Studies of broken symmetries and fundamental constants with muons and pions

这个项目集中在核物理研究的两个领域。第一个领域是实现核子低能相互作用中强相互作用的基本对称性。第二个主题是费米耦合的确定，这是电弱理论中的一个基本常数。编码在量子色动力学中的是各种各样的对称性。一般来说，这些内部对称性产生的守恒电流限制了介子和核子的相互作用。我们的实验解决了低能量过程中手性对称和同位旋对称的特征。手性对称和二旋对称的表现形式丰富而复杂，包括自发对称破缺和显对称破缺。此外，它们的实现与强子质量的起源、夸克质量的影响以及介子的特殊性质密切相关。我们提出的介子-核子相互作用和介子-核子相互作用的实验将解决我们目前对如何在介子和核子的低能相互作用中实现手性对称和同位旋对称的理解中的两个难题。电弱理论是研究电磁相互作用和弱相互作用的基本理论。该理论基于产生两种规范力的两种局部对称性。通过这些规范场和希格场之间的相互作用，玻色子混合并获得质量，从而产生介导弱相互作用的大质量w、Z玻色子和介导电磁相互作用的无质量光子。相互作用是由三个参数决定的，这些参数本身是通过测量精细结构常数、Z玻色子质量和费米耦合常数来确定的。我们正在对正介子寿命进行新的测量，以获得费米耦合常数的改进测定。费米耦合常数是所有电弱计算中的一个输入参数，与其他量结合起来，它允许对标准模型进行重要测试，并允许对物理的关键限制。这项实验研究是在加拿大温哥华的TRIUMF实验室和瑞士苏黎世附近的Paul Scherrer研究所用介子和介子光束进行的。我们的实验规模相对较小，为本科生、研究生和博士后研究人员提供了广泛的机会和良好的训练。

This project is focused on two areas of nuclear physicsresearch. The first area is the realization of the basicsymmetries of the strong interaction in the low energyinteractions of the nucleon. The second topic is thedetermination of the Fermi coupling, a fundamentalconstant in electroweak theory.Encoded in quantum chromodynamics are various symmetries.In general these internal symmetries yield conserved currentsthat constrain the interactions of pions and nucleons.Our experiments address the signatures of chiralsymmetry and isospin symmetry in low energyprocesses. The manifestation of chiral symmetry andisospin symmetry is rich and complex, and involvesspontaneous symmetry breaking and explicit symmetrybreaking. Further their realization is deeply connected withthe origins of hadron masses, the effects of quark masses,and the special nature of the pion. Our proposed experimentson muon--nucleon interactions and pion-nucleon interactionswill address two puzzles in our current understandingof how both chiral symmetry and isospin symmetry are realizedin low-energy interactions of pions and nucleons.Electroweak theory is the fundamental theoryof electromagnetic interactions and weak interactions.The theory is based on two local symmetries whichgenerate two gauge forces. Through the interactionbetween these gauge fields and the Higg's field thebosons mix and acquire mass, thus yielding the massiveW, Z bosons that mediate weak interactions, and themassless photon the mediates electromagnetic interactions.The interactions are determined by three parameters,which themselves are fixed by measurements of the finestructure constant, the Z boson mass, and the Fermi couplingconstant. We are undertaking a new measurement of thepositive muon lifetime to obtain an improved determinationof the Fermi coupling constant. The Fermi coupling constantis an input parameter in all electroweak calculations,and combined with other quantities it permitsboth important tests of the standard model and key constraintson the physics beyond.This experimental research is conducted with beamsof 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 broadopportunities and excellent training for undergraduate students,graduate students and post-doctoral researchers.