Improved Measurement of the Positive-Muon Lifetime and Determination of the Fermi Constant

Abstract

The mean life of the positive muon has been measured to a precision of 11 ppm using a low-energy, pulsed muon beam stopped in a ferromagnetic target, which was surrounded by a scintillator detector array. The result, tau(micro)=2.197 013(24) micros, is in excellent agreement with the previous world average. The new world average tau(micro)=2.197 019(21) micros determines the Fermi constant G(F)=1.166 371(6)x10(-5) GeV-2 (5 ppm). Additionally, the precision measurement of the positive-muon lifetime is needed to determine the nucleon pseudoscalar coupling g(P).

Detailed report of the MuLan measurement of the positive muon lifetime and determination of the Fermi constant

“…For the 2006/07 data taking the πE3 beamline was extended through the positron detector with the stopping target mounted in the vacuum pipe. The in-vacuum target reduced the number of upstream muon stops compared to our 2004 commissioning experiment [14]. These stops were worrisome as their µSR signals are not canceled by the opposite detector sums.…”

“…Letters describing our 2004 commissioning measurement and 2006-2007 production measurements were published in Refs. [14,15].…”

Electroweak precision tests of the Standard Model after the discovery of the Higgs boson

“…The FAST collaboration [292] used a finely segmented active target to detect the decay positrons, allowing for a higher beam rate as previous experiment. The MuLan collaboration [293,294] based their measurement on a dedicated pulsed, 100% longitudinally polarized, 29 MeV muon beam, with 5 µs-long beam-on and subsequent 22 µs beam-off time segments. The beam times and the measured count rate for one cycle is illustrated in Figure 3.40.…”

Beta decays and non-standard interactions in the LHC era

“…is the tree-level SM Fermi constant, and δ β encodes the effect of SM electroweak radiative corrections to semi-leptonic transitions, noting that the Fermi theory QED contributions have been subtracted [39,40,38,41,42,43]. The coupling G (0) F can be expressed in terms of the Fermi constant G µ = 1.166371(6) × 10 −5 GeV −2 precisely measured in muon decay [44]. In order to do so, one has to consider the low-energy effective Lagrangian describing muon decay [45],…”

“…This dependence arises because correlation measurements involve the construction of asymmetry ratios [132], and the dependence on b does not cancel in the asymmetry denominators. For example, in order to isolate A(E e ) one constructs the ratio 44) where N ± (E e ) are the spectra corresponding to events with J · p e > 0 and J · p e < 0, respectively, so that sensitivity to b does indeed appear through the denominator. In general, asymmetry measurements probeỸ where the ellipsis denotes other possible corrections of radiative and recoil order whose appearence depend on the correlation considered.…”