Measurement of the Fermi constant by FAST

An initial measurement of the lifetime of the positive muon to a precision of 16 parts per million (ppm) has been performed with the FAST1 detector at the Paul Scherrer Institute. The result is τμ=2.197083(32)(15)μs, where the first error is statistical and the second is systematic. The muon lifetime determines the Fermi constant, GF=1.166352(9)×10−5GeV−2 (8 ppm).