Speaker
Dr
David Webber
(University of Wisconsin-Madison)
Description
on behalf of the MuLan Collaboration
The Fermi constant, G_F, describes the strength of the weak force and
is determined most precisely from the mean life of the positive muon,
tau_mu. Advances in theory have reduced the theoretical uncertainty on
G_F as calculated from tau_mu to a few tenths of a part per million
(ppm). Until recently, the remaining uncertainty on G_F was entirely
experimental and dominated by the uncertainty on tau_mu. 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 x 10^12
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 tau_{mu^+}(MuLan)=2196980.3(2.2) ps. This measurement of the muon
lifetime gives the most precise value for the Fermi constant:
G_F(MuLan) = 1.1663788 (7) x 10^-5 GeV^-2 (0.6 ppm). It is also used
to extract the mu^-p singlet capture rate, which determines the
proton's weak induced pseudoscalar coupling g_P.
Author
Dr
David Webber
(University of Wisconsin-Madison)
