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 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 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 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.

The processγγπ+π− has been measured with complete particle identification. Cross-sections are presented from near threshold up to the region of thef(1270). In the mass range 0.5–0.7 GeV, crosssections are lower than the Born term predictions and show no evidence for an ε(600). The two-photon width of thef(1270) is found to be in agreement with previous results.

A measurement of the γγ total cross section, σγγ(Q2W), is presented for theQ2 range 0.1 to 100 GeV2, and for the massW of the hadronic final state between 1.5 and 10 GeV. The dependence of σγγ on bothQ2 andW is measured. The results are compared with theoretical predictions. It is found that the data are well described by a sum of quarkparton model and vector dominance contributions.

We present an experimental study of jetproduction in photon-photon interactions for 0.1≲Q 2≲120 GeV2 and jet transverse momentum,p T , up to 5 GeV/c. At alQ 2, the data show a highp T , tail, characteristic of a point-like interaction. The jet production cross-section approaches the quarkparton model (QPM) expectation as either jetp T orQ 2 increases. Overall, the data are well described in both total cross-section and event topology by the sum of a vector-dominance model and a point-like interaction, represented by the QPM.

Transverse particle momenta have been measured ine+e− annihilation into hadrons at c.m. energies between 9.4 and 31.6 GeV. The data are fully corrected for detector effects and radiation in the initial state. A comparison is made with recent QCD calculations.

Measurements of energy moments for single quark jets at c.m. energies between 12 and 31.6 GeV are presented. The data, corrected for detector effects and initial state radiation, are compared to QCD predictions in the leading log approximation. Non perturbative effects are found to be moderate, and they strongly decrease with increasing c.m. energy. Once partly corrected for the presence of these fragmentation effects, our data agree well with all features of the leading log prediction, and in particular with the variation of the strong coupling constant over a wide range of energies and momentum transfers.

The reactione+e−→μ+μ− has been studied with the PLUTO detector at the c.m. energy of 34.7 GeV, based on an integrated luminosity of 46 pb−1. The total cross section agrees with QED predictions, and the corresponding lower limits for theΛ+ andΛ− QED cutoff parameters are 267 and 126 GeV respectively (95% c.l.). A forward-backward asymmetry parameterA=−(13.4±3.1±<1)% is observed, which is in agreement with the standard model, as well as with other PETRA results obtained at the same energy.

The jet character of the hadronic final states produced ine+e− annihilations is studied in terms of jet measures such as thrust, sphericity, jet opening angle and jet masses, in the energy range 7.7 to 31.6 GeV. All distributions and averages have been corrected for detector effects and initial state radiation. The energy dependence of the averages of these jet quantities is used to estimate the contributions due to perturbative QCD and fragmentation effects. Correlations between the jet measures and the multiplicity of charged hadrons are also presented.

Multihadron events resulting frome+e− annihilation at the Υ-resonance are analysed with respect to lowest order QCD which predicts a dominant Υ decay into three vector gluons. Smearing effects due to the gluon fragmentation are so large, however, that no clear 3-jet structure or flatness is expected in the hadron final state. Instead, Monte Carlo simulations predict event structures with thrust and triplicity distributions distinctly different from those of two quark-jet and of phase space like decay mechanisms. The data are in perfect agreement with the 3-vector gluon decay as predicted by QCD and exclude dominantly phase space like or 2-quark jet decays. Also, the decay into scalar gluons is excluded by the data. An upper limit for the Υ-decay into one photon and two gluons is obtained.

The differential cross sections for Bhabha scattering and μ pair production, and the total τ pair cross section as measured by the PLUTO detector at PETRA, have been analyzed to extract information on the weak interaction of leptons. The data are compared with unified gauge theories. Since the observed electroweak effects are still consistent with zero (within errors) we can set experimental limits on neutral current parameters atQ2 values of 950 GeV2. In the framework of the standard SU(2)×U(1) model we find sin2Θ w <0.52(95% c.l.). In the context of general singleZo models we can excludeZo masses of less than 40 GeV.