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Astro & GW Colloquium

Towards measuring vacuum magnetic birefringence using a CERN LHC static field magnet

Speaker: Prof. Guido Zavattini (University of Ferrara)
Date: Wednesday 10 April 2019
Time: 14:00
Venue: Queens Buildings N/3.28

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, Delbrueck 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 \Delta n = 4.0e-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. I will discuss an experimental which may overcome this difficulty by using a LHC super-conducting static field 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.