The ratios of the numbers of Z bosons decaying to e+e−, μ+μ− and τ+τ− pairs to the number decaying to hadrons have been measured. The branching ratios and partial widths for each channel were determined and found to be equal, consistent with lepton universality. The mean leptonic branching ratio was found to be 0.0321 ± 0.0013 and the leptonic partial width to be 85.4 ± 5.3 MeV. The partial widths for hadronic decays and for invisible decays were deduced to be 1833 ± 116 MeV and 569 ± 92 MeV, respectively. The number of light neutrino types, assuming only the standard model value for the ratio ΘeΘv, was found to be 3.35 ± 0.41.

We have performed a search for supersymmetric particles using acoplanar pairs of oppositely-charged particles in decays of the Z0. In 0.53 pb−1 of integrated luminosity near the Z0 peak, we observe two events where approximately four are expected from background, allowing limits to be extended on combined photino and slepton masses, and also on combined photino and chargino masses.

In the analysis of the reactione+e−→e+e−K S 0 K s 0 clear evidence for exclusive γγ→f 2 ′ resonance production is observed. The productΓ γγ ·B(f 2 ′ →K\(\bar K\)) is measured to be 0.10 −0.03−0.02 +0.04+0.03 keV independent of ana priori assumption on the helicity structure. Our data are consistent with a pure helicity 2 contribution and we derive an upper limit for the ratioΓ γγ (0) /Γ γγ . The absence of events in the mass region around 1.3 GeV clearly proves destructivef2−a2 interference and allows to measure the relative phases betweenf2,a2 andf 2 ′ . Upper limits on the production of the glueball candidate statesf2(1720) andX(2230) as well as theK S 0 K S 0 -continuum are given.

We present results on jet production in γγ interactions where both photons are quasi-real. The invariant masses of the hadronnic system are limited to the range 4≦Wvis≦12 GeV/c2. The data approach the quark-Parton-Model (QPM) expectation at the highestp T jet values (≧4 GeV/c). Jet production at lowp T (≦1 GeV/c) can be described by a Vector Dominance derived model. The data also have a component with no apparent jet structure in the range, 1.0 ≦P T jet ≦4.0 GeV/c which can be described by phase space or by models of the QCD hard scattering processes cesses\(\gamma \gamma \to q\bar qg\) and\(\gamma \gamma \to q\bar qq\bar q\)

We observe for evidence for γγ → ηc production in the reaction e+e− → e+e−KS0K±π∓. The product Γγγ(ηc)·B(ηc → KS0K±π∓) is measured to be 0.5−0.15+0.2±0.1 keV.

We present high statistics measurements of the energy-energy correlation (EEC) and its related asymmetry (AEEC) ine+e− annihilation at a c.m. energy of 34.6 GeV. We find that the energy dependence as well as the large angle behaviour of the latter are well described by perturbative QCD calculations toOα( s 2 ). Non-perturbative effects are estimated with the help of fragmentation models in which different jet topologies are separated using (ɛ, δ) cuts, and found to be small. The extracted values of\(\Lambda _{\overline {MS} }\) lie between 100 and 300 MeV.

The differential cross section of the reactione+e-?e+e- at a c.m. energy of 34.7 GeV has been measured. The result, together with our previously measurede+e-?a+a- data, are compared with the standard model predictions. We obtain for the weak neutral current couplings the valuesgv2=0.09×0.06,ga2=0.38×0.08. A fit of the Weinberg mixing angle gives the valuegv2=0.09×0.06,ga2=0.038×0.08. The data are also used to set limits on possible deviations from the pointlike structure of leptons. An upper limit for thee+e- coupling to a heavy spin 0 boson is also given.

The Sterman-Weinberg 2-jet cross section is measured from the hadron energy flow using the thrust axis to define the axis of the jet cone. The results are compared with first order QCD predictions. Deviations due to fragmentation are observed which decrease with increasing center of mass energy. We obtain an upper limit of Λmax≈523 MeV for the QCD scale parameter using a method which is insensitive to the details of the fragmentation. Moments of energy flow are also studied.

A search for the reactionsγγ→ωω andγγ→ρ0ω has been carried out at an averagee+e− CM energy of 34.6 GeV with an integrated luminosity of 45 pb−1. Upper limits are set for these two channels over the γγ CM Energy range of 1.6 to 2.5 GeV. The cross section is determined for the exclusive channelγγ→π+2π−π0.

The differential cross section of the reactione+e−→e+e− at a c.m. energy of 34.7 GeV has been measured. The result, together with our previously measurede+e−→α+α− data, are compared with the standard model predictions. We obtain for the weak neutral current couplings the valuesg v 2 =0.09×0.06,g a 2 =0.38×0.08. A fit of the Weinberg mixing angle gives the valueg v 2 =0.09×0.06,g a 2 =0.038×0.08. The data are also used to set limits on possible deviations from the pointlike structure of leptons. An upper limit for thee+e− coupling to a heavy spin 0 boson is also given.

The processe+e−→e+e− μ+μ− has been studied using the PLUTO detector at PETRA. The data agree well with QED calculations to order α4. Comparison is also made with the predicted leptonic structure function of the real photon using the deep inelastice-γ scattering formalism.

An experimental study of two jet production by interactions of two quasi-real photons is presented. The data for production of jets with high transverse momentum squared, p T 2 >10GeV2, are found to be consistent with the fractionally charged quark-parton model. If gauge integer charged quark models are considered, then the gluon mass is less than 5 MeV at the 95% confidence level.

We present high statistics measurements of the energy-energy correlation (EEC) and its related asymmetry (AEEC) ine+e− annihilation at a c.m. energy of 34.6 GeV. We find that the energy dependence as well as the large angle behaviour of the latter are well described by perturbative QCD calculations toOα(s2). Non-perturbative effects are estimated with the help of fragmentation models in which different jet topologies are separated using (ɛ, δ) cuts, and found to be small. The extracted values of <img src="/fulltext-image.asp?format=htmlnonpaginated&src=U88701024184350P_html\10052_2005_Article_BF01413599_TeX2GIFIE2.gif" border="0" alt=" $$\Lambda _{\overline {MS} }$$ " /> lie between 100 and 300 MeV.

We have studied 419 τ pair events produced in the reactione+e−→τ+ τ− at a c.m. energy of 34.6 GeV. We measure the cross section and angular distribution, as well as the decay branching ratios. The production characteristics are consistent with the Standard Electroweak Model predictions of γ andZ0 interference. The branching ratios are generally consistent with the τ decaying according to standard weak interaction principles, but we observe somewhat more decays resulting in single charged hadrons plus neutrals than are predicted by present theory.