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

Abstract

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, ${\ensuremath{\tau}}_{\ensuremath{\mu}}=2.197\text{ }013(24)\text{ }\text{ }\ensuremath{\mu}\mathrm{s}$, is in excellent agreement with the previous world average. The new world average ${\ensuremath{\tau}}_{\ensuremath{\mu}}=2.197\text{ }019(21)\text{ }\text{ }\ensuremath{\mu}\mathrm{s}$ determines the Fermi constant ${G}_{F}=1.166\text{ }371(6)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}\text{ }\text{ }{\mathrm{GeV}}^{\ensuremath{-}2}$ (5 ppm). Additionally, the precision measurement of the positive-muon lifetime is needed to determine the nucleon pseudoscalar coupling ${g}_{P}$.