A Brief History of µSR


Jess H. Brewer

Canadian Inst. for Advanced Research
and Dept. of Physics & Astronomy,
Univ. of British Columbia, Vancouver, BC, Canada V6T 1Z1

This page is under construction at     /intro/musr/history.htm

Origins of µSR

Muons were first discovered in the 1930's and their true nature was learned in the 1940's, when they also found their first use as probes of magnetism in matter [Rasetti, 1944]. However, the story of µSR really begins with a subtle revolution in theoretical physics. In 1956 and 1957, T.D. Lee and C.N. Yang [Lee and Yang, 1956] predicted that any process governed by the weak nuclear interaction might not have a corresponding ``mirror image'' process of equal probability. Before they proposed this resolution of the ``tau-theta puzzle'' (involving the decay modes of electrically neutral strange particles now known as ``kaons''), [Fitch, 1981; Cronin, 1981] it was firmly believed by the physics community that if a reaction were viewed in a mirror, the mirror image was a priori just as likely to occur as the original process - a principle known as parity (${\cal P}$) symmetry.

Although ${\cal P}$ ``violation'' was first observed in kaon decay, credit for its experimental discovery is usually given to C.S. Wu et al., [Wu et al., 1957] who confirmed its existence in the beta-decay of 60Co.

At almost the same time, however, experiments were performed at the Nevis cyclotron by R.L. Garwin, L.M. Lederman and M. Weinrich [Garwin et al., 1957] and at the Chicago cyclotron by J.I. Friedman and V.L. Telegdi [Friedman and Telegdi, 1957] which showed a dramatic effect in the decay of pions to muons and the subsequent decay of muons to electrons and neutrinos. The Nevis experiment was the precursor of modern µSR.

Of these famous measurements confirming the hypothesis of Lee and Yang, [Lee and Yang, 1957], one also suggested that ${\cal P}$-nonconservation in $\pi \to \mu \to e$ decay might furnish a sensitive general-purpose probe of matter. The history of µSR began with that experiment [Garwin et al., 1957], which used an experimental method similar to the most common and familiar of modern µSR techniques: transverse field (TF)-µSR.

1957-1973: The Formative Years of µSR

Were it not for the enthusiasm with which muons were used in history's most rigourous test of QED (quantum electrodynamics), µSR would probably not exist today. These Herculean feats (especially the CERN ``g-2'' experiment, which is now being repeated by Vernon Hughes et al. at even higher precision!) not only produced the basic experimental apparatus and techniques needed to perform the first µSR experiments, but generated the original interest in doing them - in order to explain the subtle environmental effects confounding the fundamental measurements that were the primary objective.

Many people pursued these ``sidelines'' to discover that those annoying complications were in fact interesting areas of study in their own right, often ones for which the muon was either the best or the only available probe.

The Meson Factories

In the early 1970's new high-intensity, intermediate-energy accelerators were built at laboratories in Villigen (just outside Zurich), Switzerland, Los Alamos, NM, USA and Vancouver, BC, Canada. These new ``meson factories'' produced virtually no mesons heavier than the pion, but they produced pions (and therefore muons) in unprecedented numbers - several orders of magnitude more than previous sources - and in doing so, ushered in a new era of exponential growth in the techniques and applications of µSR.
Jess H. Brewer
Last modified: Wed Sep 2 11:06:15 PDT 1998