Study of hadronic transitions between Y' and Y with the ARGUS-detector

The hadronic transitions between the banti b mesons Y' and Y are a tool to test models based on Quantum Chromo-Dynamics (QCD) which describe these transitions by the emission of E1E1- and E1M1-gluons. The gluons fragment into light mesons, like two pions or an eta. The exclusive decays Y'->Yπ+π- -> e+e-π+π-/μ+μ-π+π- and Y' -> Yπ0π0 -> e+e-π0π0/μ+μ-π0π0 are measured using the ARGUS detector at the e+e-storage ring DORIS II at DESY in Hamburg. The branching ratios of Y' -> Yπ0π0, Y -> e+e-, Y -> μ+μ- and the ππ mass spectrum as well as all decay angular distributions of Y' -> Yπ+π- are determined. In addition an upper limit for the branching ratio of Y' -> Yeta is given with 90% confidence. π0 and eta detection require good measurement of photon energies over a long time. An essential part for the stability control is a laser monitor-system which is described in detail. The branching ratio of the decay Y'->Yπ0π0 indicates - together with the corresponding value of Y' -> Yπ+π- - that the Y' and the Y have isospin 0. The ππ mass spectrum is peaking at high masses. The angular distributions of the ππ system and of the π+ are compatible with the assumption that the ππ system is emitted in an s-wave. The angular distribution of the final lepton shows that the Y is in the same polarisation state as the produced Y after emission of the two pions. All these results are in agreement with QCD. Comparing the ππ mass spectrum to the corresponding one in phi' -> J/phiππ, one observes a significant difference in shape. This was predicted in one QCD model as an effect of the running coupling constant αsub(s). (orig.)