Sommaire:

Vacuum magnetic birefringence refers to the changes of the polarization of a light beam propagating in a transverse magnetic field in vacuum. Anisotropies of the index of refraction of a magnetized vacuum are predicted by quantum electrodynamics, but this effect is so small that has yet to be observed. These anisotropies can also be due to the presence of hypothetical low mass particles interacting with two photons. The experiment PVLAS has been designed to detect such effect by using a very sensitive ellipsometer based on a high finesse Fabry Perot cavity. In a first phase of the experiment the magnetic field was provided by a superconducting magnet. The PVLAS collaboration is presently running a new apparatus in Ferrara (Italy) and the new magnetic source is a pair of 2.5 T, 0.9 m long permanent magnet. In the talk, after presenting a brief history related to the search for a magnetic vacuum birefringence, details will be given on the detection technique and the results obtained with the new apparatus.

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