Determination of the radiative decay width of the r / c meson ARGUS Collaboration

An analysis of the reaction yy ~ r/c was l~erformed in five different decay channels of the r/c: /~sK+~r ~, K+K-Tr+~ ", 27r+21r , ~b~b and 2K+2K . A value Fry(r/c) = ( 11.3 + 4.2) keV was obtained for the radiative decay width by combining the results from the first four channels. Using our result on the two-photon width we also determined the branching ratio for the decay r/c --~ 2K+2K . 0370-2693/94/$07.00 (~) 1994 Elsevier Science B.V. All rights reserved SSDI 0 3 7 0 2 6 9 3 ( 9 4 ) 0 1 1 1 6-8 ARGUS Collaboration / Physics Letters B 338 (1994) 390-396 391 The measurements of the radiative decay widths of hadrons have proved to be one of the most important tools to study the composition and the properties of the bound states of strong interactions. While for light quarks the understanding of the binding forces is still on a more phenomenological level, in heavy quark systems, such as charmonium, fundamental tests of QCD dynamics can be made. One of the important quantities in the charmonium system is the relation between the leptonic decay width of the J/qt and the two-photon width of the r/c which is to the lowest order given by (see for example [ 1 ] ) Frr (r&) = 3e~( Ms/¢ )2. r,+e_ ( J l Cs ) " --~< QCD corrections have been calculated to order as by Barbieri et al. [2] resulting in the relation Frr(~7~) = Fete ( J / O ) " (1 ~a+O.OT~ which, together . . . . _0.05.1, with Fe+ e(J/~b) = (5.36:t:0.29) keV [31, yields the prediction Frr(r/c) = (8.6 + 0.6) keV. Furthermore, with the assumption that the hadronic r/c decays can be described by the lowest order two-gluon diagram one obtains the ratio of the hadronic to two-photon partial width Fhadrons(~'lc)/Fyr(rlc) proportional to ( a s / a ) 2. This relation could in principle be used to determine as. However, in this case the QCD radiative corrections seem to be too large to provide a reliable prediction. The radiative decay width Frt(r/~) has been measured by two different methods: in photon-photon collision experiments [4-7] and by the measurement of the process p/~ --* ~ [ 8 ]. The most significant results I DESY, IfH Zeuthen 2 Supported by the German Bundesministerium for Forschung und Technologic, under contract number 05413051P. 3 Supported by the German Bundesministerium for Forschung und Technologie, under contract number 056DDI 1P. 4 Supported by the German Bundesministerium ?dr Forschung und Technologie, under contract number 054ERI2R 5 Supported by the German Bundesministerium ?dr Forschung und Technologie, under contract number 055HD21P. 6 University of Toronto, Toronto, Ontario, Canada. 7 McGill University, Montreal, Quebec, Canada. 8 Supported by the Natural Sciences and Eagineering Research Council, Canada. 9 Supported by the German Bundesministerium ?dr Forschung und Technologie, under contract number 055KA l 1R l0 Supported by the Ministry of Science and Technology of the Republic of SIovenia and the Intemationales Bilro KfA, Jiilich. were obtained from the analysis of the decay channel r/¢ --* K~sK+cr :F, where a good identification of the/~s was achieved by determining its decay vertex. The results range from 4 to 27 keV and have large errors because of the small number of collected r& events and uncertainties in the knowledge of r/c branching ratios. The present analysis aims at improving the precision of the Fr~ (r/c) measurement by using several decay channels of the r/c meson. The data used in this analysis were collected using the ARGUS detector at the e+e storage ring DORIS II at DESY, and correspond to an integrated luminosity of 473 pb t . The beam energies varied between 4.7 and 5.3 GeV. The ARGUS detector and details about its trigger and its particle identification capabilities were described elsewhere [9]. In what follows, we describe the main features of the analysis, while details can be found in Refs. [10,11]. The two-photon production of the r/c was studied in e+e interactions via the reaction