报告人：Professor Fuqiang Wang, Purdue University 

　　报告题目：Cold atoms and hot quark-gluon plasma

　　报告时间：5月15日（星期五）上午10：30

　　报告地点：频标楼三楼会议室

　　摘要：

　　Relativistic heavy ion physics is a field to use high energy nuclear collisions to create and study the hot and dense state of deconfined quarks and gluons, called the quark-gluon plasma (QGP), believed to have existed for a brief time in our early universe after the Big Bang. One would not normally connect the hot QGP, as hot as 1012 Kelvin, to cold atoms, as cold as 10-6 Kelvin. However, they exhibit amazingly similar behavior. For example, cold atoms trapped in a prolate spheroid were observed to expand into an oblate shape in the final state [K. M. O’Hara et al., Science 298, 2179 (2002)]. Similar anisotropy was also observed in final-state particle emission in nuclear collisions [STAR Collaboration, Phys. Rev. Lett. 86, 402 (2001)]. These phenomena suggest a common underlying mechanism—both systems are strongly interacting and expand hydrodynamically, and anisotropically due to the initial geometry, despite of the different nature of their interactions, electromagnetic QED for cold atoms and strong interaction QCD (quantum chromodynamics) for QGP.

　　On the other hand, the systems differ vastly in size: ~10-5m for cold atoms and 10-14m for QGP. The former is orders of magnitude larger than the mean free path and thus its expansion is truly hydrodynamic. The situation is less clear for QGP—recent transport study [L. He et al., arXiv:1502.05572 (2015)] indicates opacity, not hydrodynamic expansion, to be responsible for the final-state anisotropy. In addition, because of the large size, quantum mechanical (uncertainty principle) effects are negligible for the cold atoms system compared to the temperature. For the much smaller QGP, quantum effects are comparable to its temperature. In this sense, the hot QGP is actually “cooler” than cold atoms relative to their respective intrinsic quantum temperatures—the hot QGP is quantum mechanical and the cold atoms are classical! In fact, quantum mechanical calculation [D. Molnar et al., arXiv:1404.4119 (2014)] indicates that the anisotropy due to quantum uncertainty principle is appreciable for QGP.

　　There are many open questions, but unfortunately heavy ion collisions cannot be tuned to test various conditions and hypotheses. Cold atoms, if they can be trapped in much smaller size than before, with tunable interaction strengths, may truly mimic heavy ion collisions and offer a promising venue to help answer these questions. In this talk I will give an overview on relativistic heavy ion physics research, the similarities and differences between QGP and cold atoms, and how we might use the latter to address fundamental questions of the former.