Two-photon physics

Recent results in γγ interactions have demonstrated the value of this branch of e/sup +/e/sup -/ physics as a testing ground for various aspects of QCD. In exclusive final states, the radiative widths of the light pseudoscalar and tensor mesons have been measured, giving insight into their quark content. The small radiative width of the scalar a/sub o/(980)/δ(980) poses problems for the interpretation of this state as a simple qq-bar meson. In the realm of glueball searches, strong limits have been placed on the formation of the iota(1440) in γγ interactions, but there is evidence of a spin 1 E(1420) in tagged γ/sup */γ collisions. In continuum π/sup +/π/sup -/ production, there are some indications of structure just above threshold. At high invariant mass, pion and kaon pair-production are in rather good agreement with perturbative QCD calculations, though this is not the case for pp-bar production. The reaction γγ → ω rho has been seen for the first time, causing some problems for the theories developed to explain γγ → rho rho. In inclusive γγ → hadrons, the technical problems involved in obtaining reliable cross-sections seem to have been overcome. Four mutually consistent determinations of the total cross-section at low Q/sup 2/ yield results in agreement with VDM expectations. Studies of event topology in the untagged and low Q/sup 2/(Q/sup 2/ < 1 GeV/sup 2/) regimes indicate the presence of a substantial number of relatively spherical events, as expected from QCD. The first evidence for D/sup */ production in γγ has been presented, indicating a large charm contribution to the cross-section