. Maggi, S. Nuzzo, A. Ranieri, G. Raso, F. Ruggieri, G. Selvaggi, L. Silvestris, P. Tempesta, A. Tricomi, G. Zito Dipartimento di Fisica, INFN Sezione di Bari, I-70126 Bari, Italy X. Huang, J. Lin, Q. Ouyang, T. Wang, Y. Xie, R. Xu, S. Xue, J. Zhang, L. Zhang, W. Zhao Institute of High Energy Physics, Academia Sinica, Beijing, The People’s Republic of China D. Abbaneo, P. Azzurri, G. Boix, O. Buchmüller, M. Cattaneo, F. Cerutti, B. Clerbaux, G. Dissertori, H. Drevermann, R.W. Forty, M. Frank, T.C. Greening, J.B. Hansen, J. Harvey, P. Janot, B. Jost, M. Kado, P. Mato, A. Moutoussi, F. Ranjard, L. Rolandi, D. Schlatter, O. Schneider, P. Spagnolo, W. Tejessy, F. Teubert, E. Tournefier, J. Ward European Laboratory for Particle Physics (CERN), CH-1211 Geneva 23, Switzerland Z. Ajaltouni, F. Badaud, A. Falvard, P. Gay, P. Henrard, J. Jousset, B. Michel, S. Monteil, J-C. Montret, D. Pallin, P. Perret, F. Podlyski Laboratoire de Physique Corpusculaire, Université Blaise Pascal, INP-CNRS, Clermont-Ferrand, F-63177 Aubière, France J.D. Hansen, J.R. Hansen, P.H. Hansen, B.S. Nilsson, A. Wäänänen Niels Bohr Institute, DK-2100 Copenhagen, Denmark A. Kyriakis, C. Markou, E. Simopoulou, A. Vayaki, K. Zachariadou Nuclear Research Center Demokritos (NRCD), GR-15310 Attiki, Greece A. Blondel, G. Bonneaud, J.-C. Brient, A. Rougé, M. Rumpf, M. Swynghedauw, M. Verderi, H. Videau Laboratoire de Physique Nucléaire et des Hautes Energies, Ecole Polytechnique, INP-CNRS, F-91128 Palaiseau Cedex, France V. Ciulli, E. Focardi, G. Parrini Dipartimento di Fisica, Università di Firenze, INFN Sezione di Firenze, I-50125 Firenze, Italy A. Antonelli, M. Antonelli, G. Bencivenni, G. Bologna, F. Bossi, P. Campana, G. Capon, V. Chiarella, P. Laurelli, G. Mannocchi, F. Murtas, G.P. Murtas, L. Passalacqua, M. Pepe-Altarelli Laboratori Nazionali dell’INFN (LNF-INFN), I-00044 Frascati, Italy A.W. Halley, J.G. Lynch, P. Negus, V. O’Shea, C. Raine, A.S. Thompson Department of Physics and Astronomy, University of Glasgow, Glasgow G12 8QQ,United Kingdom S. Wasserbaech Department of Physics, Haverford College, Haverford, PA 19041-1392, U.S.A. R. Cavanaugh, S. Dhamotharan, C. Geweniger, P. Hanke, G. Hansper, V. Hepp, E.E. Kluge, A. Putzer, J. Sommer, K. Tittel, S. Werner, M. Wunsch Kirchhoff-Institut für Physik, Universität Heidelberg, D-69120 Heidelberg, Germany R. Beuselinck, D.M. Binnie, W. Cameron, P.J. Dornan, M. Girone, N. Marinelli, J.K. Sedgbeer, J.C. Thompson Department of Physics, Imperial College, London SW7 2BZ, United Kingdom V.M. Ghete, P. Girtler, E. Kneringer, D. Kuhn, G. Rudolph Institut für Experimentalphysik, Universität Innsbruck, A-6020 Innsbruck, Austria E. Bouhova-Thacker, C.K. Bowdery, A.J. Finch, F. Foster, G. Hughes, R.W.L. Jones, M.R. Pearson, N.A. Robertson Department of Physics, University of Lancaster, Lancaster LA1 4YB, United Kingdom I. Giehl, K. Jakobs, K. Kleinknecht, G. Quast, B. Renk, E. Rohne, H.-G. Sander, H. Wachsmuth, C. Zeitnitz Institut für Physik, Universität Mainz, D-55099 Mainz, Germany A. Bonissent, J. Carr, P. Coyle, O. Leroy, P. Payre, D. Rousseau, M. Talby Centre de Physique des Particules, Université de la Méditerranée, INP-CNRS, F-13288 Marseille, France M. Aleppo, F. Ragusa Dipartimento di Fisica, Università di Milano e INFN Sezione di Milano, I-20133 Milano, Italy A. David, H. Dietl, G. Ganis, K. Hüttmann, G. Lütjens, C. Mannert, W. Männer, H.-G. Moser, R. Settles, H. Stenzel, W. Wiedenmann, G. Wolf Max-Planck-Institut für Physik, Werner-Heisenberg-Institut, D-80805 München, Germany J. Boucrot, O. Callot, M. Davier, L. Duflot, J.-F. Grivaz, Ph. Heusse, A. Jacholkowska, J. Lefrançois, J.