Mulan: a part-per-million measurement of the muon lifetime and determination of the Fermi constant

TL;DR

The MuLan experiment achieved a highly precise measurement of the positive muon lifetime and Fermi constant, significantly improving the accuracy of fundamental constants in particle physics.

Contribution

It introduced innovative experimental techniques to measure muon lifetime with unprecedented precision, refining the values of key physical constants.

Findings

Muon lifetime measured to 1.0 ppm accuracy

Fermi constant determined with 0.5 ppm precision

Enhanced precision supports tests of the Standard Model

Abstract

The part-per-million measurement of the positive muon lifetime and determination of the Fermi constant by the MuLan experiment at the Paul Scherrer Institute is reviewed. The experiment used an innovative, time-structured, surface muon beam and a near-4pi, finely-segmented, plastic scintillator positron detector. Two 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. The experiment obtained a muon lifetime 2 196 980.3(2.2) ps (1.0 ppm) and a Fermi constant 1.166 378 7(6) 10^-5 GeV^-2 (0.5 ppm). The thirty-fold improvement in the muon lifetime has proven valuable for precision measurements in nuclear muon capture and the commensurate improvement in the Fermi constant has proven valuable for precision tests of the standard model.