Experimental Studies of Broken Symmetries with Muons, Pions and Photons

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0072200Gorringe Quantum chromodynamics (QCD) is the accepted theory of thestrong interactions of quarks and gluons. At low energies the quarksand gluons are confined in baryons (three quarks) and mesons(quark--antiquark pairs), and understanding the properties andinteractions of baryons and mesons in terms of QCD is of fundamental importance. QCD possess a number of elementary symmetries. They include isospin symmetry, whereby the interactions are unaltered by u,d quark flavor rotations, and chiral symmetry, whereby the interactions are unaltered by separate flavor rotations for left, right handed quarks. The realization of these symmetries is rich and varied, involving so-called spontaneous, explicit and anomalous symmetry breaking effects. Understanding their realization is central to understanding the low energy interactions of baryons and mesons. This experimental research addresses the manifestation of symmetries in low energy processes. At the TRIUMF Laboratory in Vancouver, Canada we are investigating chiral and isospin symmetries with beams of muons and pions. For example the weak interaction between muons and protons is a key test of chiral symmetry breaking and the strong interaction between pions and protons is central to understanding the breaking of isospin. In addition, at the Jefferson Laboratory in Newport News, Virginia we are collaborating on a precision measurement of the decay rate of the neutral pion - a unique test of anomalous symmetry breaking in QCD.