  The PVLAS collaboration is at present running, at the Laboratori Nazionali di
Legnaro of I.N.F.N., Padova, Italy, a very sensitive optical ellipsometer
capable of measuring the small rotations or ellipticities which can be acquired
by a linearly polarized laser beam propagating in vacuum through a transverse
magnetic feld (vacuum magnetic birefringence). The apparatus will also be able
to set new limits on mass and coupling constant of light scalar/pseudoscalar
particles coupling to two photons by both producing and detecting the
hypothetical particles. The axion, introduced to explain parity conservation in
strong interactions, is an example of this class of particles, all of which are
considered possible dark matter candidates. The PVLAS apparatus consists of a
very high finesse (> 140000), 6.4 m long, Fabry-Perot cavity immersed in an
intense dipolar magnetic field (~6.5 T). A linearly polarized laser beam is
frequency locked to the cavity and analysed, using a heterodyne technique, for
rotation and/or ellipticity acquired within the magnetic field.
