Quantification of GR effects in muon g-2, EDM and other spin precession experiments

TL;DR

This paper provides a full general relativistic calculation to quantify GR effects in muon g-2 and EDM experiments, finding negligible impact on muon g-2 but significant effects on EDM measurements using the frozen spin method.

Contribution

It offers the first comprehensive GR correction analysis for spin precession experiments using Fermi-Walker transport in a Schwarzschild background.

Findings

GR effects are negligible in muon g-2 experiments

GR effects are detectable in EDM frozen spin experiments

Corrections are necessary for high-precision EDM measurements

Abstract

Recently, Morishima, Futamase and Shimizu published a series of manuscripts, putting forward arguments, based on a post-Newtonian approximative calculation, that there can be a sizable general relativistic (GR) correction in the experimental determination of the muon magnetic moment based on spin precession, i.e., in muon g-2 experiments. In response, other authors argued that the effect must be much smaller than claimed. Further authors argued that the effect exactly cancels. Also, the known formulae for de Sitter and Lense-Thirring effect do not apply due to the non-geodesic motion. All this indicates that it is difficult to estimate from first principles the influence of GR corrections in the problem of spin propagation. Therefore, in this paper we present a full general relativistic calculation in order to quantify this effect. The main methodology is the purely differential geometrical tool of Fermi-Walker transport over a Schwarzschild background. Also the Larmor precession due to the propagation in the electromagnetic field of the experimental apparatus is included. For the muon g-2 experiments the GR correction turns out to be very small, well below the present sensitivity. However, in other similar storage ring experimental settings, such as electric dipole moment (EDM) search experiments, where the so-called frozen spin method is used, GR gives a well detectable effect, and should be corrected for. All frozen spin scenarios are affected which intend to reach a sensitivity of 0.1 microradians/second for the spin precession in the vertical plane.