Measuring vacuum magnetic birefringence using an LHC magnet: the VMB@CERN

by Guido Zavattini (FE)

Tuesday 11 Dec 2018, 15:00 → 16:00 Europe/Rome

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, Delbr\"uck scattering, {g-2} and vacuum magnetic birefringence (VMB). This last effect deriving from quantum fluctuations appears at a macroscopic level and involves only photons in the initial and final states.

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 $\Delta n = 4.0\times 10^{-24}$ @ 1~T. Past efforts have been based on the use of Fabry-Perot based polarimeters. The reason for the lack of success has been recently narrowed down to intrinsic mirror noise from the Fabry-Perot cavity.

An experiment is being proposed which may overcome these difficulties by using a LHC superconducting magnet together with a novel polarisation modulation scheme for the polarimeter.  Using the proposed setup, vacuum magnetic birefringence should be detected with an SNR = 1 in less than 1 day.

Zavattini_VMB@CERN_small.pdf