Systematic Effects Limiting the Sensitivity of "Magic Energy" Proton EDM Rings

Proposals to measure a possible Electric Dipole Moment (EDM) of protons in an electro-static storage ring are studied by a world-wide community. The machine is operated at the so-called "magic energy" to satisfy the "frozen spin" condition such that, without imperfections and with the well known magnetic moment of the particle, the spin is always oriented parallel to the direction of movement. The effect of a finite EDM is a build-up of a vertical spin component. Any effect, other than a finite EDM, leading as well to a build-up of a vertical spin limits the sensitivity of the experiment. Such "systematic effects" are caused by machine imperfections, such as magnetic fields inside the magnetic shield surrounding the ring, and misalignments of electro-static elements or of the RF cavity. Operation of the machine with counter-rotating beams helps mitigating some of the effects. The most dangerous effects are those, which cannot be disentangled from an EDM by combining measurements from both counter-rotating beams, such as an average residual radial magnetic field penetrating the magnetic shield or a combination of magnetic fields and misalignments of electric elements.