Physics of the Muon EDM Experiment:

We are proposing a new method to carry out a dedicated search for a permanent electric dipole moment (EDM) of the muon with a sensitivity at a level of 10^{-24} e cm. The experimental design exploits the strong motional electric field sensed by relativistic particles in a magnetic storage ring. As a key feature, a novel technique has been invented in which the g-2 precession is compensated with radial electric field.

The standard model of particle physics is a very successful theoretical framework, which describes all confirmed observations to date. However, the model leaves important questions concerning the physical nature of observed processes unexplained, although it provides an accurate description of them. Among the not yet understood phenomena are the reasons for parity violation, the particle masses, the violation of the CP symmetry. CP violation is the only known mechanism that could explain the matter antimatter asymmetry found in the universe. In order to obtain a deeper insight, speculative models have been suggested which often are connected to observable deviations from standard theory predictions, particularly violations of assumed symmetries or yet unknown properties of particles. The spectrum of these theories includes super symmetry (SUSY), left-right symmetry, multi Higgs scenarios and many other important approaches. This strongly motivates sensitive searches for forbidden decays, e.g. the lepton number violating muon electron conversion, the decay muon --> electron + gamma and precise measurements of particle characteristics such as the muon magnetic moment anomaly.

A fundamental particle is described by only a few parameters such as its mass, its intrinsic angular momentum, its electric charge (electric monopole moment), its magnetic dipole moment, and a set of conserved quantum numbers, like lepton flavor. A permanent EDM has not been observed for any of them. It would violate both parity (P) and time reversal (T) invariance. If CPT is assumed to be a valid unbroken symmetry, a permanent EDM would hence be a signature of CP violation.

The standard model of particle physics predicts a CP violating EDM in fundamental particles at the multi loop level of amplitude more than five orders of magnitude below the sensitivity of present experiments. Therefore searches for a permanent particle EDM render excellent opportunities to test models beyond standard theory where in some cases they predict effects as large as the presently known experimental bounds. Despite the non-observation of a positive EDM signal such research has nevertheless been most successful in steering the development of theoretical particle physics over many decades and also was the incentive for the ever increasing precision in the experiments themselves. The absence of any observed finite EDM for the neutron, for example, has disfavored more speculative models than any other experimental approach so far.

Searches for EDMs have been performed with highest sensitivity for electrons (e), neutrons (n), and Hg. Further there are experimental limits for muons muon, tauons and protons (p). The experiments include beams of neutral atoms, molecular beams, stored neutrons and stored charged particles. In heavy atoms and particularly in polar molecules there are substantial enhancement factors due to the utilization of the rather strong internal electric fields within these systems.

The upper bound extracted from electron and neutron experiments already disfavor super-symmetric models with CP violating phases of order unity and suggest variants with phases of order alpha / pi. Other significant restrictions of the parameter space would also be an option, however at a loss of generality. A muon EDM experiment at 10^{-24} e cm will be competitive here. Moreover, it will provide valuable complementary information, because the muon belongs to the second generation of particles where in the quark sector CP violation occurs. In two Higgs doublet models with large ratios for the vacuum expectation values of the involved Higgs field and for left right symmetry a muon EDM could be as large as a few 10^{-24} e cm and some special models allow values up to 10^{-21} e cm .

Sensitive searches are presently being carried out on neutral objects, i.e. neutrons, atoms and molecules. This choice was strongly influenced by the Ramsey-Purcell-Schiff theorem which states that for point-like charged objects in electromagnetic equilibrium, the net electric field averages to zero. The widely known loopholes so far were weak and strong nuclear forces, weak electron-nucleon forces and relativistic forces. It is recognized now that this theorem is also not applicable to particles in a storage ring, particularly to the method proposed here, where motional fields are employed, because it is not possible to factorize particle velocity and electric field, which constituted the basis of the theorem. Therefore these electric fields, which are very strong for relativistic particles, can be beneficially exploited. Such fields can be three orders of magnitude larger than technically achievable fields between electrodes, where 5.5 MV/m are regarded an upper limit.

It should be mentioned that the muon is the only second generation particle for which a precision EDM experiment is feasible. Since CP violation is associated with the second and third generation of quarks only, the muon may have a unique window for new physics in the lepton sector. Therefore, even if there were an EDM observed in another system, a measurement on the muon would be extremely important to understand the nature of the effect.

The Muon g-2 Experiment, E821, now being conducted at BNL, has been designed to probe physics beyond the standard model. This includes super-symmetry with large tan(beta) where the muon EDM also has sensitivity. The experiments complement each other, because the magnetic anomaly and the EDM are related to each other as real and imaginary parts of the same physical quantity. The recent new limit on the muon magnetic anomaly corresponds (assuming the CP-violating phase to be of order 1) to an electric dipole moment of order 3.5 X 10^{-22} e cm, which is well within the reach of this experiment, and demonstrates, how the explored parameter space can be expanded.