Scientists at Shandong University recently observed a significant enhancement of electron-positron pair (e+e-) production at low transverse momentum in high-energy nuclear collisions. It is the first time scientists are able to observe photon-photon collisions.
High-energy nuclear collisions create Quark-Gluon Plasma (QGP), a new state of matter composed of quarks and gluons and believed to exist in the first few microseconds of the Big Bang. Such experiment provides an ideal laboratory for studying the state of matter dominated by strong interaction. Since heavy nuclei are highly charged, high-energy nuclear collisions also produce the strongest electromagnetic field in nature (1014 Telsa). Quark-Gluon Plasma with trapped strong electromagnetic fields can be used to study many fundamental phenomena, such as chiral magnetic effect, quantum anomalous transport, and magnetohydrodynamics at Fermi-meter scale. These theoretical stipulations have been at the frontier of high-energy nuclear physics in recent years. However, direct evidence of the existence of strong electromagnetic fields in QGP has not yet been found in high-energy nuclear collision experiments.
By comparing data and theoretical model calculation, the research group showed that the significant enhancement comes from photon-photon collisions originating from charged nuclei passing each other at a speed close to the speed of light, and the observed transverse-momentum broadening may stem from deflection of e+e− pair in a strong electromagnetic field trapped in the conducting QGP. This provides a new tool for studying the properties of QGP and magnetohydrodynamics under extreme condition. The research was published on Physics Review Letters 121 (2018) 132301https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.121.132301.
In 1924, Fermi pointed out that the electromagnetic field generated by high-speed charges is very similar to the electromagnetic field of photons, and is therefore equivalent to a large number of quasi-real photons moving in the direction of charge motion. Photons do not carry charge and usually do not interact among themselves. However, quantum mechanics allows photon interactions through vacuum fluctuation. These quasi-real photons are produced copiously in high-energy nuclear collisions at sufficiently short distance that they interact among themselves and produce electron-positron pairs. Professor Zhangbu Xu, associate professor Chi Yang, post-doctor Qian Yang at Shandong University and collaborators analyzed the high-energy Au+Au and U+U collisions collected by the STAR experiment at Brookhaven National Laboratory in US, and measured the yields and momentum spectra of e+e− pairs to identify such rare physics process. For the first time, scientists are able to observe photon-photon collisions, which produce the low transverse-momentum e+e− pairs, accompanying the creation of QGP. The group also proposed that the e+e− pairs generated by this process will deflect in the electromagnetic field in QGP, resulting in a broadening of their transverse momentum spectrum. A simulation of this broadening effect was conducted to evaluate the sensitivity to the magnetic field. It shows that a magnetic field of the order of 1014 T is required to produce enough momentum broadening to match the experimental data. This significantly exceeds the strongest man-made magnetic field (about 100 T), and is comparable to or even higher than the magnetic field of the magnetar (107 to 1014 T). The finding provides a direct experimental tool for the study of strong electromagnetic fields in QGP, an important step for discovering many magnetohydrodynamic effects in QGP. The simulation results and experimental data are published in Physical Review Letters.
Shandong University joined the STAR International Collaboration in 2008 and is the only group with a research program on spin physics in STAR outside USA. In the past two years, the young scientists from the SDU team have played important roles in this scientific collaboration. The international collaboration has five physics working groups, each specialized in different areas of research. Associate Research Professor Yang Chi, an expert on dilepton experiment, is the convener of the dilepton related physics group. Professor Yi Li is the convener of the jet physics group. Detectors are essential for experimental physics research. Shandong University has played an important role in detector construction as well. Prof. Qinghua Xu is the lead investigator in carrying out the most crucial upgrade project for the collaboration: the inner sector of TPC (iTPC) upgrade project of STAR detector. It is a joint project by Shandong University, Shanghai Institute of Applied Physics of Chinese Academy of Science and University of Science and Technology of China. At the same time, the STAR group at Shandong University is also undertaking the research and development of a STAR tracking detector at forward angle using Silicon and sTGC technologies. The upgrades will greatly improve the precision and acceptance of related physical measurements.
The STAR Collaboration consists of 66 institutions from 14 countries and a total of over 600 collaborators. Eight institutions from Mainland China are members of this international collaboration: Fudan University, Huzhou University, Central China Normal University, Tsinghua University, Shandong University, University of Science and Technology of China, Institute of Modern Physics and Shanghai Institute of Applied Physics of Chinese Academy of Science. The research of STAR Group in Shandong University is supported by the National Natural Science Foundation of China, the Ministry of Science and Technology, and Shandong University.
Written by: Ding Xiaoyang
Edited by: Xie Tingting