SEMINAR Thursday July 10th, 2003 - h 15:00 Auditorium B. Touschek   Sergio Di Matteo  INFM/LNF Structural and magnetic symmetries as detected by x-ray resonant scattering and dichroism. Abstract The first half of the seminar is devoted to introduce the resonant processes, for both dichroism and anomalous diffraction, in order to clarify the physics behind these techniques. After showing the connection to ordinary diffraction, we derive, on the basis of symmetry considerations, the physical quantities observable in the processes. Thanks to the high brilliance of modern synchrotron radiation facilities it is possible to detect small magnetic symmetry changes due to high-order magnetic distributions and toroidal moments. Some example are given for the case of LaMnO3, V2O3, Cr2O3 and the superconducting cuprates, where time-reversal symmetry breaking theoriescan be tested.