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

Abstract

We present a detailed report of the method, setup, analysis and results of a precision measurement of the positive muon lifetime. The experiment was conducted at the Paul Scherrer Institute using a time-structured, nearly 100%-polarized, surface muon beam and a segmented, fast-timing, plastic scintillator array. The measurement employed two target arrangements; a magnetized ferromagnetic target with a ∼4 kG internal magnetic field and a crystal quartz target in a 130 G external magnetic field. Approximately 1.6 × 10 12 positrons were accumulated and together the data yield a muon lifetime of τµ(MuLan) = 2 196 980.3(2.2) ps (1.0 ppm), thirty times more precise than previous generations of lifetime experiments. The lifetime measurement yields the most accurate value of the Fermi constant GF (MuLan) = 1.166 378 7(6) × 10 −5 GeV −2 (0.5 ppm). It also enables new precision studies of weak interactions via lifetime measurements of muonic atoms.

Investigating the near-criticality of the Higgs boson

“…From the measurement τ µ = (2196980.3 ± 2.2) ps [100] we find G µ = 1.1663781(6) 10 −5 / GeV 2 . This is 1σ lower than the value quoted in [100] because we do not follow the convention of including in the definition of G µ itself the last term of (2.27), which is the contribution from dimension-8 SM operators. The computation of ∆r 0 requires the subtraction of the QED corrections by matching the result in the SM with that in the Fermi theory.…”

“…We extract G µ from τ µ via 27) where F (ρ) = 1 − 8ρ + 8ρ 3 − ρ 4 − 12ρ 2 ln ρ = 0.9981295 (for ρ = m 2 e /m 2 µ ) is the phase space factor and ∆q = ∆q (1) +∆q (2) = (−4.234+0.036)×10 −3 are the QED corrections computed at one [102] and two loops [103]. From the measurement τ µ = (2196980.3 ± 2.2) ps [100] we find G µ = 1.1663781(6) 10 −5 / GeV 2 . This is 1σ lower than the value quoted in [100] because we do not follow the convention of including in the definition of G µ itself the last term of (2.27), which is the contribution from dimension-8 SM operators.…”

Direct measurement of αQED(m Z 2 ) at the FCC-ee

“…This method does not rely on the experimental determination of the vacuum polarization ∆α (5) had . Here, the point is not to extrapolate This letter is organized as follows.…”

“…These constraints were obtained under two assumptions: (i) the precision of the pertaining theoretical calculations will match the expected experimental accuracy by the time of the FCC-ee startup; and (ii) the determination of standard-model input parameters -four masses: m Z , m W , m top , m Higgs ; and three coupling constants: α s (m 2 Z ), G F , α QED (m 2 Z ) -will improve in order not to be the limiting factors to the constraining power of the fit. The determinations of the Higgs boson mass from the LHC data [4] and of the Fermi constant from the muon lifetime measurement [5] are already sufficient for this purpose. It is argued in refs.…”

Constraints and consequences of reducing small scale structure via large dark matter-neutrino interactions

“…JHEP04 (2015)170 π → eν [39,40], the ratio of Γ(π → eν)/Γ(π → µν) [41][42][43] and the measurement of the muon lifetime [44]. |U µ4 | 2 limits (dotted, purple) are derived from peak searches in π → µν [45] and K → µν [46,47], the muon lifetime measurement [44], the energy spectrum in muon decay (labeled "TWIST") [48,49], and the unitarity of the measurements of the CKM matrix [50].…”