Abstract
Sensitive magneto-optical polarimetry was proposed by E. Iacopini and E. Zavattini in 1979 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) and/or MilliCharged Particles (MCP). Today ALPs are considered a strong candidate for Dark Matter. Starting in 1992 the PVLAS collaboration, financed by INFN, Italy, attempted to measure VMB conceptually following the original 1979 scheme 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 at the Laboratori Nazionali di Legnaro, Legnaro, Italy and the second using two rotating permanent 2.5 T dipole magnets at the INFN section of Ferrara. At present PVLAS is the experiment which has set the best limit in VMB reaching a noise floor within a factor 7 of the predicted QED signal: Δn(QED)=2.5×10−23 @ 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. In this review, a detailed description of the PVLAS effort and the comprehension of its limits leading to a new proposal will be given.
Highlights
From Maxwell’s equations in vacuum the velocity of light is related to the magnetic permeability μ0 and vacuum permittivity ε0 through the relationPublisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. c= √1 . (1) ε0μ0Since 20 May 2019, the velocity of light in vacuum c is defined to be c = 299, 792, 458 m/s and ε0 and μ0 are derived from the measurement of the fine structure constant α: α =e2 4πε0hc e2cμ0 4πh (2)being e and halso defined
A Faraday rotation resulting from a time variation of an eventual small longitudinal component of the magnetic field along the light path would appear at the first harmonics of the rotation frequency of the magnetic field
Rotating Superconducting Magnet The magnet used in this phase was the original 1 m long superconducting dipole magnet developed by Mario Morpurgo at CERN [77] as a prototype for the CERN D2 Proposal [78,79], the first proposal to measure Vacuum Magnetic Birefringence (VMB) using the optical technique proposed a couple of years before [52]
Summary
From Maxwell’s equations in vacuum the velocity of light is related to the magnetic permeability μ0 and vacuum permittivity ε0 through the relation. The formulation of Einstein’s energy-mass relation, Heisenberg’s Uncertainty Principle and Dirac’s relativistic equation of the electron opened the doors to vacuum fluctuations leading to nonlinear electrodynamics in vacuum such as light-by-light (LbL) elastic scattering and vacuum magnetic birefringence (VMB). Allowing 4-field interactions, this Lagrangian leads to light-by-light (LbL) scattering [9,10], to a reduction of the velocity of light in the presence of an external field and to Vacuum Magnetic Birefringence (VMB) [11,12,13,14,15,16], namely a difference in the indices of refraction for light polarized parallel and perpendicular to an external magnetic field Bext. Note that both n ,⊥ > 1 and light-by-light scattering is permitted
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