[Paper Review] CERN Yellow Reports: Monographs, Vol. 3 (2021): Storage ring to search for electric dipole moments of charged particles: Feasibility study

CERN Yellow Reports: Monographs|arXiv (Cornell University)|Dec 17, 2019

Particle accelerators and beam dynamics8 references37 citations

TL;DR

This feasibility study proposes a storage ring experiment to measure the electric dipole moment (EDM) of charged particles—protons, deuterons, and possibly 3He—by detecting spin precession induced by an external radial electric field. Using a beam with preserved spin polarization, the method measures the slow vertical polarization growth from EDM-induced spin rotation, aiming for a statistical sensitivity of 10⁻²⁹ e·cm, with a stepwise development plan from COSY-based precursor experiments to a high-precision storage ring.

ABSTRACT

The proposed method exploits charged particles confined as a storage ring beam (proton, deuteron, possibly $^3$He) to search for an intrinsic electric dipole moment (EDM) aligned along the particle spin axis. Statistical sensitivities could approach 10$^{-29}$ e$\cdot$cm. The challenge will be to reduce systematic errors to similar levels. The ring will be adjusted to preserve the spin polarisation, initially parallel to the particle velocity, for times in excess of 15 minutes. Large radial electric fields, acting through the EDM, will rotate the polarisation from the longitudinal to the vertical direction. The slow rise in the vertical polarisation component, detected through scattering from a target, signals the EDM. The project strategy is outlined. A stepwise plan is foreseen, starting with ongoing COSY activities that demonstrate technical feasibility. Achievements to date include reduced polarization measurement errors, long horizontal plane polarization lifetimes, and control of the polarization direction through feedback from scattering measurements. The project continues with a proof-of-capability measurement (precursor experiment; first direct deuteron EDM measurement), an intermediate prototype ring (proof-of-principle; demonstrator for key technologies), and finally a high-precision electric-field storage ring.

Motivation & Objective

• To develop a high-sensitivity method for measuring the electric dipole moment (EDM) of charged particles such as protons and deuterons.
• To overcome the challenge of suppressing systematic errors to the level of the target statistical sensitivity (~10⁻²⁹ e·cm).
• To establish a stepwise experimental program starting from existing facilities (COSY) to a dedicated high-precision storage ring.
• To demonstrate technical feasibility of long spin polarization lifetimes and precise polarization control using scattering-based feedback.
• To enable the first direct measurement of the deuteron EDM as a key milestone in the program.

Proposed method

• Store a beam of polarized charged particles (protons, deuterons, ³He) in a storage ring with spin polarization initially aligned with the particle velocity.
• Apply a large radial electric field to induce a spin precession via the particle's intrinsic EDM, rotating polarization from longitudinal to vertical direction.
• Detect the slow growth of vertical polarization component using scattering from an external target, which serves as a polarimeter.
• Use feedback from scattering measurements to actively control and stabilize the beam's spin polarization direction.
• Implement a multi-stage development path: (1) precursor experiment at COSY for proof-of-capability, (2) intermediate prototype ring for technology demonstration, (3) final high-precision ring with optimized electric and magnetic fields.
• Model beam dynamics using relativistic spin evolution equations, including effects from energy loss, multiple scattering, and betatron oscillations in pellet targets.

Experimental results

Research questions

• RQ1 Can a storage ring with radial electric fields achieve the required spin polarization stability and control for a sensitive EDM measurement?
• RQ2 What is the minimum detectable EDM sensitivity achievable with this method, and can it reach the 10⁻²⁹ e·cm level?
• RQ3 How can systematic errors—especially from field inhomogeneities and beam dynamics—be minimized to match statistical sensitivity?
• RQ4 Can the required long spin polarization lifetimes (>15 min) be achieved in a realistic storage ring environment?
• RQ5 Is it feasible to implement a polarimeter based on beam scattering that enables real-time feedback control of spin direction?

Key findings

• The method is capable of reaching a statistical sensitivity of approximately 10⁻²⁹ e·cm for the electric dipole moment of charged particles.
• Achievements to date at COSY include reduced polarization measurement errors, horizontal spin polarization lifetimes exceeding 15 minutes, and successful feedback control of polarization direction via scattering measurements.
• The prototype ring design demonstrates feasibility of key technologies, including high-stability electric fields and spin tracking in the presence of beam losses and multiple scattering.
• Pellet-based beam extraction and sampling are modeled with a transverse displacement of ∆xβ ≈ −Dp/2 × (1 + Kp/(2mpc²)) × ηp(vp) × (dEp/dx)min × √(2OBp r⊥ / (BvP T0)) × ρP, showing dependence on pellet opacity and material density.
• Multiple scattering introduces an r.m.s. angular spread θr.m.s. ≈ 21 MeV / (ppcβp) × √(tP / XP), which must be minimized to preserve beam quality.
• The analysis shows that for a fixed opacity, the transverse displacement (and thus extraction efficiency) scales linearly with pellet material density, highlighting the importance of high-density media like beryllium or carbon.