Detailed Report of the MuLan 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 using advanced experimental setups, resulting in the most accurate determination of the Fermi constant, significantly improving previous measurements and enabling new weak interaction studies.

Contribution

It provides the most precise muon lifetime measurement and Fermi constant determination, utilizing innovative experimental techniques and analysis methods.

Findings

Muon lifetime measured as 2196980.3(2.2) ps with 1.0 ppm precision

Fermi constant determined as 1.1663787(6) x 10^{-5} GeV^{-2} with 0.5 ppm accuracy

Measurement enables new precision studies of weak interactions

Abstract

We present a detailed report of the method, setup, analysis and results of a precision measurement of the positive muon lifetime. The experiment was conducted at the Paul Scherrer Institute using a time-structured, nearly 100%-polarized, surface muon beam and a segmented, fast-timing, plastic scintillator array. The measurement employed two target arrangements; a magnetized ferromagnetic target with a ~4 kG internal magnetic field and a crystal quartz target in a 130 G external magnetic field. Approximately 1.6 x 10^{12} positrons were accumulated and together the data yield a muon lifetime of tau_{mu}(MuLan) = 2196980.3(2.2) ps (1.0 ppm), thirty times more precise than previous generations of lifetime experiments. The lifetime measurement yields the most accurate value of the Fermi constant G_F (MuLan) = 1.1663787(6) x 10^{-5} GeV^{-2} (0.5 ppm). It also enables new precision studies of weak interactions via lifetime measurements of muonic atoms.