Speaker
Prof.
David Hertzog
(University of Washington)
Description
The Fermi constant $G_\mathrm{F}$ governs the rates of all weak interaction processes and,
along with the fine structure constant $\alpha$ and the $Z$-boson mass $M_Z$, it is
one of the principal input parameters to the Standard Model. Owing to the purely leptonic
nature of the muon decay process, $G_\mathrm{F}$ is extracted most precisely from
measurements of the muon lifetime $\tau_\mu$. In 1999, the publication of missing radiative
corrections effectively eliminated the largest, purely theoretical uncertainty in extracting
$G_\mathrm{F}$ from $\tau_\mu$. At present, the precision in $G_\mathrm{F}$ is limited by
experimental uncertainty in $\tau_\mu$. We report a measurement of the positive muon lifetime to a
precision of one part-per-million, a better than twenty-fold improvement over the previoius generation
of experiments. The new result will improve precision in $G_F$ to better than 0.8 parts-per-million.
The MuLan experiment was conducted at the Paul Scherrer Institute in Villigen, Switzerland, using a
pulsed surface muon beam, in-vacuum muon-stopping targets, and a large acceptance, finely segmented
scintillator array. We will describe our measurement method and report our final result.
Author
Dr
Vladimir Tishchenko
(University of Kentucky)
