TWIST- The TRIUMF weak interaction symmetry test the Michel parameters from μ+ decay

Muon decay spectrum: Leading logarithmic approximation

“…The actual size of the effect of radiative corrections on Michel parameters can be derived there in a similar way, starting from the analytical formulas for theoretical predictions and applying specific experimental conditions. The planned accuracy of the TWIST experiment [9,10] is σ exp (ρ) = 1 · 10 −4 , σ exp (η) = 30 · 10 −4 , σ exp (ξP µ ) = 1.3 · 10 −4 , σ exp (δ) = 1.4 · 10 −4 .…”

“…Precise determinations of those observables test the Standard Model and can be used to search for possible "new physics" effects [6]. This motivates ongoing studies of the positron transverse [7] and longitudinal polarization [8] and angular/energy distributions [9,10]. Muon lifetime is also being remeasured [11,12].…”

“…The experiment TWIST at Canada's National Laboratory TRIUMF is designed to measure the positron spectrum from polarized µ + decays with a precision of 10 −4 [9,10]. To match this, and help search for "new physics" effects, the Standard Model prediction must include at least the leading O α 2 effects in the positron distribution.…”

On spin asymmetry in muon and tau decays

“…In particular it is so in the TWIST experiment. 13,14 Obviously, a sufficiently strong magnetic field destroys the structure of any QED jet (consisting either of one electron plus some photons or of several charged particles, like e + e − e − ), and the observed trajectories of all charged particles become separated from each other. In this case the treatment of events with several charged particles suggested in Ref.…”

Virtual and soft pair corrections to polarized muon decay spectrum

“…The experiment T WIST [1,2] is currently running at Canada's National Laboratory TRI-UMF. It is going to measure the muon decay spectrum [3,4] with the accuracy level of about 1 · 10 −4 .…”