J. B. Dainton

Pion pair production in photon-photon interactions

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.

Tests of the standard model with lepton pair production ine+e− reactions

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 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.

Photoproduction of ϕ mesons by linearly polarized photons of energy 20–40 GeV and further evidence for a photoproduced high-massKK enhancement

A study of ϕ-meson photoproduction by partially polarized photons of energy 20–40 GeV is reported. The production mechanism is found to conserves-channel helicity and to proceed via natural-parity exchange in thet channel. In the photoproduction of high-massK+K− states with photons of energy 20–70 GeV, there is evidence for an enhancement at a mass of 1.76 GeV with width 0.08 GeV.

Tensor meson excitation in the reaction γγ→K S 0 K S 0

AbstractIn the analysis of the reactione+e−→e+e−KS0Ks0 clear evidence for exclusive γγ→f2′ resonance production is observed. The productΓγγ ·B(f2′→K

Jet Production in Photon-Photon Interactions

\bar K

A study of energy-energy correlations ine+e− annihilations at\(\sqrt s = 34.6\) GeV

) 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 ratioΓγγ(0)/Γγγ. 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 andf2′. Upper limits on the production of the glueball candidate statesf2(1720) andX(2230) as well as theKS0KS0-continuum are given.

Study ofηπ+π− in the ϱ′(1600) mass region photoproduced in the reaction γp→ηπ+π−p at photon energies of 20 to 70 GeV

We present an experimental study of jetproduction in photon-photon interactions for 0.1≲Q2≲120 GeV2 and jet transverse momentum,pT, up to 5 GeV/c. At alQ2, the data show a highpT, tail, characteristic of a point-like interaction. The jet production cross-section approaches the quarkparton model (QPM) expectation as either jetpT orQ2 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.

Measurements of Tau-Lepton production and decay

AbstractWe 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 highestpTjet values (≧4 GeV/c). Jet production at lowpT (≦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 ≦PTjet≦4.0 GeV/c which can be described by phase space or by models of the QCD hard scattering processes cesses