Collaborative Research: The MuLAN Project -- Development of Instrumentation for a New High-Precision Determination of the Fermi Coupling Constant

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0079449Carey The MuLan Project will develop the instrumentation needed tomeasure the lifetime of the positive muon to unprecedentedprecision, namely, a factor of 20 better than the current worldaverage. The muon lifetime fixes the value of the Fermi constant,which governs the strength of all weak-interaction processes, justas the fine structure constant governs the strength of allelectromagnetic-interaction processes. The Fermi constant is oneof 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 newbeam chopper system in order to create intense, short bursts ofmuons, which will stop in thin targets. A positive muon stopped inappropriate materials decays as if it were in vacuum. Such targetswill be surrounded by a nearly hermetic set of 180 fast-scintillatortiming detectors, each coupled to a state-of-the-art waveformdigitizer, and all readout by a high-speed data acquisition system. Online analysis of the nearly 100 terabytes of data will take placewith a small array of fast microprocessors. A second use of the instrumentation is in conjunction with anongoing effort at PSI to measure the negative muon lifetime inhydrogen gas. The negative muon can decay exactly in the samemanner as the positive muon, or in hydrogen it can interact with aproton by the weak-interaction process known as "capture." Thecapture rate is predicted very accurately by theory; however,current experiments, which have various interpretation problems, arein disagreement with theory by a very significant amount. If thisdisagreement is confirmed, this difference raises the excitingpossibility of pointing to new and unaccounted-for physics. Bymeasuring the difference between the positive and negative muonlifetimes, 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.