Grant: Subatomic Physics Envelope - Project

The TRIUMF Weak Interaction Symmetry Test (TWIST) is a search for tiny deviations from the pattern of muon decays predicted by the very successful Standard Model of particle interactions. The Standard Model, a theory which agrees with many observations over many years, is nonetheless believed by most subatomic physicists to be only an approximation to a more basic and precise model whose properties are as yet not known. The Standard Model is incomplete and leaves several questions unanswered, for example, why just three 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 the first or lowest generation make up the materials of our everyday lives. The muon is the lightest and most accessible fundamental charged particle of a higher generation. It nearly always decays into an electron and two very 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 the direction and energy of the positive electron (or positron) produced for each of billions of positive decays. The pattern 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 a more 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.