Feasibility Study for an EDM Storage Ring Contact

This project exploits charged particles confined as a storage ring beam (proton, deuteron, possibly He) to search for an intrinsic electric dipole moment (EDM, ~ d) aligned along the particle spin axis. Statistical sensitivities can approach 10−29 e·cm. The challenge will be to reduce systematic errors to similar levels. The ring will be adjusted to preserve the spin polarization, initially parallel to the particle velocity, for times in excess of 15 minutes. Large radial electric fields, acting through the EDM, will rotate the polarization (~ d × ~ E). The slow rise in the vertical polarization component, detected through scattering from a target, signals the EDM. The project strategy is outlined. It foresees a step-wise plan, 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 the 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 the high precision electric-field storage ring. http://pbc.web.cern.ch/edm/edm-default.htm ar X iv :1 81 2. 08 53 5v 2 [ ph ys ic s. ac cph ] 1 8 Ja n 20 19 1 Science context and objectives Symmetry considerations and symmetry-breaking patterns have played an important role in the development of physics in the last 100 years. Experimental tests of discrete symmetries (e.g. parity P, chargeconjugation C, their product CP, time-reversal invariance T, the product CPT, baryonand/or lepton number) have been essential for the development of the Standard Model (SM) of particle physics. Subatomic particles with nonzero spin (regardless whether of elementary or composite nature) can only support a nonzero permanent electric dipole moment (EDM) if both time-reversal (T) and parity (P) symmetries are violated explicitly while the charge symmetry (C) can be maintained (see e.g. [1]). Assuming the conservation of the combined CPT symmetry, T-violation also implies CP-violation. The CP-violation generated by the Kobayashi-Maskawa (KM) mechanism of weak interactions contributes a very small EDM that is several orders of magnitude below current experimental limits. However, many models beyond the Standard Model predict EDM values near the current experimental limits. Finding a non-zero EDM value of any subatomic particle would be a signal that there exists a new source of CP violation, either induced by the strong CP violation via the θQCD angle or by genuine physics beyond the SM (BSM). In fact, the best upper limit on θQCD follows from the experimental bound on the EDM of the neutron. CP violation beyond the SM is also essential for explaining the mystery of the observed baryonantibaryon asymmetry of our universe, one of the outstanding problems in contemporary elementary particle physics and cosmology. A measurement of a single EDM will not be sufficient to establish the sources of any new CP-violation. Complementary observations of EDMs in multiple systems will thus prove essential. Up to now measurements have focused on neutral systems (neutron, atoms, molecules). We propose to use a storage ring to measure the EDM of charged particles. The storage ring method would provide a direct measurement of the EDM of a charged particle comparable to or better than present investigations on ultra-cold neutrons. The neutron investigations measure the precession frequency jumps in traps containing magnetic and electric fields as the sign of the electric field is changed. These experiments are now approaching sensitivities of 10−26 e·cm [2] and promise improvements of another order of magnitude within the next decade. Because proton beams trap significantly more particles, statistical sensitivities may reach the order of 10−29 e·cm [3] with a new, all-electric, high-precision storage ring. Indirect determinations for the proton produce model-dependent EDM limits near 2 × 10−25 e·cm using Hg [4]. Thus storage rings could take the lead as the most sensitive method for the discovery of an EDM. It should be noted that the rotating spin-polarized beam used in the EDM search is also sensitive to the presence of an oscillating EDM resulting from axions or axion-like fields, which correspond to the dark-matter candidates of a pseudo-scalar nature. These may be detected through a time series analysis of EDM search data or by scanning the beam’s spin-rotation frequency in search of a resonance with an axion-like mass in the range from μeV down to 10−24 eV [5, 6].