PVLAS

Home page of the Experiment: http://www.ts.infn.it/experiments/pvlas/

The experimental exploration of the complex structure of the quantum vacuum is the research goal of the PVLAS collaboration (Universities of Trieste, Pisa, Ferrara, Padova, and I.N.F.N. Legnaro Laboratories). Several effects predicted by Quantum Electrodynamics (QED), such as photon-photon scattering and particle production from a two-photon interaction, contribute to this structure. Since any neutral light particle couples to two photons, with a strength depending on its particular nature, a possible detection strategy can be pursued by optical techniques: among such particles there could be those considered to be possible dark matter candidates, and the axion is an example of this class of objects.

These interactions are explored by the PVLAS experiment by sending a linearly polarised laser beam through a transverse magnetic field, and by measuring changes in the polarisation state of the light. The PVLAS collaboration is at present running at the Laboratori Nazionali di Legnaro of I.N.F.N. a very sensitive optical ellipsometer capable of measuring the small rotations or ellipticities which can be acquired by the beam as a consequence of the interaction with the external field. The apparatus consists of a very high finesse (F~140000), 6.4 m long, vertical Fabry-Perot optical resonant cavity. The magnetic field is provided by a superconducting, 1 m long, ~6.5 T, dipole magnet housed in a warm bore, liquid He, cryostat. The magnet-cryostat assembly can be rotated to provide the time-modulation of the effect necessary for heterodyne detection, and it is located within a granite tower-like structure holding the optics setup. A linearly polarised laser beam is frequency locked to the cavity and its polarisation state is studied, using a heterodyne technique, for rotation and/or ellipticity acquired within the magnetic field. Light transmitted through crossed polarisers is detected by a photodiode and the polarisation parameters are extracted from a Fourier analysis of the resulting current signal. These measured parameters, besides giving direct access to such fundamental quantities as the fine structure constant and the Compton wavelength of the electron, contain mass and coupling constant to two photons of possible produced particles