-J. Veillet, I. Videau, C. Yuan Laboratoire de l’Accélérateur Linéaire, Université de Paris-Sud, INP-CNRS, F-91898 Orsay Cedex, France G. Bagliesi, T. Boccali, G. Calderini, L. Foà, A. Giassi, F. Ligabue, A. Messineo, F. Palla, G. Sanguinetti, A. Sciabà, G. Sguazzoni, R. Tenchini, A. Venturi, P.G. Verdini Dipartimento di Fisica dell’Università, INFN Sezione di Pisa, e Scuola Normale Superiore, I-56010 Pisa, Italy G.A. Blair, G. Cowan, M.G. Green, T. Medcalf, A. Misiejuk, J.A. Strong, P. Teixeira-Dias, J.H. von Wimmersperg-Toeller Department of Physics, Royal Holloway & Bedford New College, University of London, Egham, Surrey TW20 OEX, United Kingdom R.W. Clifft, T.R. Edgecock, P.R. Norton, I.R. Tomalin Particle Physics Dept., Rutherford Appleton Laboratory, Chilton, Didcot, Oxon OX11 OQX, United Kingdom B. Bloch-Devaux, P. Colas, S. Emery, W. Kozanecki, E. Lançon, M.-C. Lemaire, E. Locci, P. Perez, J. Rander, J.-F. Renardy, A. Roussarie, J.-P. Schuller, J. Schwindling, A. Trabelsi, B. Vallage CEA, DAPNIA/Service de Physique des Particules, CE-Saclay, F-91191 Gif-sur-Yvette Cedex, France N. Konstantinidis, A.M. Litke, G. Taylor Institute for Particle Physics, University of California at Santa Cruz, Santa Cruz, CA 95064, USA C.N. Booth, S. Cartwright, F. Combley, M. Lehto, L.F. Thompson Department of Physics, University of Sheffield, Sheffield S3 7RH, United Kingdom K. Affholderbach, A. Böhrer, S. Brandt, C. Grupen, A. Ngac, G. Prange, U. Sieler Fachbereich Physik, Universität Siegen, D-57068 Siegen, Germany G. Giannini Dipartimento di Fisica, Università di Trieste e INFN Sezione di Trieste, I-34127 Trieste, Italy J. Rothberg Experimental Elementary Particle Physics, University of Washington, Seattle, WA 98195 U.S.A. S.R. Armstrong, K. Cranmer, P. Elmer, D.P.S. Ferguson, Y. Gao, S. González, O.J. Hayes, H. Hu, S. Jin, J. Kile, P.A. McNamara III, J. Nielsen, W. Orejudos, Y.B. Pan, Y. Saadi, I.J. Scott, J. Walsh, Sau Lan Wu, X. Wu, G. Zobernig Department of Physics, University of Wisconsin, Madison, WI 53706, USA Also at CERN, 1211 Geneva 23, Switzerland. Now at Université de Lausanne, 1015 Lausanne, Switzerland. Also at Dipartimento di Fisica di Catania and INFN Sezione di Catania, 95129 Catania, Italy. Deceased. Also Istituto di Cosmo-Geofisica del C.N.R., Torino, Italy. Supported by the Commission of the European Communities, contract ERBFMBICT982894. Supported by CICYT, Spain. Supported by the National Science Foundation of China. Supported by the Danish Natural Science Research Council. Supported by the UK Particle Physics and Astronomy Research Council. Supported by the US Department of Energy, grant DE-FG0295-ER40896. Now at Departement de Physique Corpusculaire, Université de Genève, 1211 Genève 4, Switzerland. Supported by the US Department of Energy, grant DE-FG03-92ER40689. Also at Rutherford Appleton Laboratory, Chilton, Didcot, UK. Permanent address: Universitat de Barcelona, 08208 Barcelona, Spain. Supported by the Bundesministerium für Bildung, Wissenschaft, Forschung und Technologie, Germany. Supported by the Direction des Sciences de la Matière, C.E.A. Supported by the Austrian Ministry for Science and Transport. Now at SAP AG, 69185 Walldorf, Germany. Also at Department of Physics, Tsinghua University, Beijing, The People’s Republic of China. Now at Département de Physique, Faculté des Sciences de Tunis, 1060 Le Belvédère, Tunisia. Now at Groupe d’ Astroparticules de Montpellier, Université de Montpellier II, 34095 Montpellier, France. Also at Dipartimento di Fisica e Tecnologie Relative, Università di Palermo, Palermo, Italy. Now at CERN, 1211 Geneva 23, Switzerland. Now at ISN, Institut des Sciences Nucléaires, 53 Av. des Martyrs, 38026 Grenoble, France. Now at INFN Sezione di Roma II, Dipartimento di Fisica, Universitá di Roma Tor Vergata, 00133 Roma, Italy. Now at LBNL, Berkeley, CA 94720, U.S.A.