Prospects for Electric Dipole Moment Measurement Using Electrostatic Accelerators

Abstract

Electrostatic accelerators have played a glorious role in physics, especially for low energy atomic and nuclear physics and electron microscopy. But circular accelerators have depended almost exclusively on the far greater bending force possible with static magnetic, rather than electric, fields. There is a potential exception to this magnetic bending monopoly for experimental high energy elementary particle physics — it is the possibility of measuring the electric dipole moments (EDMs) of charged elementary particles, such as proton, deuteron, or electron, using an electrostatic storage ring. Any such non-zero EDM would demonstrate violation of both parity (P) and time-reversal (T) invariance. One way of understanding the preponderance of matter over anti-matter in the present-day universe pre-supposes the existence of violations of P and T substantially greater than are allowed by the “standard model” of elementary particle physics. This provides the leading motivation for measuring EDMs. Currently, only upper limits are known for these EDMs. The very same smallness that makes it important to determine them makes their measurement difficult. Accepting as obvious the particle physics motivation, this paper concentrates on the accelerator physics of the (not very) high energy electrostatic accelerators needed for EDM measurements. Developments already completed are emphasized. Impressive advances have been made in the diagnostic tools, spin control and polarimetry that will make EDM measurement possible. Ring design for minimizing spin decoherence and limiting systematic EDM errors is presented. There have, however, been worrisome indications from low energy rings, concerning beam current limitations. A prototype ring design is proposed for investigating and addressing this concern.