Thursday, June 22, 2006, 16:00
WHGA Auditorium
G. Cantatore, Univ. Trieste
Abstract:
The vacuum element can be used as a target in a photon-photon collider
in order to study its properties. Some of these properties are
predicted by Quantum Electrodynamics, while additional, unexpected
properties might be linked to the existence of yet- undiscovered
axion-like particles interacting with two photons. In this low energy
case, 1-2 eV real photons from a polarized laser beam are scattered off
virtual photons provided by a magnetic field. Information on the
scattering processes can be obtained by measuring changes in the
polarization state of the probe photons. In the PVLAS (Polarizzazione
del Vuoto con LASer) experiment, running at the Legnaro Laboratory of
INFN, near Padova, Italy, a linearly polarized laser beam is sent
through a 5 Tesla magnetic field in vacuum, where it is reflected back
and forth, by means of a Fabry-Perot resonator, about ~100000 times
over a distance of 1 m. An heterodyne ellipsometer allows simultaneous
detection of birefringencies and of rotations of the polarization
plane. The sensitivity of the instrument allows detection of rotation
or of ellipticity angles of about 1e-9 rad in an hour of data taking.
The measurement technique employed by PVLAS will be illustrated, and
recent observations of an anomalous rotation of the polarization plane
due to the magnetized vacuum will be presented. The interpretation of
these effects in terms of axion-like particles, along with recent
changes and upgrades of the apparatus, will also be discussed. A
photon-regeneration-type experiment will be briefly illustrated as a
possible physical check of the particle interpretation of the vacuum
rotation effect.