Letter of Intent to measure Vacuum Magnetic Birefringence: the VMB@CERN experiment

Non linear electrodynamic effects have been predicted since the formulation of the Euler effective Lagrangian in 1935. These include processes such as light-by-light scattering, Delbrück scattering, g-2 and vacuum magnetic birefringence. This last effect deriving from quantum fluctuations appears at a macroscopic level. Although experimental efforts have been active for about 40 years (having begun at CERN in 1978) a direct laboratory observation of vacuum magnetic birefringence is still lacking: the predicted magnetic birefringence of vacuum is ∆n = 4.0 × 10^−24 @ 1 T. Key ingredients of a polarimeter for detecting such a small birefringence are a long optical path within an intense magnetic field and a time dependent effect. To lengthen the optical path a Fabry- Perot interferometer is generally used. Interestingly, there is a difficulty in reaching the predicted shot noise limit of such polarimeters. The cavity mirrors generate a birefringence-dominated noise whose ellipticity is amplified by the cavity itself limiting the maximum finesse which can be used. This Letter of Intent proposes an experiment which overcomes this difficulty by using a LHC super- conducting magnet together with a novel polarisation modulation scheme for the polarimeter. The proposing authors all come from previous experimental efforts to measure vacuum magnetic birefrin- gence and represent the maximum expertise in the field. Using the proposed setup, vacuum magnetic birefringence should be detected with an SNR = 1 in less than 1 day. The first detection of VMB would result in a direct observation of the fluctuations of the electron- positron field, and it would pave the way both to an accurate test of QED and to the observation of higher order effects.