The Precise Measurement of the μ+ Lifetime

The lifetime of the positive muon (τμ+) can be directly associated with the Fermi Coupling Constant (GF), which is one of the most basic parameters of the Standard Model. However, the current experimental accuracy of the τμ+ is ∼30 ppm and it has not been improved for more than 15 years. We propose a new experiment for a pulsed muon facility such as RIKEN-RAL to measure the muon lifetime with multi-decay per one time window method. The advantage of our setup, no time window limitation, enables us to test the exponential decay law (EDL) in the long decay time region at the same time. The preliminary analysis set a new upperlimit for the EDL deviation in the muon decay. We accumulated ∼1010 muon decays and analysis is in progress.