Glen Marshall
TRIUMF Laboratory

First Data from the TWIST Experiment

TWIST is an experiment to measure the parameters describing the energy and 
angular dependence of positrons (e+) emitted from the decay of polarized 
positive muons (mu+). It uses a beam of highly polarized surface muons 
guided axially into a spectrometer consisting of a large, uniform 2-T 
solenoid magnetic field and a precisely constructed, low-mass array of planar 
drift chambers. 

The measured distribution of decay e+ is predicted by the Standard Modael, and 
can be parametrized by four quantities, rho, eta, delta, and xi, known as 
Michel parameters. The eventual goal of TWIST is to provide a stringent test 
of the Standard Model by reducing the uncertainties of the Michel parameter 
measurements by up to an order of magnitude. The first data have been 
obtained, and analysis is underway to extract values of rho and delta to a 
precision better than that currently available.

The experiment will be described, and preliminary data presented, accompanied 
by descriptions of systematic uncertainties.