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Search for electric dipole moments using storage rings

Periodic Reporting for period 3 - srEDM (Search for electric dipole moments using storage rings)

Reporting period: 2019-10-01 to 2021-03-31

"According to our present understanding, 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 furthermore 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), CP-violation is observed, but its size (essentially only 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.

Electric dipole moments 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 derived 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. It is worth noting that - 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 hadron (p, d) 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 (dEDM).

The EDM measurement principle, the time development of the (p ord d) 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 the holy grail of a dual-beam precision all-electric 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"" and, by overcoming the technological and metrological challenges, it will provide many new advances in accelerator and detector technology.

In contrast to other EDM searches, the srEDM project can only be pursued in Europe.
"The srEDM project comprises five work packages with two general deliverables: demonstration of key-technologies for the storage-ring method and first storag-ring EDM measurements at COSY. In the following, the major work and achievements are briefly discussed along the work packages:

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 experimentally tested at COSY and, for the first time, 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. Preparations are under way to refurbish a dipole magnet (""ANKE D2"") with a huge gap, previously installed in the COSY ring, with electric field plates, so that it can be used 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 in the laboratory and installed into COSY for use with the RF-Wien filter experiment.
(iv) An upgrade program for COSY has started in order to improve the precision of the beam position determination. As part of this, the magnetic center of the COSY quadrupoles with respect to beam position monitors is determined by the so called beam-based alignment method.

(2) Beam polarimetry:
(i) The vector analyzing power and the un-polarized elastic cross section of deuterons elastically scattered off a carbon target has been measured at COSY for six different beam energies starting from 170 MeV up 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 detector based on LYSO scintillators is under development and tests have advanced to a stage that the polarimeter can now be installed into COSY. The 52 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, it is important to simulate the orientation of the so called invariant spin axis in order 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 the Cooler Synchrotron (COSY) storage ring has been realized in order to control both 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 knowledge of the position of quadrupole magnets with respect to beam position monitors. This will lead to smaller systematic errors for the EDM measurements once the beam passes through the center of the magnets.

(5) Proof-of-principle and first EDM measurement:
(i) A radiofrequency (RF) Wien filter has been developed and constructed in order to allow for an EDM polarization build-up in a conventional (B-field deflector) ring like COSY. It has been installed in COSY in May 2017 and was commissioned with a polarized deuteron beam subsequently in February 2018.
(ii) A first deuteron test measurement has been conducted in May 2018. Currently the obtained results are analyzed and a second beamtime is in preparation.
(1) Progress beyond state-of-the-art:
(i) The COSY storage ring will be optimized for use as a precision experiment.
(ii) A polarimeter based on LYSO scintillators with SiPM readout for the precision measurement of the deuteron beam polarization will be available in COSY.
(iii) Simulation programs will be benchmarked and ready to use 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 EDM limit for deuterons will be available, obtained from the precursor experiment at COSY.
(ii) As an additional new outcome, a proof-of-principle for an axion dark-matter search will be provided.
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