Institute of Nuclear Physics PAS: a new model for the expansion of quantum-rotating plasma

The Institute of Nuclear Physics of the Polish Academy of Sciences (IFJ PAN) in Kraków informed about the research in a press release sent to PAP.

Each proton and each neutron is composed of several quarks bound by strong interactions carried by intermediary particles (gluons). When heavy ions built of protons and neutrons, accelerated to velocites very close to the velocity of light, collide with each other, they are usually destroyed and transform into an exotic fluid: quark-gluon plasma.

Due to its negligible viscosity, this plasma is considered to be the "most perfect fluid in the Universe". But new experimental measurements suggest that the particles leaving the plasma exhibit nontrivial arrangement of their spin directions. In order to explain these results, a group of scientists from the Institute of Nuclear Physics PAS and the GSI Helmholtz Centre for Heavy Ion Research in Darmstadt (Germany) has presented a new model of relativistic flows of quark-gluon plasma, taking into account the phenomena arising from the quantum spin of the particles forming it.

The article describing the new model was published in the journal Physical Review C (DOI: https://doi.org/10.1103/PhysRevC.97.041901)

For tens of microseconds after the Big Bang, quark-gluon plasma filled the entire Universe. But it cooled down rapidly and gluons stuck the quarks together into groups - the particles of which our world is built. As a result, today the quark-gluon fluid can only be seen as the effect of high-energy collisions of heavy ions. Collisions of this type are currently being carried out in just a few accelerator centres in the world.

The flow of fluids and gases is the subject of hydrodynamics, a field that has been developed for centuries. Following the theory of relativity, classical hydrodynamics was expanded with relativistic phenomena, occurring when fluid flows at velocities close to the velocity of light. After the birth of quantum theory, hydrodynamics was expanded with descriptions of the flow of particles with spin.

Spin is a feature of elementary particles associated with the properties of their wave functions relative to rotation. It can only take on discrete values, e.g. 0, 1/2, 1, 3/2, etc. The direction of spin of particles with spin 1/2 can be equal to +1/2 or -1/2 with respect to any axis. The non-zero polarization of particles with spin 1/2 means that the produced particles are more likely to have one spin direction (+1/2 or -1/2).

"Hydrodynamics is an excellent tool for describing many physical phenomena. We have broadened its scope of applicability" - explains Prof. Wojciech Florkowski (Institute of Nuclear Physics PAS), who has developed a new flow model in collaboration with the group of Prof. Bengt Friman (GSI). "We are the first to present a coherent description of relativistic particle flows with spin 1/2" - he adds.

Work on the model of relativistic flows with spin was inspired by recent measurements of the polarization of spins of particles known as Lambda hyperons (these are conglomerates of three quarks: up, down and strange, with a total spin of 1/2), recorded in heavy-ion collisions. Physicists have long been performing experiments to better understand the polarization of Lambda hyperons. The measurements, however, were subject to considerable uncertainty. Only recently the experiments carried out at the Brookhaven National Laboratory on Long Island near New York showed that the spins of the Lambda hyperons formed in collisions of heavy nuclei are indeed polarized.

It has been known for a long time that the spin of a quantum object contributes to its total momentum. For example, in ferromagnetic materials, the Einstein-de Haas effect can be observed: when a non-polarized system is placed in a magnetic field, the spin of the particles it is composed of starts to orientate according to the magnetic field. This means that in order to maintain the total angular momentum, the system must begin to rotate. Observation of the polarization of the Lambda hyperons formed as a result of quark-gluon plasma transformations thus indicates the role of spin in shaping the flow of this plasma that`s difficult to ignore.

The model presented by the group of physicists from the Institute of Nuclear Physics PAS and GSI is a generalization of the hydrodynamics of perfect fluid. Since spin exists in the described systems, the principle of angular-momentum conservation should have been included in the theoretical description.

"Just like temperature is associated with the principle of conservation of energy, velocity with the principle of conservation of momentum, and electric potential with the principle of conservation of charge current, so in the systems we describe spin polarization is associated with the principle of conservation of momentum" - explains Prof. Florkowski. "When you take this principle into account, you get additional equations that better describe the evolution of the system".

Quark-gluon plasma is such an exotic state of matter that its technological applications will remain out of the question for decades or even hundreds of years. However, according to the Institute of Nuclear Physics PAS, these studies have important implications today. Relativistic flows of particles with spin are in fact a new window to the world of strong interactions, which, among other things, bind quarks in protons and neutrons. Thus, strong interactions play a very important role in the Universe, but they are extremely complicated to describe. Researchers hope that it will be possible better understand these effects in relativistic flows with spin.

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