MECO & PRISM/PRIME: Experimental Searches for Coherent µ-e Conversion with Sensitivity at least 10-17

Dr. James L. Popp
Department of Physics
Idaho State University

The design, cost, and schedule for MECO (Muon-Electron COnversion experiment) has occupied physicists and engineers for more than a decade; research funded by the NSF and DOE. MECO has enjoyed uninterrupted support from funding agencies and the National Science Board throughout this time. MECO is a landmark experiment to build a high-intensity pulsed muon beamline at the AGS, in Brookhaven National Laboratory, and a high-rate detector system to directly observe the rare, and as yet unseen, process µ-N to e-N, coherent conversion of a muon into an electron in the field of a nucleus. The search for muon and electron number violation, i.e. lepton flavor violation (LFV), has continued since the discovery of the muon, with no evidence found for such processes. There is strong theoretical motivation to continue this search since the possibility of LFV exists in essentially all extensions of the Standard Model (SM). In fact, many theories aimed at grand unification suggest the branching fraction for coherent conversion relative to muon capture on the nucleus to be only a few orders of magnitude below current measured branching fraction limits: 10-12 - 10-13. The discovery of such a process would indicate the existence of a new force mediated by new gauge bosons with non-diagonal lepton flavor couplings, or a new class of heavy particles with lepton flavor mixing in this new sector (e.g., supersymmetry).  Still other mechanisms may exist, for example, due to previously unrecognized extra dimensions.  A search for coherent µ-e conversion is experimentally attractive since there is only one particle in the final state, avoiding the background complications associated with other processes like µ- to e-gamma. In August of this year, on the verge of starting construction, this experiment was cancelled, abruptly.  This talk will examine the MECO experiment and how the knowledge gained and lessons learned can be applied to the PRISM and PRIME projects in Japan to continue the pursuit of this and other important LFV experiments.