The structure of the photon and its interactions

The OPAL experiment at LEP has performed a variety of measurements of photon-photon and electron-photon scattering at the electron-positron collider LEP to gain a deeper insight into the structure of the photon and its interactions. This review presents a summary of these results.Comment: 18 pages with 20 eps files. Invited talk given at the Workshop on Photon Interactions and the Photon Structure, Lund, Sweden, September 10-13, 1998 to be published in the proceeding

Spin asymmetry at large x_F and k_T

We suggest that the large single spin asymmetries observed at high momentum fractions x_F and transverse momenta k_T of the pion in p^\uparrow p -> \pi(x_F,k_T)+X arise from the coherence of the soft interactions with the hard parton scattering process. Such coherence can be maintained if x_F -> 1 as k_T -> \infty, while k_T^2(1-x_F) ~ \Lambda_{QCD}^2 stays fixed. Analogous coherence effects have been seen experimentally in the Drell-Yan process at high x_F. We find that the p^\uparrow p -> \pi X production amplitudes have large dynamic phases and that helicity flip contributions are unsuppressed in this limit, giving rise to potentially large single spin asymmetries.Comment: 3 pages, 1 figure. Talk given at EPS HEP 2007, Mancheste

Vector-Meson Electroproduction from Generalized Vector Dominance

Including destructively interfering off-diagonal transitions of diffraction-dissociation type, we arrive at a formulation of GVD for exclusive vector-meson production in terms of a continuous spectral representation of dipole form. The transverse cross-section sigma_T for gamma* p -> V p behaves asymptotically as 1/Q^4, while R_V = sigma_L/sigma_T becomes asymptotically constant. Contributions violating s-channel helicity conservation stay at the 10-15% level established in low-energy photoproduction and diffractive hadron--hadron interactions. The data for phi- and rho-meson production for 0 \lsim Q^2 \lsim 20 GeV^2 from HERA are found to be in agreement with these predictions.Comment: 16 pages, latex, 3 figure