Studies of Broken Symmetries and Fundamental Constants with Muons and Pions

This project is focused on two areas of nuclear physics research. The first area is the realization of the basic symmetries of the strong interaction in the low energy interactions of the nucleon. The second topic is the determination of the Fermi coupling, a fundamental constant in electroweak theory. Encoded in quantum chromodynamics are various symmetries. In general these internal symmetries yield conserved currents that constrain the interactions of pions and nucleons. Our experiments address the signatures of chiral symmetry and isospin symmetry in low energy processes. The manifestation of chiral symmetry and isospin symmetry is rich and complex, and involves spontaneous symmetry breaking and explicit symmetry breaking. Further their realization is deeply connected with the origins of hadron masses, the effects of quark masses, and the special nature of the pion. Our proposed experiments on muon--nucleon interactions and pion-nucleon interactions will address two puzzles in our current understanding of how both chiral symmetry and isospin symmetry are realized in low-energy interactions of pions and nucleons. Electroweak theory is the fundamental theory of electromagnetic interactions and weak interactions. The theory is based on two local symmetries which generate two gauge forces. Through the interaction between these gauge fields and the Higg's field the bosons mix and acquire mass, thus yielding the massive W, Z bosons that mediate weak interactions, and the massless photon the mediates electromagnetic interactions. The interactions are determined by three parameters, which themselves are fixed by measurements of the fine structure constant, the Z boson mass, and the Fermi coupling constant. We are undertaking a new measurement of the positive muon lifetime to obtain an improved determination of the Fermi coupling constant. The Fermi coupling constant is an input parameter in all electroweak calculations, and combined with other quantities it permits both important tests of the standard model and key constraints on the physics beyond. This experimental research is conducted with beams of pions and muons at the TRIUMF Laboratory in Vancouver, Canada, and the Paul Scherrer Institute near Zurich, Switzerland. The relatively small-scale of our experiments offers both broad opportunities and excellent training for undergraduate students, graduate students and post-doctoral researchers.