Physics Background of TWIST

Muons are unstable particles which are fundamental elements of the Standard Model of subatomic particles and interactions. Discovered in 1937, muons were the first of the inexplicable particles which didn't seem to fit in. Today, they continue to present us with puzzles. For example, why do they behave so much like electrons, while having a mass which is so different? Why are they part of a family of three such particles - rather than a family of two or of four?

The Standard Model of subatomic particles and interactions purports to provide an exact description of the muon decay spectrum. This is the same model which describes the interactions and indeed the existence of the quarks, and which has been incredibly successful in describing a wide variety of subatomic phenomena. In spite of this success, it is widely believed that the theory is only approximately correct. In much the same way that Newtonian mechanics - which served very well for 300 years - turned out to be only an approximation to Einstein's more precise relativistic theory, the Standard Model of particles and interactions is expected to be flawed in some important respects. Indeed virtual armies of scientists have been deployed in an attempt to discover phenomena which reveal the shortcomings of the standard theory.

Muon decay is a particularly interesting phenomenon. In particular, the quarks are not involved in this decay. Because of this, the "strong" interaction does not play a role, which makes possible a precise quantum treatment of the decay.

Furthermore, the decay is thought to respect certain symmetries. For example, the neutrinos which are emitted are expected to have an angular momentum (that is, a spin direction - analogous to the clockwise or counter-clockwise spin of a top) which is always directed opposite to the linear momentum. This "feature" of the Standard Model has been incorporated because it describs things very well. There is no a priori reason to expect that neutrinos violating this symmetry principle cannot exist. If they do exist, it would be taken as evidence of an additional component of the weak interaction, mediated by a new particle which would be responsible for "right-handed" weak interactions.

It is likely that a careful examination of the muon decay spectrum would be the best test for the existence of such right-handed interactions. It is remarkable that to date, a precise measurement of the entire decay distribution has not been undertaken.

In TWIST, a group of physicists from Canada, the United States, and Russia have proposed to make a precise measurement of the muon decay distribution for the purpose of determining the electroweak coupling constants which determine the underlying symmetries. Deviations from the expected decay distribution at the level of a few parts in 10,000 could reveal flaws in the Standard Model - such as yet undetected right-handed interactions.

The scientific impact of such a discovery would be enormous, competing in impact with the biggest of the collider experiments. While this would normally be weighed against the odds against finding a hidden flaw in the Standard Model, in this case - because the right-handed interactions have been excluded only by fiat - there is no reason to bet against TWIST making a dramatic discovery.

You may wish to visit "The Particle Adventure", an excellent introduction to particle physics sponsored by the Particle Data Group at Lawrence Berkeley National Laboratory in the United States.

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