Abstract

Sensitive magneto-optical polarimetry was proposed in 1979 as a means to detect vacuum electrodynamic non linearity, in particular Vacuum Magnetic Birefringence (VMB). This process is predicted in QED via the fluctuation of electron-positron virtual pairs but can also be due to hypothetical Axion Like Particles (ALPs). Today ALPs are considered a strong candidate for Dark Matter. Starting in 1992 the PVLAS collaboration, financed by INFN, Italy, attempted to measure VMB with a polarimeter based on an optical cavity permeated by a time dependent magnetic field and heterodyne detection. Two setups followed differing basically in the magnet: the first using a rotating superconducting 5.5 T dipole magnet and the second using two rotating permanent 2.5 T dipole magnets. At present PVLAS is the experiment which has set the best limit in VMB reaching a noise floor within a factor 10 of the predicted QED signal of ∆n(QED) = 2.5e-23 @ 2.5 T.  It was also shown that the noise floor was due to the optical cavity and a larger magnet is the only solution to increase the signal to noise ratio. The PVLAS experiment ended at the end of 2018. A new effort, VMB@CERN, which plans to use a spare LHC dipole magnet at CERN with a new modified optical scheme is now being proposed and tested. In this seminar a description of the PVLAS effort and the comprehension of its limits leading to a VMB@CERN will be given.