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Search for electric dipole moments using storage rings

Periodic Reporting for period 4 - srEDM (Search for electric dipole moments using storage rings)

Reporting period: 2021-04-01 to 2022-09-30

The early Universe contained the same amount of matter and anti-matter and, if the Universe had behaved symmetrically as it developed, every particle would have been annihilated by one of its antiparticles, leaving behind no matter but only photons.

One of the great mysteries in the natural sciences is therefore why there is matter at all present in our world rather than only light, and why matter dominates over anti-matter in the visible Universe. The reason for this apparent inequality must be sought in a difference between matter and anti-matter, related to the breaking of a fundamental symmetry called charge-parity (CP)-violation (CPV).

In the Standard Model of elementary particle physics (SM), CPV is observed, but its size (due to the electroweak interaction with essentially no contribution from the strong interaction) is greatly insufficient to explain the matter anti-matter asymmetry and further sources of CPV must be sought. These could manifest themselves in electric dipole moments (EDMs) of elementary particles, which occur when the centroids of positive and negative charges are mutually and permanently displaced. An EDM observation would also be an indication for physics beyond the SM.

EDMs are searched for in different systems (neutrons (n), atoms, molecules and even bulk material), but, up to now, no EDM has been observed - only ever smaller upper limits have been deduced, including indirekt results for the proton (p) and the electron (e). A new line of EDM-search has recently been suggested for charged particles (protons and deuterons (d), and 3-helium) in storage rings, aiming at direct limits with unprecedented sensitivity. Once EDMs have been discovered - results for different particles (e, n, p, d) are required to pin down the CPV sources.

The srEDM-project will lay the foundations for direct EDM searches of charged hadrons in a completely new class of precision storage rings by developing the required key technologies. It will exploit the existing conventional storage ring COSY (based on magnetic-field deflection) of Forschungszentrum Jülich (Germany) and it will also provide a first measured EDM limit for deuterons.

The EDM measurement principle, the time development of the polarization vector (which is oriented along the EDM) subject to a perpendicular electric field, is simple, but the smallness of the EDM together with the potentially overwhelming impact of magnetic dipole moment (MDM) effects make this an enormously challenging project. A staged approach is required – from R&D for key-technologies towards a dual-beam all-electric precision storage ring with simultaneously circulating clock-wise and counter clock-wise beam bunches, running in the so called "frozen spin" condition – to achieve the highest EDM sensitivity.

The srEDM project has an outstanding science case with the potential to solve "the puzzle of our existence" (the baryon asymmetry of our Universe) and the search for axion/axion-like particles (candidates for Dark Matter in our Universe). By overcoming the technological and metrological challenges, it will also provide many new advances in accelerator and detector technology.
srEDM comprises 5 WPs with two general deliverables: demonstration of key-technologies for the storage-ring method and first EDM measurements at COSY. The major work and achievements are as follows:

1) Accelerator developments:
(i) A new method based on the spin tune response of a machine to artificially applied longitudinal magnetic fields, called “spin tune mapping”, has been developed. The technique was tested at COSY and the angular orientation of the stable spin axis at two different locations in the ring has been determined to an unprecedented accuracy of better than 2.8 µrad.
(ii) Electrostatic deflector elements with 10 mm radius, made of stainless steel, have been developed and tested at gap distances from 1 mm to 0.05 mm at electric fields, ranging from 15 to 90 MV/m. A large dipole magnet with a huge gap will be equipped with electric field plates to investigate combined electric and magnetic field conditions required for a deuteron EDM ring.
(iii) A compact beam position monitor (Rogowski-type) has been developed, calibrated and installed into COSY for use with the RF-Wien filter experiment.
(iv) COSY has been upgraded in order to improve the precision of the beam position determination, i.p. the magnetic center of the COSY quadrupoles with respect to beam position monitors was determined by the beam-based alignment method.

(2) Beam polarimetry:
(i) The vector analyzing power of deuterons elastically scattered off a carbon target has been measured at COSY for six different beam energies (170 MeV to 380 MeV). Such data are needed to determine the deuteron beam polarization, whose time development is the observable for the deuteron EDM search.
(ii) A precision polarimeter based on LYSO scintillators has been developed and installed into COSY. The detector modules can stop elastically scattered deuterons and protons up to 300 MeV. The modules are very compact, due to modern high pixel density SiPM readout.

(3) Beam simulations:
(i) Existing spin tracking simulation programs (COSY Infiniity, Bmad) have been extended to simulate spin motion in the presence of an electric dipole moment. The appropriate EDM kicks and electric field elements (static, radiofrequency) have been implemented and benchmarked with data. Different possible scenarios (frozen spin method, quasi frozen spin method) have been investigated to explore the achievable sensitivity.
(ii) For the precursor experiment at COSY simulations for the orientation of the invariant spin axis have been performed to determine a possible additional (experimentally measured) tilt due to an EDM.

(4) Feasibility studies at COSY:
(i) The successful use of feedback from a spin polarization measurement to the revolution frequency of a 0.97 GeV/c bunched and polarized deuteron beam in COSY) has been realized in order to control the precession rate (≈ 120 kHz) and the phase of the horizontal polarization component.
(ii) Beam-based alignment has been implemented at COSY in order to obtain a precise information of the position of quadrupole magnets with respect to beam position monitors.

(5) Proof-of-principle and first EDM measurement:
(i) A radiofrequency (RF) Wien filter has been developed and constructed iand installed in COSY n order to allow for an EDM polarization build-up in a conventional (B-field deflector) ring like COSY.
(ii) Two deuteron EDM experiments at COSY have been conducted, which are currently analyzed.
(1) Progress beyond state-of-the-art:
(i) The COSY storage ring will be optimized for use as a precision experiment.
(ii) A polarimeter for the precision measurement of the deuteron beam polarization is available in COSY.
(iii) Simulation programs are benchmarked for the analysis of EDM measurements.
(iv) A conceptual design of the all-electric prototype EDM storage ring will be finished.

(2) Expected results until end of project:
(i) A first result for an axion dark-matter search will be provided.
(ii) A deuteron precursor EDM experiment at COSY will be conducted.
Inside View of the Radiofrequency Wien Filter for the srEDM Experiment at COSY
Photograph of the Cooler Storage Ring of Forschungszentrum Jülich, where the srEDM experiments are c
Principle of the EDM Experiment in a Storage Ring