On August 3, the journalNaturepublished a cover article on the latest research results from STAR international Collaboration at Brookhaven National Laboratory. In this study, a fast swirling effect (coined "global polarization") of the quark gluon plasma (QGP) was observed for the first time in the ultra-relativistic heavy ion collision. The effect was discovered by a measurement of the spin polarization of the Lambda hyperon with respect to the interaction plane. QGP "global polarization" theory was first proposed by Professor Liang Zuotang at Shandong University.

The theory, published in thejournal Physical Review Letters(Phys. Rev. Lett. 94, 102301), initiated a new research direction by adding the spin degree of freedom into the equations and observables in addition to the regular space, time and momentum. The discovery by STAR confirmed that theoretical prediction.

The relativistic heavy ion collider (RHIC) of the Brookhaven National Laboratory found on Long Island, New York in the east coast of the United States, accelerates two gold nuclei close to the speed of light in opposite directions to create collisions that simulate first few microseconds of the Big Bang. The produced material with extreme density and temperature, similar to those in the early universe, is known as the quark gluon plasma (QGP) and has been confirmed by high energy heavy ion collision experiments. This scientific research program is currently at the frontier of the high-energy nuclear physics field.

In 2004, Prof. Liang Zuotang postulated that the initial system involved in the off-center heavy ion collision had a significant orbital angular momentum relative to the collision reaction plane, which would inevitably lead to a series of important observables. This would be an important way for studying the orbital angular momentum and polarization effects of quantum chromodynamics (QCD). They applied the particle scattering method, based on the fundamental strong interaction theory (QCD), to the transfer from momentum gradient to the quark polarization. Moreover, they calculated the polarization of the generated quark system, and demonstrated that these global orbital angular momenta can be transformed into the QGP polarization via interaction. For the first time, the concept "QGP global polarization" was conceived and opened the research of QGP to include studying the spin degree of freedom.

The experimental results of STAR discovered the existence of QGP global polarization, and it is the only experiment that has so far proven that the QGP fluid has an extreme high vorticity. Compared to the previous highest vorticity produced in the helium superfluid at the nanometer scale, the spinning speed of this vorticity is more than a dozen orders of magnitude higher.

The STAR collaboration is composed of more than 500 physicists from 54 institutions in 14 countries. Shandong University became a member of the STAR collaboration in 2008, and has been involved in spin research including the hyperon polarization under the leadership of Professor Xu Qinghua. This group is one of the few experimental groups in RHIC focusing on studying the spin structure of nucleons using the world’s only collider with polarized proton beams. The research group used the large barrel time projection chamber (TPC) to observe and reconstruct Lambda hyperon decay. The correlation between the angular distribution of the decay daughters and the hyperon spin is sensitive to the parton distribution in a proton.

The group led the first publication in 2009 studying Lambda hyperon polarization in proton and proton beam collisions. In addition to Shandong University, Shanghai Institute of Nuclear and Applied Physics (SINAP), University of Science and Technology of China (USTC), Institute of Modern Physics, Tsinghua University and Central China Normal University (CCNU) are also members of the STAR collaboration.

The experimental results show that QGP spin-physics research is a fruitful frontier of heavy ion collision physics, and a large number of scholars have continued to research on different aspects of this subject. Since 2014, the STAR group at Shandong University has led the STAR inner Time Projection Chamber (iTPC) upgrade program, the most important detector upgrade project for RHIC Beam Energy Scan Phase II (BES-II).

The project was partially funded by the key international cooperation projects from the National Natural Science Foundation of China (NSFC) and the 973 Project from Ministry of Science and Technology (MoST). The key component of the iTPC is being produced at the campus of the Shandong University in Jinan. The iTPC upgrade will increase the acceptance of particles streaming from the collisions and improve the accuracy of particle identification. It will play an indispensable role in the study of the QGP properties, including the global hyperon polarization, the QCD phase transition and the search for the QCD critical point.

Source: the School of Physics

Translated by: Yang Chi

Edited by: Xie Tingting