A new part-per-million measurement of the positive muon lifetime and determination of the Fermi Constant

Abstract

The Fermi Constant, GF, describes the strength of the weak force and is determined most precisely from the mean life of the positive muon, tµ. Advances in theory have reduced the theoretical uncertainty on GF as calculated from tµ to a few tenths of a part per million (ppm). Until recently, the remaining uncertainty on GF was entirely experimental and dominated by the uncertainty on tµ. We report the MuLan collaboration's recent 1.0 ppm measurement of the positive muon lifetime. This measurement is over a factor of 15 more precise than any previous measurement, and is the most precise particle lifetime ever measured. The experiment used a time-structured low-energy muon beam and an array of plastic scintillators read-out by waveform digitizers and a fast data acquisition system to record over 2 × 1012 muon decays. Two different in-vacuum muon-stopping targets were used in separate data-taking periods. The results from these two data-taking periods are in excellent agreement. The combined results give tµ+(MuLan) = 2196980.3(2.2) ps. This measurement of the muon lifetime gives the most precise value for the Fermi Constant: GF(MuLan) = 1.1663788(7) × 10-5 GeV-2 (0.6 ppm). The lifetime is also used to extract the µ-p singlet capture rate, which determines the proton's weak induced pseudoscalar coupling gP.