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。第一个方面是在低能核子相互作用中实现强相互作用的基本对称性。第二个主题是费米耦合的确定，费米耦合是电弱理论中的一个基本常数。在量子色动力学中编码了各种对称性。一般来说，这些内部对称性产生了限制π介子和核子相互作用的守恒电流。我们的实验解决了低能过程中手征对称性和同位旋对称性的特征。手征对称性和disospin对称性的表现形式丰富而复杂，涉及自发对称性破缺和显式对称性破缺。此外，它们的实现与强子质量的起源、夸克质量的影响以及π介子的特殊性质有着深刻的联系。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理解中的两个难题，即手征对称性和同位旋对称性如何在低能的介子和核子相互作用中实现。电弱理论是电磁相互作用和弱相互作用的基本理论。该理论基于两种局域对称性，产生两种规范力。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无质量W，Z玻色子，以及介导电磁相互作用的无质量光子。相互作用由三个参数决定，这三个参数本身由精细结构常数，Z玻色子质量和费米耦合常数的测量确定。我们正在进行一个新的测量的正μ子寿命，以获得一个改进的测定费米耦合常数。费米耦合常数是所有电弱计算中的输入参数，它与其他物理量结合起来，既是标准模型的重要检验，也是物理学的关键约束。这项实验研究是在加拿大温哥华的TRIUMF实验室和苏黎世附近的Paul Scherrer研究所用π介子和μ介子束进行的，瑞士：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适应性和良好的培训。

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.