During 2014 the PVLAS experiment has started data taking with a new apparatus installed at the INFN Section of Ferrara, Italy. The main target of the experiment is the observation of magnetic birefringence of vacuum. According to QED, the ellipticity
generated by the magnetic birefringence of vacuum in the experimental apparatus is expected to be $psi^{rm(QED)} approx 5times10^{-11}$. No ellipticity signal is present so far with a noise floor $psi^{rm(noise)} approx 2.5times10^{-9}$ after 210 hours of data taking. The resulting ellipticity limit provides the best model independent upper limit on the coupling of axions to $gammagamma$ for axion masses above $10^{-3}$eV.
The PVLAS collaboration is presently assembling a new apparatus (at the INFN section of Ferrara, Italy) to detect vacuum magnetic birefringence (VMB). VMB is related to the structure of the QED vacuum and is predicted by the Euler-Heisenberg-Weisskop
f effective Lagrangian. It can be detected by measuring the ellipticity acquired by a linearly polarised light beam propagating through a strong magnetic field. Using the very same optical technique it is also possible to search for hypothetical low-mass particles interacting with two photons, such as axion-like (ALP) or millicharged particles (MCP). Here we report results of a scaled-down test setup and describe the new PVLAS apparatus. This latter one is in construction and is based on a high-sensitivity ellipsometer with a high-finesse Fabry-Perot cavity ($>4times 10^5$) and two 0.8 m long 2.5 T rotating permanent dipole magnets. Measurements with the test setup have improved by a factor 2 the previous upper bound on the parameter $A_e$, which determines the strength of the nonlinear terms in the QED Lagrangian: $A_e^{rm (PVLAS)} < 3.3 times 10^{-21}$ T$^{-2}$ 95% c.l. Furthermore, new laboratory limits have been put on the inverse coupling constant of ALPs to two photons and confirmation of previous limits on the fractional charge of millicharged particles is given.
Although quantum mechanics (QM) and quantum field theory (QFT) are highly successful, the seemingly simplest state -- vacuum -- remains mysterious. While the LHC experiments are expected to clarify basic questions on the structure of QFT vacuum, much
can still be done at lower energies as well. For instance, experiments like PVLAS try to reach extremely high sensitivities, in their attempt to observe the effects of the interaction of visible or near-visible photons with intense magnetic fields -- a process which becomes possible in quantum electrodynamics (QED) thanks to the vacuum fluctuations of the electronic field, and which is akin to photon-photon scattering. PVLAS is now close to data-taking and if it reaches the required sensitivity, it could provide important information on QED vacuum. PVLAS and other similar experiments face great challenges as they try to measure an extremely minute effect. However, raising the photon energy greatly increases the photon-photon cross-section, and gamma rays could help extract much more information from the observed light-light scattering. Here we discuss an experimental design to measure photon-photon scattering close to the peak of the photon-photon cross-section, that could fit in the proposed construction of an FEL facility at the Cabibbo Lab near Frascati (Rome, Italy).
