Charged particle EDM (cpEDM) measurement


Within the PBC study group, CERN has joined an international effort studying rings to measure a possible very small Electric Dipole Moment (EDM) of charged particles. Most proposals rely on operation with “frozen spin” such that, in the absence of an EDM, an initial longitudinal polarization of injected bunches is maintained. A finite EDM generates a rotation from the horizontal plane into the vertical direction. The “frozen spin” condition can be fulfilled for any particle species and beam energy by a suitable ratio between electric and magnetic fields bending the beam. For particles with positive anomalous magnetic moment, the frozen spin conditions is met in a purely electro-static machine operated at a specific energy (so called “magic energy”) which for protons is 233 MeV.

Machine imperfections, e.g. uncontrolled magnet fields and residual misalignments, generate a vertical polarization in the absence of an EDM. These so-called “systematic effects” limit the sensitivity, i.e., the smallest EDM that can be detected. The WG is involved in the analysis of systematic effects to improve their understanding and to estimate the sensitivity of a realistic machine for different schemes under discussion.

There is agreement that a smaller prototype ring should be constructed as intermediate step to gain experience with large rings using electro-static elements to bend and focus the beam and to study sensitivity limitations and their mitigations. The WG contributes to the design of such a Prototype Ring (PTR).


Contribution to an international effort to design an EDM PTR: The WG is planning to coordinate a WP on ring design and to contribute to lattice design, transfer and injection schemes, provide expertise on precise alignment in particular in the vertical plane, the analysis of performance limitations and cost estimates.

Continuation of studies of systematic effects in a possible high precision EDM ring: The aim of thorough studies of systematic effects is to come to a realistic estimate of the sensitivity for the different proposed schemes.

Working Group Members

Conveners: Christian Carli, Frank Rathmann (Forschungszentrum Jülich)

Core Members: Jan Borburgh, Yann Dutheil, Helene Mainaud-Durand (list being finalized)


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