SEMINAR      Thursday, 20 April 2006 - h. 15:00  Auditorium B. Touschek     U. Gastaldi (INFN - Legnaro) PVLAS, an experiment to probe vacuum with polarized light: results and prospects Abstract The PVLAS experiment operates at LNL an ellipsometer (with a 6.4 m long vertical Fabry-Perot cavity which embraces a rotating 5.5 Tesla superconducting dipole magnet) that can measure ellipticity and rotation induced by the magnetic field onto linearly polarized laser light. The sensitivity of the instrument is of the order of 10 -7 rad sqrtHz -1 with infrared light stored in the Fabry-Perot. Measurements have been made with infrared laser light until the end of 2004 and in march 2006, and with green laser light during 2005. With a residual pressure less than 10 -7 mbar in the FP the apparatus gives both ellipticity and rotation signals at the 10 -7 rad level. These signals can be interpreted as being generated largely by vacuum ellipticity and dichroism induced by the transverse magnetic field. If this interpretation is correct, a tool has become available to characterize physical properties of vacuum as if it were an ordinary transparent medium. A microscopic effect responsible for the induced dichroism could be the existence of ultralight spin zero bosons with mass of the order of 10 -3 eV, that would couple to two photons and would be created in the experiment by interactions of photons of the laser beam with virtual photons of the magnetic field. The coupling g m?? of these bosons to two photons would be of the order of 10 -6 GeV -1 . Axions and dilatons are respectively pseudoscalar and scalar bosons that could exist with masses of the order of 10 -3 eV and with very or extremely weak couplings to known matter, so that the mass scale M =1/g m?? is expected to be much larger than 10 6 GeV. Results and prospects of the experiment will be presented.