Storage Ring Proton EDM

Yannis K. Semertzidis, BNL, October 2011 for the Storage Ring EDM Collaboration The storage ring EDM collaboration by the time of the workshop is expecting to have submitted a proposal to DOE for a proton EDM experiment sensitive to 10−29e · cm, see [1]. The method utilizes polarized protons at the, so-called “magic” momentum of 0.7 GeV/c in an all-electric storage ring. At this momentum, the proton spin and momentum vectors precess at the same rate in any transverse electric field. When the spin is kept along the momentum direction the radial electric field acts on the EDM vector precessing the proton spin vertically building up its vertical component for the duration of the storage time. The storage time is limited to 10s by the estimated horizontal spin coherence time (hSCT) of the beam within the admittance of the ring. Further improvements in the hSCT using stochastic cooling might be possible to extend the experimental sensitivity by another order of magnitude. The collaboration has developed more sophisticated methods of “freezing” the spin along the momentum direction by applying a combination of magnetic and electric fields [1], [2] but at this point we believe the proton EDM method at the magic momentum is the simplest one to implement. The physics reach of a proton EDM experiment with sensitivity of 10−29e ·cm is 3000 TeV for contact interactions and 300 GeV for SUSY-like New Physics. It can test the electro-weak baryogenesis models of the mechanism responsible for the observed baryon asymmetry in our universe. If an EDM value is found to be consistent with zero at the planned sensitivity level it can exclude the baryogenesis models. However, if a non-zero value is found, as it is also the case with the neutron EDM, it will need additional information to confirm beyond any doubt that the CP-violating source is New Physics and not just originating from the θQCD term in the SM. That question may then be answered by probing the deuteron EDM. The protons at their magic momentum behave as ideal Dirac-like particles, with an effective gyromagnetic ratio factor equal to 2. Every particle with a positive anomalous magnetic moment has a “magic” momentum, the value of which is p = m √ a . The magic momentum concept was first used in the last muon g-2 experiment at CERN [3] and in the muon g-2 experiment at BNL [4]. The proton EDM proposal went through two very successful technical reviews: one in December 2009 and another in March 2011 coming out with a strong recommendation to submit the proposal to DOE. The collaboration came up with a proposal to build an allelectric ring with radius of ∼ 40 m, using an E-field of ∼ 10 MV/m between plates separated by 3 cm. The strength of the storage ring EDM method comes from the fact that we can store a large number of highly polarized particles for a long time, achieve a large hSCT and probe the transverse spin components as a function of time with a high sensitivity polarimeter. The polarimeter uses elastic nuclear scattering off a solid carbon target placed in a straight section of the ring serving as the limiting aperture. The storage ring EDM collaboration has over the years developed the method and improved our understanding and confidence on it. Over the years