Scientists present new observations to understand the phase transition in quantum chromodynamics
New findings published in ”Nature“ on the formation of matter / Experiments provide information on the beginnings of the universe
The building blocks of matter in our universe were formed in the first 10 microseconds of its existence, according to the currently accepted scientific picture. After the Big Bang about 13.7 billion years ago, matter consisted mainly of quarks and gluons, two types of elementary particles whose interactions are governed by quantum chromodynamics (QCD), the theory of strong interaction. In the early universe, these particles moved (nearly) freely in a quark-gluon plasma. Then, in a phase transition, they combined and formed hadrons, among them the building blocks of atomic nuclei, protons and neutrons. In the current issue of the science journal ‘Nature’, an international team of scientists presents an analysis of a series of experiments at major particle accelerators which sheds light on the nature of this transition. The scientists determined with precision the transition temperature and obtained new insights into the mechanism of cooling and freeze-out of the quark-gluon plasma into the current constituents of matter such as protons, neutrons, and atomic nuclei. The team of researchers consists of scientists from the GSI Helmholtzzentrum für Schwerionenforschung in Darmstadt, and from the universities of Heidelberg, Münster, and Wroclaw (Poland).
Analysis of experimental results confirm the predicted value of the transition temperature / Onehundred and twenty thousend times hotter than the interior of the sun
A central result: The experiments at world-wide highest energy with the ALICE detector at the Large Hadron Collider (LHC) at the research center CERN produce matter where particles and anti-particles coexist, with very high accuracy, in equal amounts, similar to the conditions in the early universe. The team confirms, with analysis of the experimental data, theoretical predictions that the phase transition between quark-gluon plasma and hadronic matter takes place at the temperature of 156 MeV. This temperature is 120,000 times higher than that in the interior of the sun.
„Snowballs in hell“
The physicists analyzed more precisely the yields of a number of particles and anti-particles. „Our investigations revealed a number of surprizing discoveries. One of them is that light nuclei and their anti-particles are produced at the same temperature as protons and anti-protons, although their binding energies are about 100 times smaller than the energy corresponding to the transition temperature“, explains Prof. Dr. A nton Andronic who recently joined the University of Münster from the GSI Helmholtzzentrum für Schwerionenforschung. The scientists presume that such „loosely bound objects“ are formed at high temperature first as compact multi-quark objects which only later develop into the observed light nuclei and anti-nuclei. The existence of such multi-quark states was proposed a long time ago but no convincing evidence was found.
“Confinement”: Charm quarks travel freely in the fireball
Another remarkable observation concerns a phenomenon long known but poorly understood: Normally, quarks are confined into the interior of protons and neutrons; isolated quarks have never been observed, a property which scientists describe as „confinement“. In the interior of the fireball formed in nuclear collisions at high energy this confinement is lifted (deconfinement). The new study shows that charmonium states such as J/psi mesons, consisting of a pair of charm and anti-charm quarks, are produced far more often at LHC energies compared to observations at lower energies, such as at the „Relativistic Heavy Ion Collider“ in the USA. Because of the higher energy density at LHC the opposite, namely a reduction of J/psi mesons through dissociation was expected. In contradistinction, enhancement was predicted 18 years ago by two of the team members (Prof. Dr. Peter Braun-Munzinger, GSI, and Prof. Dr. Johanna Stachel, Universität Heidelberg) because of deconfinement of the charm quarks. The consequences of the prediction were worked out in detail in a series of publications by the whole team. The now observed enhanced production of J/psi particles confirms the prediction: J/psi mesons can only be produced in the observed large quantities if their constituents, the charm- and anticharm quarks, can travel freely in the fireball over distances of a trillionth of a centimeter – corresponding to about ten times the size of a proton. „These observations are a first step towards understanding the phenomenon of confinement in more detail“, underlines Prof. Dr. Krzysztof Redlich of the University of Wroclaw (Poland).
Experiments at CERN and at Brookhaven National Laboratory
The data were obtained during several years of investigations in the framework of the experiment „ALICE“ at the Large Hadron Collider accelerator at the research center CERN near Geneva. In „ALICE“, scientists from 41 countries investigated in collisions between two lead nuclei the state of the universe within microseconds after the Big Bang. The highest ever man-made energy densities are produced in such collisions. These result in the formation of matter (quarks and gluons) as it existed at that time in the early universe. In each head-on collision more than 30,000 particles (hadrons) are produced which are then detected in the ALICE experiment. The actual study also used data from experiments at lower energy accelerators, the „Super Proton Synchrotron“ at CERN and the „Relativistic Heavy Ion Collider“ at the US-Brookhaven National Laboratory on Long Island, New York.
The investigations were supported in the framework of the ‘Collaborative Research Center’ 1225 „Isolated quantum systems and universality under extreme conditions (ISOQUANT)“ by the German Research Foundation (DFG). Furthermore, they were supported by the Polish National Science Center (NCN) (Maestro grant DEC-2013/10/A/ST2/00106).
Elementary particles and heavy ions physics at University of Wroclaw
The research is carried out in the Department of Elementary Particles Theory of the Institute of Theoretical Physics which was first conducted by Prof. J.Lukierski, then Prof. L.Turko and now, for several years, by Prof. K.Redlich, the correspondent member of Polish Academy of Science. The Department is involved in an extensive international colaboration and takes part in ALICE experiment at CERN ( in the range of the Polish team led by Prof. T. Siemiarczuk from the National
Nuclear Research Centre in Warsaw) and in the experiment NA61/SHINE at CERN. The Department comprises both the Polish physicists as well as the scientists from Germany, ,Japan, China and Ukraine. The intensive neutrino physics research under the direction of Prof. J. Sobczyk is also conducted in the Institute of Theoretical Physics, regarding the cooperation with the leading experiment T2K in Japan.
Andronic A., Braun-Munzinger P., Redlich K. und Stachel J. (2018): Decoding the phase structure of QCD via particle production at high energy. Nature volume 561, pages 321–330 (2018) issue; DOI: 10.1038/s41586-018-0491-6, https://www.nature.com/articles/s41586-018-0491-6.