A new part-per-million measurement of the positive muon lifetime and determination of the Fermi Constant

TL;DR

This paper reports a highly precise measurement of the positive muon lifetime at 1.0 ppm, significantly improving the accuracy of the Fermi Constant, which is fundamental for understanding the weak force.

Contribution

The study presents the most precise muon lifetime measurement to date, reducing experimental uncertainty and refining the value of the Fermi Constant with advanced experimental techniques.

Findings

Muon lifetime measured as 2196980.3(2.2) ps

Fermi Constant determined as 1.1663788(7) × 10^{-5} GeV^{-2}

First measurement to achieve 0.6 ppm precision

Abstract

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 times 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) \times 10^{-5} {GeV}^{-2} (0.6 ppm). The lifetime is also used to extract the mu^-p singlet capture rate, which determines the proton's weak induced pseudoscalar coupling g_P.