MuLan Measurement of the Positive Muon Lifetime and Determination of the Fermi Constant

TL;DR

This paper presents a highly precise measurement of the positive muon lifetime and the Fermi constant using a large dataset from the MuLan experiment, improving fundamental constants in particle physics.

Contribution

The study provides the most accurate measurement to date of the muon lifetime and Fermi constant, utilizing innovative experimental techniques and a large decay sample.

Findings

Muon lifetime measured as 2196980.3(2.2) ps

Fermi constant determined as 1.1663787(6) x 10^{-5} GeV^{-2}

Achieved 1.0 ppm precision in lifetime measurement

Abstract

We report results from the MuLan measurement of the positive muon lifetime. The experiment was conducted at the Paul Scherrer Institute using a time-structured surface muon beam and a segmented plastic scintillator array. Two different in-vacuum muon stopping targets were used: a ferromagnetic foil with a large internal magnetic field and a quartz crystal in a moderate external magnetic field. From a total of 1.6 x 10^{12} decays, we obtained the muon lifetime tau_mu = 2196980.3(2.2) ps (1.0 ppm) and Fermi constant G_F = 1.1663787(6) x 10^{-5} GeV^{-2} (0.5 ppm).