We report on the first observation of the beauty baryon Λb in an exclusive decay channel at the CERN pp collider. Using 4.7 pb-1 of muon data collected in the 1988/89 collider runs we reconstruct 16±5 Λb's in the decay mode Λb→ J ψΛ above a background of 9±1 events, corresponding to a significance of about five standard deviations. We measure the Λb mass to be mΛb=5640±50±30 MeV/c2. Using the beauty cross-section measured by UA1 we deduce for the product of the production fraction and branching ratio fΛb Br (Λb→ J ψΛ)=(1.8±1.0)×10-10. Our sample contains three-muon event in which the beauty particle opposite to the Λb is tagged by the third muon. We also observe an indication of a signal in the decay channel B0→ J ψK0* with a significance of three standard deviations.
Bibliographical noteFunding Information:
We are thankful to the management and staff of CERN and of all participating institutes for their vigorous support of the experiment. The following funding agencies have contributed to this programme: Fonds zur F6rderung der Wissenschaftlichen For-schung, Austria; Valtion luonnontieteellinen toimikunta, Suomen Akatemia, Finland; Institut National de Physique Nucl6aire et de Physique des Particules and Institut de Recherche Fon-damentale (CEA), France; Bundesministerium fiir Forschung und Technologic, FRG; Istituto Nazionale di Fisica Nucleare, Italy; Science and Engineering Research Council, UK; Stichting Voor Fundamenteel Onderzoek der Ma-terie, The Netherlands; Centro de Investigaciones Energetica, Medioam-bientales y Tecnologicas (CIEMAT), Spain; Department of Energy, USA. Thanks are also due to the following people who have worked with the collaboration in the preparations for and data collection on the runs described here: L. Baumard, F. Bernasconi, D. Brozzi, V. Cec-coni, L. Dumps, G. Fetchenhauer, G. Gallay, S. La-zic, J.C. Michelon, S. Pavlon and L. Pollet.