应物理科学与技术学院、理论物理交流平台和萃英学院邀请，德国汉堡电子同步加速器实验室著名粒子物理学家Ahmed Ali教授将做客“格致·创新”论坛第九期。欢迎广大师生届时参加！

题目：Multiquark Hadrons - the Next Frontier of QCD

主讲人：Ahmed Ali, DESY, Hamburg, Germany

世间：2018年4月17日 (星期二) 10:00

地点：格致楼3016报告厅

摘要：

For many decades after the invention of the quark model in 1964 there was no evidence that hadrons are formed from anything other than the simplest pairings of quarks and antiquarks, mesons being formed of a quark-antiquark pair and baryons from three quarks. In the last decade, however, in experiments from both $e^+e^-$ and hadron colliders, many observed states do not fit into this picture. These new particles are called generically “exotics”, and represent the next frontier of QCD. They can be either mesons or baryons. Remarkably, they all decay into at least one meson formed of either a $coverline{c}$ or $boverline{b}$ pair.  In this talk, some of these new discoveries are reviewed, first from an experimental point of view, then subsequently various theoretical scenarios are discussed. These exotics can be explained if the new mesons contain two-quarks and two-antiquarks (tetraquarks), while the baryons contain four-quarks plus an antiquark (pentaquarks). The theoretical explanations for these states take three divergent tracks: tightly bound objects, just as in the case of normal hadrons, but with more constituents, or loosely bound “molecules” similar to the deuteron, but formed from two mesons, or a meson or baryon, or more wistfully, they are not multiquark states but appear due to kinematic effects caused by different rescatterings of virtual particles.  Both the tightly and loosely bound models predict the masses and related quantum numbers of new, as yet undiscovered states. Thus, future experimental discoveries are needed along with theoretical advances to elucidate the structure of these new exotic states. Some benchmarks measurements for the future will be discussed.

主讲人简介：

Ahmed Ali 是德国汉堡电子同步加速器实验室（DESY）的著名粒子物理学家。他对高能物理学的研究超过四十年，对该领域产生了重要影响，对强子喷注物理、多夸克态物理及包含粲、底、顶夸克的重味物理方面的贡献尤为突出。他70年代用量子色动力学（QCD）研究正负电子湮灭产生强子喷注现象的工作，对成型阶段的微扰QCD理论的检验起到重要作用。在重味物理领域，他的研究工作促使欧洲核子中心发现B介子的粒子与反粒子混合效应，以及CLEO实验发现B介子辐射衰变过程中电磁企鹅图的贡献。最近几年他主要研究多夸克粒子的性质，例如四夸克态和五夸克态。到目前为止，他发表的文章被引用次数达到13500，h因子为58。