TWIST - Precision measurement of muon decay parameters (TRIUMF E614)

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The TRIUMF Weak Interaction Symmetry Test (TWIST) is a search for tiny deviations from the pattern ofmuon decays predicted by the very successful Standard Model of particle interactions. The Standard Model, atheory which agrees with many observations over many years, is nonetheless believed by most subatomicphysicists to be only an approximation to a more basic and precise model whose properties are as yet notknown. The Standard Model is incomplete and leaves several questions unanswered, for example, why justthree generations, and why do the particles have the masses they do?The Standard Model classifies the most fundamental particles into three "generations"; only particles of thefirst or lowest generation make up the materials of our everyday lives. The muon is the lightest and mostaccessible fundamental charged particle of a higher generation. It nearly always decays into an electron and twovery light, elusive, neutral particles called neutrinos. High quality beams of muons are produced at TRIUMF,and a very high precision detector system in a high solenoidal magnetic field is used to accurately measure thedirection and energy of the positive electron (or positron) produced for each of billions of positive decays. Thepattern or symmetry of these decays is precisely predicted by the Standard Model.If found, a deviation from expectations based on the Standard Model would provide clues to the character of amore basic description of the smallest particles in our universe. On the other hand, if no deviation is found,some hypothesized features of a more basic description can be eliminated from consideration.