Seminars 李政道研究所-粒子核物理研究所联合演讲

Spin alignment of vector mesons and strong fields in heavy-ion collisions

by Prof. Qun Wang(王群) (University of Science and Technology of China)





There is an intrinsic connection between rotation and spin polarization since they are related to the conservation of total angular momentum and can be converted from one to another, as demonstrated in the Barnett effect and the Einstein-de Haas effect in materials. The same effects also exist in high energy heavy-ion collisions (HIC) in which the huge orbital angular momentum (OAM) along the direction normal to the reaction plane can be partially converted to the global spin polarization of hadrons. The global spin polarization of $\Lambda$  has been measured through their weak decays in Au+Au collisions. As spin-one particles, vector mesons can also be polarized in heavy ion collisions in the same way as hyperons.  But their spin polarization cannot be directly measured through their strong decays which respect parity symmetry. Instead, the 00-component of the spin density matrix (ρ00, the spin alignment) can be measured through the angular distribution of its decay daughters.  The first measurement by STAR collaboration on non-vanishing ρ00 in heavy ion collisions is considered as another experimental milestone after the measurement of non-vanishing global Lambda polarization. The STAR's data on phi-mesons's ρ00 show a surprisingly large positive deviation from 1/3 which cannot be explained by other conventional mechanisms. We proposed that this puzzle can be resolved by including the fluctuation of strong interaction fields which can be directly detected by ρ00. We believe this is the first time that fluctuations of strong interaction fields are shown to be accessible by an experimental measurement.



Prof. Wang obtained his Bachelor of Science in physics at Shandong University in 1988, and Ph.D in particle and nuclear physics at Shandong University in 1997. From July 1997 to January 2000, he was an associate professor at the Department of Physics, Shandong University. From February 2000 to May 2003, he was a Humboldt Fellow in the Institute for Theoretical Physics, University of Frankfurt, Germany. From June 2003 and June 2005, he was a visiting professor in the GSI Virtual Institute, a joint research program between the Institute of Theoretical Physics of University of Frankfurt and the German National Laboratory for Heavy Ion Research. He was a senior research fellow (visiting professor) at Brookhaven National Lab in 2014. Prof. Wang was awarded the Distinguished Young Scholars in 2012 by National Science Foundation of China (NSFC). Since June 2005, Prof. Wang has been a professor at the Department of Modern Physics, University of Science and Technology of China (USTC).

Prof. Wang's research field is theoretical high energy nuclear physics. His research interest is in application of quantum chromodynamics (QCD) in dense and hot matter, quantum field theory at finite temperature and density, phenomenology of heavy ion collision, non-equilibrium dynamics in strong interaction matter. He is also interested in the theory of low energy nuclear reaction.

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