CPEDM: A Storage Ring Facility for Charged-Particle EDM Searches

Abstract

Scientific Background: Symmetries and Physics Beyond the Standard Model Permanent electric dipole moments (EDM) of particles violate both time reversal (T) and parity (P) invariance and, on the basis of the CPT (C represents charge conjugation) theorem, they also violate the combined symmetry CP (Charge-Parity Violation: CPV). Such a symmetry breaking is thought to be responsible for the different behavior of particles and antiparticles, leading, for example, to the apparent matter– antimatter asymmetry in the Universe. CPV is found in the electroweak part of the standard model (SM) of particle physics but—because SM-CPV is much too weak to explain the matter– antimatter asymmetry—other sources must be sought. An obvious observable to investigate is an EDM, because the SM-EDM values are unmeasurably small with current experimental techniques, finding an EDM would very probably also indicate new physics, not contained in the SM [1]. After a possible discovery of an EDM, different systems will have to be investigated in order to identify the CPV-source. Because of its exceptional science case, EDMs are searched for in various systems, hitherto, for example, for the electron bound in atoms and molecules or the free neutron, but only impressive upper limits have been obtained so far [2]. Recently, it has been proposed to use polarized charged particles, like protons, deuterons, and 3He, confined in a storage ring [3]. The measurement principle is based on the time development of the polarization vector—which is parallel to the EDM—subject to a radial electric field: a beam of particles, originally polarized in the horizontal plane along the direction of the momentum vector, slowly develops a vertical component. In spite of its simplicity, this represents an enormously challenging project due to the smallness of the expected effect. As of late, oscillating EDMs as an additional observable have come into focus [4]: axions and axion-like particles (ALPs) induce oscillating EDMs with an oscillation frequency proportional to their mass. Since these yet unobserved particles are well-motivated candidates for dark matter (DM) with largely unconstrained masses, they are also searched for with different approaches: EDM storage rings are very well suited to allow for these searches over a wide range of masses/oscillation frequencies. In order to observe axions/ALPs, the stored particle spins have to precess at the oscillation frequency of the axion field to produce a resonant build-up of the vertical polarization. In contrast to static EDM measurements where the spins have to be “frozen” parallel to the momentum vector in the horizontal plane, this principle can already be applied in storage rings with magnetic bending, such as COSY (COoler SYnchrotron at Forschungszentrum Jülich, Germany). In addition, oscillating EDM searches are considered to be less sensitive to systematic effects compared to static EDMs in a ring. A schematic overview of the EDM science case is given in Figure 1.