Measurement of the Positive Muon Lifetime and Determination of the Fermi Constant to Part-per-Million Precision

TL;DR

This paper reports an ultra-precise measurement of the positive muon lifetime, significantly improving the accuracy of the Fermi constant and providing insights into weak interactions involving protons.

Contribution

The study achieves the most precise muon lifetime measurement to date, refining the value of the Fermi constant and extracting the muon-proton capture rate.

Findings

Muon lifetime measured to 1.0 ppm precision

Fermi constant determined with 0.6 ppm accuracy

Muon-proton capture rate extracted

Abstract

We report a measurement of the positive muon lifetime to a precision of 1.0 parts per million (ppm); it is the most precise particle lifetime ever measured. The experiment used a time-structured, low-energy muon beam and a segmented plastic scintillator array to record more than 2 x 10^{12} decays. Two different stopping target configurations were employed in independent data-taking periods. The combined results give tau_{mu^+}(MuLan) = 2196980.3(2.2) ps, more than 15 times as precise as any previous experiment. 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.