Improved Measurement of the Positive-Muon Lifetime and Determination of the Fermi Constant

The mean life of the positive muon has been measured to a precision of 11 ppm using a low-energy, pulsed muon beam stopped in a ferromagnetic target, which was surrounded by a scintillator detector array. The result, ${\tau }_{\mu }=2.197013(24)\mu \mathrm{s}$, is in excellent agreement with the previous world average. The new world average ${\tau }_{\mu }=2.197019(21)\mu \mathrm{s}$ determines the Fermi constant $G_F=1.166371(6)\times {10}^{-5}{\mathrm{GeV}}^{-2}$ (5 ppm). Additionally, the precision measurement of the positive-muon lifetime is needed to determine the nucleon pseudoscalar coupling $g_P$.