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.