摘要：

In high energy collisions of large, heavy nuclei (e.g., Au or Pb), a new state of matter consisting of liberated quarks and gluons is formed at a temperature of a few trillion Kelvins. This "Quark-Gluon Plasma" (QGP), discovered at the Relativistic Heavy Ion Collider (BNL) and the Large Hadron Collider (CERN, Switzerland), is found to exhibit amazing collective behavior as a nearly "perfect" fluid, which flows with close-to-zero viscous dissipation. Unexpectedly, in recent years, similar collective behaviors have also been revealed in small, elementary systems like proton-proton (pp) or proton-nucleus (pA) collisions, which were thought to be too small and dilute to form a QGP fluid. This discovery has triggered wide debates on what is the fundamental scale for a QCD system to exhibit emergent collectivity. After reviewing key findings and state-of-the-art theoretical understandings of the QGP formation in AA, pA and pp at the LHC, I will introduce a new concept for the possible emergence of QCD collectivity in systems as small as those initiated by a single parton propagating in the vacuum, and present new results from the CMS experiment. I will then discuss implications to a series of fundamental questions related to the ultimate partonic "droplet", nature of non-perturbative QCD and the QCD vacuum, as well as future opportunities in addressing these questions at the high luminosity LHC.

报告人简介：

Prof. Wei Li received his Ph.D. in experimental particle and nuclear physics at MIT in 2009. Following a postdoc position at MIT working on the first relativistic heavy ion physics program on the CMS experiment at the LHC, he joined the Rice faculty in 2012. Prof. Li's research interest focuses on understanding emergent behaviors in quantum chromodynamics and nuclear matter under extremes conditions, as well as the development of future particle acceleration and detection technology. Prof. Li has been a leader of the CMS high-energy nuclear program since its inception, serving as co-convener of the group. He has also been leading the time-of-flight detector upgrade project for nuclear physics program at the future high-luminosity LHC as the project director. Among many achievements, his group has pioneered a surprising discovery of quark-gluon plasma in proton-proton and proton-nucleus collisions. Prof. Li received an Early Career Award from the U.S. Department of Energy, a Sloan Research Fellowship and a Presidential Early Career Award for Scientists and Engineers (PECASE) from the White House.