On systematic Effects of charged Particle Electric Dipole Moment Measurement Rings

Storage rings to measure a possible Electric Dipole Moment (EDM) of charged particles and, in particular Protons, are studied by an international community. Most proposals foresee to operate a ring with electrostatic bendings at the “magic energy”. At this particular energy and with the well-known Magnetic Dipole Moment (MDM) only, an initial longitudinal polarization of a bunch is maintained. A finite EDM generates a small vertical polarization.

Any effect occurring with an MDM only and caused by imperfections may limit the sensitivity, i.e., the smallest EDM that can be identified of the facility. Some of these systematic effects can be mitigated, e.g., by simultaneous circulation of beams in CW and CCW directions or additional bunches polarized mainly in radial direction. The most delicate systematic effect, which cannot be disentangled from an EDM by using counter-rotating beams, in case electro-static quadrupoles are used for focusing, are residual radial magnetic fields. The hybrid ring proposal uses magnetic quadrupoles. The fast spin rotations expected with this scheme can in principle be disentangled from a finite EDM with counter-rotating beams. Several other effects occur due to a combination of several machine imperfection.

A review of known systematic effects occurring in charge particle EDM rings will be given with quantifications for special cases. An estimate of the sensitivity of the proposed facilities would be a be very challenging thorough simulation of a ring with initial imperfections as positioning errors and residual magnetic stray fields, followed by beam based corrections and finally an estimate of the vertical spin build up without EDM with all mitigations measures in place.