Precise measurement of muon lifetime

Positive muon decays to a positron and two neutrinos almost 100% by week interaction. In muon lifetime measurements, time spectrum of positrons from muon decay were measured after muon stopping in a target, and the lifetime was determined by the slops of time spectra. In order to obtain muon lifetime, a precise experiments with high statistics was required. At the RIKEN-RAL, a new experimental method was developed, and the muon lifetime measurement was performed.

Weak interaction, electromagnetic interaction and muon decay are described by Weak sector of the Standard Model with several parameters to be determined by experiments. One of important parameters is Fermi coupling constant (G_F), which is described by the following equation, and is

determined by muon lifetime.

In the equation, m_e, m_μ, m_w are masses of electron, muon and W-boson. The 2nd, 3rd an 4th terms are precisely calculated QED corrections in the theory, and G_F is therefore determined with the precision of less than 1ppm by precise muon lifetime measurement.

The experiment was conducted at port-2 by using the apparatus shown in the figure. Positive muons were stooped in holmium (Ho) target, and the decay positrons were detected with Multi Wire Proportional Counters (MWPC) with 640 channels.