Future prospects for measuring vacuum magnetic birefringence with a (quasi) static magnetic field

• Guido Zavattini (Ferrara)

Room: 100/-1-A900 - Auditorium Joliot Curie 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 Delta n_QED = 0.5 10^-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 brief introduction of the PVLAS effort will be given and the comprehension of its limits leading to a possible new effort will be discussed.