Muon anomalous magnetic moment (g-2) and electric dipole moment at J-PARC

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The Standard Model (SM) of particle physics, about half a century old, seems to have survived the many stringent experimental tests, culminating in the recent discovery of the Higgs particle. However, it leaves some critical questions unanswered, motivating the continuing search for evidence of physics beyond: new particles from accelerators or extraterrestrial sources, new interactions that "don't fit" with expected conservation laws or other SM predictions, and so on. A tantalizing result has been obtained by experimenters at Brookhaven National Laboratory (BNL) in the US using subatomic particles called muons that are easily produced in particle accelerators, but are not a constituent of normal matter. They measured the muon's anomalous magnetic moment, a_mu, with an incredible precision of 0.54 parts per million. But the value obtained for a_mu did not quite match SM expectations. Striving for yet improved precision that might represent convincing discovery of new physics, the BNL experiment is being recreated with improvements at the US Fermi National Laboratory (FNAL) with many of the same techniques. A much different method to measure a_mu, but with similar goals, has been proposed at J-PARC in Japan. The J-PARC scheme requires significant development of several experimental aspects, perhaps the most challenging being the production of a beam of ultra-cold muons, where the speeds and directions of the individual muons in the beam have only a tiny deviation from their average. It is proposed to create this beam by laser ionization of the hydrogen-like atomic bound state of a positive muon and an electron (muonium, or Mu), after formation in silica aerogel and subsequent diffusion at room-temperature thermal velocities into vacuum. In addition to enabling the J-PARC scheme, an ultra-cold polarized muon beam of variable low energy would open up new applications in materials science with the techniques of muon spin rotation in thin layers, surfaces, and material interfaces. In the past year, significant progress has been achieved, with production of viable amounts of thermal Mu in vacuum demonstrated in experiments at TRIUMF in Vancouver. The next step is to develop further the ultra-cold muon source by laser ionization of this abundant source of Mu in pulsed beam experiments at Rutherford Appleton Laboratory in the UK. Along with the control of systematic uncertainties made possible by J-PARC methods, this should open the way for independent experimental confirmation of whether the deviation from the SM observed at BNL is the result of a statistical fluctuation, an unidentified systematic error, or if it is indeed evidence of physics beyond the Standard Model.