First Data from the TWIST Experiment

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

The measured distribution of decay positrons  is predicted by the Standard Model, and can 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 will be presented, accompanied by discussion of systematic uncertainties.