Friday, September 21, 2018

Scientists Present New Observations to Understand the Phase Transition in Quantum Chromodynamics

ALICE detector at LHC. Credit: CERN/Saba, A.

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).

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.

The physicists analyzed more precisely the yields of a number of particles and anti-particles. 

"Our investigations revealed a number of surprising 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. Anton 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.

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).

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).

Nuclear physicists at the University of Münster not only participated in the ALICE experiment from the start but were also part of the predecessor experiments at CERN and the Relativistic Heavy Ion Collider. The group at the Institute for Nuclear Physics, first under the direction of Prof. Dr. Rainer Santo, then Prof. Dr. Johannes Wessels and now Prof. Dr. Anton Andronic, played a leading role in the design, construction and operation of the ALICE Transition Radiation Detector TRD which is primarily used to identify electrons and positrons. In addition, the Münster group played a significant role in the physics analyses for the ALICE experiment.

Credit: uni-muenster.de

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