重味物理:通通往新物理之门

Heavy flavour physics:A portal to new physics

作为高能物理的重要分支,重味物理在精确检验标准模型和寻找可能的新物理信号方面发挥着重要作用.近20年来,随着B介子工厂、北京正负电子对撞机和西欧大型强子对撞机的运行,重味物理的研究取得了重大进展.两个B介子工厂发现的B介子系统衰变中的电荷-宇称联合对称性(charge conjugation and parity,CP)破坏,将有助于进一步研究宇宙中正反物质不对称性.中性正反粒子的质量差有助于进一步判定新物理的能标;重味介子非轻衰变的研究使得对量子色动力学以及因子化的研究达到新的高度;更有趣的是,目前重味物理中出现的各种反常,如RK(*)和R(D(*)),可能是新物理存在的迹象.除此之外,重味物理还是研究新强子态的重要场所.近年来在北京谱仪(Beijing spectrometer,BES-III)、B介子工厂以及LHC底夸克侦测器(large hadron collider beauty,LHCb)上发现的众多奇特强子态为研究夸克模型和量子色动力学提供了新的动力.2018年,高精度对撞机Belle-II将开机运行,国际直线对撞机、环形正负电子对撞机、超级Z工厂也在积极推进,它们在重味物理上都具有各自的优势.未来这些高精度、高能量对撞机会使重味物理进入新的黄金时期.

Heavy flavour physics, as one of the most important parts in the standard model （SM）, plays important roles in testing the SM and probing the effects of new physics beyond SM. Thanks to the two B factories, Beijing spectrometer （BES） and large hadron collider beauty （LHCb） experiment, in the past decades, there is great development in heavy flavour physics, in both experimental and theoretical sides. In heavy flavour physics, the first work is to test SM by measuring three angles （α,β, and γ） and sides of Cabbibo-Kobayashi-Maskawa （CKM） triangle. Though the current results are in agreement with SM, the fit becomes less constrained in the presence of new physics, and Ο（20%） contributions of new physics are still allowed. The charge conjugation and parity （CP） asymmetries measured by two B factories helped us to study the matter-antimatter asymmetry in the Universe, and the discovered CP asymmetry in B → K5J/ψ had led to the nobel prize for Kobayashi and Maskawa. In addition, the mass differences between the neutral meson and their own anti-meson can present us the information of the scale of the new physics, because possible new particles might contribute to D0, B0d and Bs0 meson mixing through box diagrams. The nonleptonic decays of B mesons afford us a new plate for studying the quantum chromodynamics （QCD） and factorization concept, as well as searching for the strong phases, which are important factors in CP asymmetries. Importantly, the rare decays induced by the flavour-changing neutral-current has attracted much interest, because the new physics can affect the observables remarkably through penguin diagrams. In addition, it is of interest to note that the recent anomalies, such as RK（＊）, R（D（＊）） and P＇5, might be the hints of new physics beyond SM. In order to explain these anomalies, many models have been proposed, however, all of them should be further tested in the future on both experimental and theoretical sides. Moreover, heavy flavor physics also provides us an important plate for studying the new exotic states. Many new discovered exotic states in BES-III, B factories and LHCb, such as X（3872）, Zc（3900） and Pc（4450, 4380）, motivate us to further study the quark model and QCD. In 2018, Belle-II will take data, where the total luminosity is predicted to be 50ab-1 and 10l~ B meson pair could be produced. The proposed International Linear Collider, Circular Electron-Positron Collider and Hyper-Z Factory, can also produce large number of B mesons, as well as Bs, Be and Ab, when the collision energy is set to be O（rnz）. These high precision or energy machines, together with the current LHCb experiment and its next generation, will lead heavy flavor physics to a new golden era.