Ellis John, Fairbairn Malcolm, Tunney Patrick
1Theoretical Particle Physics and Cosmology Group, Physics Department, King's College London, London, WC2R 2LS UK.
2Theoretical Physics Department, CERN, 1211 Geneva 23, Switzerland.
Eur Phys J C Part Fields. 2018;78(3):238. doi: 10.1140/epjc/s10052-018-5725-0. Epub 2018 Mar 20.
We explore the constraints imposed by the cancellation of triangle anomalies on models in which the flavour anomalies reported by LHCb and other experiments are due to an extra U(1) gauge boson Z . We assume universal and rational U(1) charges for the first two generations of left-handed quarks and of right-handed up-type quarks but allow different charges for their third-generation counterparts. If the right-handed charges vanish, cancellation of the triangle anomalies requires all the quark U(1) charges to vanish, if there are either no exotic fermions or there is only one Standard Model singlet dark matter (DM) fermion. There are non-trivial anomaly-free models with more than one such 'dark' fermion, or with a single DM fermion if right-handed up-type quarks have non-zero U(1) charges. In some of the latter models the U(1) couplings of the first- and second-generation quarks all vanish, weakening the LHC constraint, and in some other models the DM particle has purely axial couplings, weakening the direct DM scattering constraint. We also consider models in which anomalies are cancelled via extra vector-like leptons, showing how the prospective LHC Z constraint may be weakened because the branching ratio is suppressed relative to other decay modes.
我们探讨了三角形反常消除对一些模型所施加的限制，在这些模型中，大型强子对撞机底夸克实验（LHCb）及其他实验所报告的味反常是由额外的U(1)规范玻色子Z引起的。我们假设前两代左旋夸克和右旋上型夸克具有通用且合理的U(1)电荷，但允许第三代对应夸克具有不同的电荷。如果右旋电荷为零，当不存在奇异费米子或仅存在一个标准模型单态暗物质（DM）费米子时，三角形反常的消除要求所有夸克的U(1)电荷都为零。存在一些非平凡的无反常模型，其中有不止一个这样的“暗”费米子，或者如果右旋上型夸克具有非零的U(1)电荷，则存在单个DM费米子。在后者的一些模型中，第一代和第二代夸克的U(1)耦合全部消失，从而削弱了大型强子对撞机（LHC）的约束，而在其他一些模型中，DM粒子具有纯轴矢耦合，从而削弱了直接DM散射的约束。我们还考虑了通过额外的类矢量轻子来消除反常的模型，展示了未来LHC对Z的约束可能如何被削弱，因为相对于其他衰变模式，其分支比受到了抑制。
