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.

A search has been performed for the production of charged Higgs bosons in e+ e− annihilation at center of mass energies up to 46.8 GeV. From the absence of both hadronic and leptonic decay signatures, we exclude them up to a mass of 19 GeV/c2 independent of the hadronic and leptonic branching ratios.

We have measured the total normalized cross section R for the process e+e− → hadrons at centre-of-mass energies between 14.0 and 46.8 GeV based on an integrated luminosity of 60.3 pb−1. The data are well described by the standard SU(3)c⊗SU(2)L⊗U(1) model with the production of the five known quarks. No open production of a sixth quark with charge 2/3 or 1/3 occurs below a centre-of-mass energy of 46.6 or 46.3 GeV, respectively. A fitting procedure which takes the correlations between measurements into account was used to determine the electroweak mixing angle sin2θw and the strong coupling constant αs(S) in second-order QCD. We applied this procedure to the CELLO data and in addition included the data from other experiments at PETRA and PEP. Both fits give consistent results. The fit to the combined data yields αs(342 GeV2) = 0.165±0.030, and sin2θw = 0.236±0.020. Fixing sin2θw at the world average value of 0.23 yields αs(342GeV2) = 0.169±0.025.

Using the CELLO detector at the PETRA e+e−-storage ring, a search for hadronic events with low thrust and anisolated lepton has been carried out. The rate of such events and the thrust distribution of inclusive lepton events show no evidence for a threshold of new sources of inclusive lepton events at the highest PETRA energies. The CELLO data thus do not support the observations of the MARK-J and JADE collaborations of an excess of muon events of this type at √s > 46.3 GeV.

We present a measurement of the hadronic structure function F2γ(x, Q2) of the photon in the Q2 range from 10 to 100 GeV2. Data were taken with the PLUTO detector at the e+e- storage ring PETRA. This measurement and previous PLUTO measurements in the Q2 range of 1.5 to 16 GeV2 are compared with higher order QCD calculations. The structure function is consistent with the predicted log Q2 behaviour when charm contributions are subtracted. The x dependence can be well described for 0.1 < x < 0.9 by the regularization scheme of Antoniadis and Grunberg. Within their scheme the data yield a value of ΛMS = 183 + 65/ −40(stat.) + 46/ −36(sys.) MeV for the QCD scale parameter.

A major effort has been devoted by the CELLO Collaboration at PETRA in the last years in order to detect signals from new phenomena coming from the prediction of the supersymmetric theories. The negative results of this wide search put limits for the mass of various supersymmetric particles; scalar partners of electrons, muons, taus and quarks; photinos, winos and zinos.

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\(\gamma \gamma \to q\bar qq\bar q\)

A search for single photons, produced in e+e− collisions together with particles interacting only weakly with matter, has been performed using the CELLO detector operating at the PETRA storage ring. From the absence of any signal, an upper limit is set at 15 (90% CL) on the number of light neutrino species, and lower limits on various supersymmetric particle masses are derived. For massless photinos, mass degenerate scalar partners of the left- and right-handed electrons are excluded below 37.7 GeV/c2 (90% CL).

The production of excited quarks has been looked for in e+e− interactions at PETRA at center of mass energies up to 46.8 GeV using the CELLO detector. The different final state topologies considered are four-jets, two-jets and two-photons, three-jets, two-jets and one-photon. No deviation from standard QCD predictions is observed, thus yielding limits on the coupling constants and the masses of these possible quark states.

Using the CELLO detector at PETRA a search has been made for excited leptons by studying e+e− interactions which yield l+l− γγ, l+l− γ and γγ final states, where l=e,μorτ. Good agreement with QED is observed and new limits are set on e∗, μ∗, and τ∗ production.

A search for light leptoquarks decaying into a lepton and a quark has been performed using the CELLO detector at the storage ring PETRA. Decay modes have been looked for where the lepton is either a muon or a neutrino, the latter being detected by a high missing transverse momentum. Except for one event previously reported no further two-jet events with two isolated muons were observed. No evidence for pair production of these particles has been found in other final states configurations. Our results are interpreted within a model where leptoquarks appear as a coloured Goldsone multiplet of charges 2/3.A 95% CL upper limit of 0.30 pb is set on the pair production cross section of such leptoquarks, thus excluding masses between 7 and 20.5 GeV.

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.

We have searched for monojet production in e+e−-annihilation with the CELLO detector operating at the highest PETRA energies. No events were found, which makes it unlikely that the recently observed monojets in pp collisions originate from unusual Z0 decays. The upper limits on monojet cross sections are compared with various models, thus yielding upper limits on the production cross sections of Higgs bosons and higgsinos.