A polarisation modulation scheme for measuring vacuum magnetic birefringence with static fields

G Zavattini,A Ejlli,G Ruoso,F Della Valle

doi:10.1140/epjc/s10052-016-4139-0

Abstract

A novel polarisation modulation scheme for polarimeters based on Fabry-Perot cavities is presented. The application to the proposed HERA-X experiment aiming to measuring the magnetic birefringence of vacuum with the HERA superconducting magnets is discussed.

Highlights

Vacuum magnetic birefringence is a non linear electrodynamic effect in vacuum closely related to light-by-light elastic scattering
Ideas to use high field superconducting magnets, such as those used in the LHC and HERA accelerators, have been suggested but their use is limited by the difficulty in modulating, in one way or another, their magnetic fields
The numbers seem to be within reach and we believe that this scheme could be a viable solution when using high field static magnetic fields generated by superconducting magnets

Summary

Introduction

Vacuum magnetic birefringence is a non linear electrodynamic effect in vacuum closely related to light-by-light elastic scattering. The Fabry–Perot cavity is necessary to increase the optical path L within the magnetic field region, whereas the variable magnetic field is necessary to induce a time dependent effect Both of these aspects significantly increase the sensitivity of the polarimeters. Ideas to use high field superconducting magnets, such as those used in the LHC and HERA accelerators, have been suggested but their use is limited by the difficulty in modulating, in one way or another, their magnetic fields To work around this problem, proposals of rotating the polarisation have been considered [15], but the presence of the Fabry–Perot cavity, whose mirrors always present an intrinsic birefringence whose induced ellipticity is orders of magnitude larger than the ellipticity due to vacuum magnetic birefringence, have made this idea unfeasible. A novel modulation scheme is presented that might profitably be employed with large superconding magnets

Preliminary considerations

Method

Conclusion