Collaborative Research: The MuLan Project-Development of instrumentation for a new high-precision determination of the Fermi coupling constant

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0079735Hertzog The MuLan Project will develop the instrumentation needed to measure the lifetime of the positive muon to unprecedented precision, namely, a factor of 20 better than the current world average. The muon lifetime fixes the value of the Fermi constant, which governs the strength of all weak-interaction processes, just as the fine structure constant governs the strength of allelectromagnetic-interaction processes. The Fermi constant is one of the fundamental parameters of the Standard Model of particlephysics. A low-energy, continuous-wave muon beam at the Paul Scherrer Institute (PSI) in Switzerland will be modified by a new beam chopper system in order to create intense, short bursts of muons, which will stop in thin targets. A positive muon stopped inappropriate materials decays as if it were in vacuum. Such targets will be surrounded by a nearly hermetic set of 180 fast-scintillator timing detectors, each coupled to a state-of-the-art waveform digitizer, and all readout by a high-speed data acquisition system. Online analysis of the nearly 100 terabytes of data will take place with a small array of fast microprocessors. A second use of the instrumentation is in conjunction with an ongoing effort at PSI to measure the negative muon lifetime inhydrogen gas. The negative muon can decay exactly in the same manner as the positive muon, or in hydrogen it can interact with aproton by the weak-interaction process known as "capture." The capture rate is predicted very accurately by theory; however,current experiments, which have various interpretation problems, are in disagreement with theory by a very significant amount. Ifthis disagreement is confirmed, this difference raises the exciting possibility of pointing to new and unaccounted for physics. Bymeasuring the difference between the positive and negative muon lifetimes, we will determine the muon capture rate reliably and at aprecision more than four times better than current measurements. The MuLan timing detectors, beamline and custom electronics areexpected to play a major role in this effort.