Volume 31 Issue 10 - November 15, 2019 PDF
New discovery in heavy-ion collisions: the fastest fluid in the world
STAR Collaboration
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There are many international collaborations combining the knowledge from physicists and engineers to explore our understanding together on the fundamental interactions in Nature including electromagnetic, weak, strong and gravity. The Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory (BNL) in US is the most important research facility on studying the strong interaction, aka Quantum Chromodynamics (QCD), in the world, and STAR (Figure 1) is one of the most important detectors at RHIC. In the heavy-ion collision experiments, the gold nuclei will be accelerated to close to the speed-of-light and collide to each other to create a hot and dense environment. A new form of matter – Quark-Gluon Plasma (QGP) then will be created. We can study the properties of QGP and QCD via the particles which traveled through this mysterious environment collected by the STAR detector. 
Figure 1: The schematic of the STAR detector. The gold nuclei will be accelerated to close to the speed of light and collide in the center of the detector. The produced particles will be collected by the detector system [1].
The polarization of Λ particle which is determined by its spin and orbital angular momentum will provide valuable information on the QGP as demonstrated in Fig. 2. In the low Au-Au collision energy, we surprisingly observed a clear non-zero Λ polarization using the proton and pion decay channel as shown in Fig. 3. Then, the vorticity of QGP can be estimated from the result and it is higher than all other known fluids by many orders of magnitude. This study opened a new door for studying the behavior of QGP and understanding the fundamental properties of QCD. With collecting more experimental data, we will be able to provide more useful information on the Λ global polarization in the future [1].
Figure 2: The global polarization of quark-gluon plasma created via Au-Au collisions can be obtained by measuring the polarization of Λ particle [1].

Figure 3: The averaged polarization of Λ particle in different Au-Au collision energies measured by STAR and there is an obvious non-zero polarization in the low collision energy region [1].
National Cheng Kung University officially joined the STAR collaboration in 2015 and is heavily participating many important detector R&D projects and physics analyses. In the on-going “Forward Detector Upgrade in STAR” project, we are also playing an important role in the silicon tracker including the R&D on the sensors and supporting structure. The Forward Detector System will start taking data in 2022 and will provide us more information on understanding of the global polarization and QGP.

  1. The STAR Collaboration, “Global Λ hyperon polarization in nuclear collisions” Nature, 548, 62, 2017
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