W. Fetscher

Muon Decay: Complete Determination of the Interaction and Comparison with the Standard Model

Abstract Present experiments determine the most general (local, derivative-free, lepton-number conserving) leptonic four-fermion interaction hamiltonian of the normal and inverse muon decay. Numerical results are given for all ten complex coupling constants and nine T -violating amplitudes with respect to the “helicity projection form”.

Muon Decay: Measurement of the Transverse Positron Polarization and General Analysis

Abstract The components of the transverse polarization of the positrons in the decay μ + → e + ν ν have been measured as a function of the positron total energy. Their energy average is〈PT1〉=0.016±0.023, 〈PT2〉=0.007±0.023 (statistical plus systematic errors). The energy dependence of PT1 and PT2 yields the μ decay parameters α A , β A (with their linear combination η= (α−2β) A ) and α′ A , β′ A , respectively. In an analysis based only on the most general, local, derivative-free four-fermion point interaction, and including all corresponding measurements, the complete set of parameters describing muon decay is determined for the first time.

An apparatus for the measurement of the transverse polarization of positrons from the decay of polarized muons

Abstract An apparatus for the measurement of the two transverse polarization components P T 1 and P T 2 of the positrons from polarized μ + decay is described. It makes use of a stroboscopic method which cancels many systematic asymmetries and allows one to use muon stop rates in the order of up to 10 8 s −1 . The experiment aims at an accuracy of Δ P T 1 =Δ P T 2 =3×10 −3 and by that at an improvement of the existing limit by nearly one order of magnitude. With these results one will obtain an improved model-independent value for the Fermi coupling constant G F and more stringent limits to possible violations of time reversal invariance in a purely leptonic reaction.

Does the positron from muon decay have transverse polarization

Abstract The transverse polarization of the e+ in μ+ decay has been measured for the first time. This measurement allows the initial determination of the four μ decay parameters: α A and β A [with their linear combination η = (α − 2β) A ] plus, as the first direct measurement of time reversal noninvariant parameters in a purely leptonic interaction, α′ A and β′ A . The results are consistent with zero and in agreement with the standard V - A form of the charged leptonic weak interaction.

Muon decay: Measurement of the positron polarization and implications for the spectrum shape parameter η, V-A and T-invariance

Abstract A new measurement of the longitudinal polarization PL of the e+ from μ+ decay is presented here. Combined with our previously published result, we obtain PL = 0.998 ± 0.045 (statistical plus systematic error). We also report on the first measurement of PL(θ) where θ is the angle between the e+ line of flight and the μ+ polarization. The implications on the “low energy spectrum shape parameter η″, on the ratio of A coupling to V coupling and on time-reversal are discussed. The results are in agreement with the standard V-A form of the charged leptonic weak interaction.

Measurement of the polarization vector of the e+ from the decay of polarized μ+ as a test of time reversal invariance

The complete polarization vector of the positrons from the decay of polarized muons has been measured for the first time with one experimental setup. The μPT experiment at the Paul Scherrer Institute determines the three polarization components simultaneously with the same apparatus by making use of three different effects (spatial and temporal dependence of annihilation-in-flight with polarized electrons as well as muon decay asymmetry). The use of a stroboscopic method greatly reduces systematic errors. The energy dependence of the transverse polarization component PT1, which lies in the plane spanned by muon spin and positron momentum, yields the low energy parameter η and thus an improved model-independent value of the Fermi coupling constant. A non-zero value of the transverse component PT2, which is perpendicular to the above-mentioned plane, would be the first observation of time reversal violation in a purely leptonic decay. The preliminary results are PT1 = (5 +/- 16) × 10−3, PT2 = (1 +/- 16) × 10−3 and PL = (1.09 +/- 0.15).

A tracking detector for low energy electrons

Abstract A multiwire proportional chamber has been developed to study parameters of a detector for tracking of low-energy electrons (0.1–1 MeV). The geometry and materials used have been optimized to minimize the energy loss and the multiple Coulomb scattering. The detector is equipped with an electronic readout and event selection system, consisting of amplifier/comparator stage, a fast trigger unit and a data multiplexer. The fast trigger system identifies the desired event within 30 ns. Based on this prototype, the full-scale detectors (50×100 cm 2 ) will be constructed to determine the electron tracks in the neutron decay experiments.

Measuring the Michel parameter ξ

Unlike the majority of Michel parameters which are consistent with the Standard Model V-A interaction, the experimental value of ξ″(=0.65±0.36) [1] is poorly known. Our experiment will measure the longitudinal polarization, PL, of positrons emitted from the decay of polarized muons. The value of PL, equal to unity in the Standard Model, will decrease for high energy positrons emitted antiparallel to the muon spin if the combination of Michel parameters ξ″/ξξ′ − 1 deviates from the Standard Model value of zero.

Muon decay: is it really non-neutrino physics?

Muon decay, after more than 60 years of experimental and theoretical investigations, has been established as a firm basis for the standard model of electroweak interactions. The Lorentz structure of the decay interaction has been determined completely from existing experiments in a model-independent way. Neutrino experiments have played a decisive role in that analysis. Precision experiments in muon decay continue to be an important and sensitive testing ground for deviations from the standard model.

New facility for fundamental physics with polarized cold neutrons

A new facility for particle physics with polarized cold neutrons has been taken into operation at the spallation source SINQ at PSI. After extraction of the first beam, its intensity and polarization have been measured as a function of the neutron wavelength. The beam characteristics are among the best in the world for studies of neutron decay. An experimental area was constructed with infrastructure support for convenient experimentation. The physics program will focus on detailed investigations of the free neutron decay process, in particular fundamental symmetries of the weak interaction. The first approved experiment is a novel search for time reversal violation.