Particle Physics Seminar

In the presence of an external magnetic field, vacuum can behave as an optically anisotropic medium and induce both an ellipticity and a dichroism on a linearly polarized light beam propagating transversely with respect to the field. Induced ellipticity and dichroism can be
linked with photon-photon scattering and with the production of
light, neutral, scalar/pseudoscalar particles.
A direct ellipticity and dichroism measurement would lead to a determination of the mass and coupling constant to two photons of the produced particles.
The PVLAS collaboration is operating a high-sensitivity (~10^{-7 sup> (Hz)-1/2) optical ellipsometer capable of detecting
very small changes in the light polarization state induced by a strong transverse magnetic field. The apparatus is based on a high finesse (~105), high quality factor, 6.4 m long Fabry- Perot (FP) optical resonator and on a 5.5 T superconducting, 1.1 m long, dipole magnet housed in a rotating cryostat. This ellipsometer is capable of measuring, using the heterodyne technique, both ellipticities and dichroisms in an independent way, down to levels below 10-8 rad, for about one hour of data taking time.
At present, all data taking runs have shown the presence of both a birefringence and dichroism signal. Diagnostic tests, and measurements with Nitrogen and Neon gas, indicate that this signal is generated within the Fabry-Perot cavity.}