Seminare de DPNC ================ Mercredi, 13 dec. 1999 17.00 - Auditoire Stuckelberg Universite de Geneve Departement de physique nucleaire et corpusculaire 24, quai Ernest-Ansermet 1211 Geneva 4 Tel. 022 702 6273 Fax 022 781 2192 ****************************************************************************** Sujet: High energy photon-photon interactions: a fresh insight into diffraction Par: Prof. Sandy Donnachie, University of Manchester Resume: The pomeron-exchange picture of diffraction on purely hadronic interactions will be recalled briefly. The parameters of the soft phenomenological pomeron will be specified and non-perturbative QCD models of the pomeron outlined. The breakdown of this simple picture in reactions with a hard scale, such as deep inelastic electron scattering at high energy, will be analysed. Alternative explanations will be discussed, including one with a second non-perturbative pomeron. The unique aspects of high energy photon-photon interactions as a probe of the detailed dynamics will be developed and predictions made for cross sections at LEP II.