What we do...

View of Alexandr Nikolaev, Swansea University, UK..

The matter which surrounds us in the everyday life is very complex from the physical point of view and has several structure levels: molecules, which are built from atoms and electronic bonds; atoms, which consist of nucleus and electron orbitals; nucleus, which are made of protons and neutrons (i.e. nucleons); nucleons, which consist of quarks and gluons. To get more knowledge of the physics on the quark level heavy nucleus are being accelerated close to the speed of light and then collided on the modern experimental facilities, such as Relativistic Heavy Ion Collider (RHIC) at BNL or the Large Hadron Collider (LHC) at CERN. State of matter right after the collision is very similar to what happened during the Early Universe stage and is characterized by non-trivial properties: the quarks are strongly coupled, but the ratio of shear viscosity over entropy density is surprisingly small. The theory, describing what happens between the quarks in such a hot matter or at the very first stage of heavy ions collision, is Quantum Chromodynamics (QCD). There is a formulation of QCD, called lattice QCD, which allows a direct calculations of the physics of strong interactions by numerical simulation on powerful supercomputers. Such calculations together with experimental high energy physics are complementary to each other and may provide us a better understanding of sub-atomic, strong interaction level of physics.

View of Rainer Stiele, INFN, Italy..

Recently and in the near future, there is and will be big experimental effort to explore the conditions and nature of the transition between normal nuclear matter and the elementary particles it is made of. These experiments are the energy scan program at the "Relativistic Heavy Ion Collider" (located at Brookhaven National Laboratory, USA) and will be performed at the future “Nuclotron-based Ion Collider fAcility” at the “Joint Institute for Nuclear Research” (Russia) and the “Facility for Antiproton and Ion Research” (Germany). These laboratory experiments at low and intermediate densities are complemented by searching for gravitational waves of inspiraling neutron stars that allow us to learn about the nature of matter at the largest densities. All these experimental measurements require a theoretical counterpart to interpret and analyse their results. We are performing our investigations with a framework which is based on two fundamental properties of strongly interacting matter, namely the mass of constituent quarks and confinement in nucleons and the liberation of light quarks in the transition to the quark-gluon plasma. The interaction between quarks that gives them their mass can be described in two ways, either as a point interaction or by exchange of a meson. The former leads to what is called the Nambu-Jona-Lasinio model and the latter to the Quark-Meson model. We work in our collaboration with these frameworks to calculate the course of lines of constant entropy per particle and properties along these trajectories like the speed of sound and baryon number susceptibilities, connected to the fluctuations of conserved charges.

View of Radka Sochorová, Czech Technical University in Prague, Czech Republic..

The quark-gluon plasma, which is produced in the early phase of an ultrarelativistic nuclear collision and which is similar to matter at the beginning of the universe, is a form of matter, in which ordinary hadrons do not exist anymore, and in which quarks and gluons become free. Hadrons, such as protons and neutrons for example, form atomic nuclei (e.g. helium) and atoms then form molecules (for example H2O – water). Quarks and gluons are the smallest, indivisible, parts of matter. Because the plasma expands after collision, its energy density drops. After some time, it changes into hadron gas. We call this process hadronization. Hadrons interact strongly until no more scattering between hadrons occur because the energy density is too low. The hadron decoupling process is called freeze-out. Firstly, chemical freeze-out occurs and then at a lower temperature kinetic freeze-out occurs. So, it is very important to create theoretical models that are able to describe the freeze-out hypersurface and to include the effects that can affect the shape of the freeze-out hypersurface, for example longitudinal and transverse expansion or decays of resonances. On this freeze-out hypersurface the transverse momentum spectra are formed. We can consider a variety of mechanisms for the production of hadrons and their clusters, e.g. coalescence model and the thermal static model. The coalescence model postulates that light nuclei are formed only at late times of the fireball evolution by recombination of protons and neutrons with close positions and velocities on the kinetic freeze-out surface. On the other hand, thermal model assumes that there is perfect chemical equilibrium above the chemical freeze-out temperature. So thermal model describes yields of all hadron species with the chemical freeze-out temperature. It means that light nuclei seem to behave like ALL other hadrons. But this is very surprising, because it is hard to imagine that loosely bound sizeable nuclei can exist in the hot and dense hadron gas. From previous studies we know that both models predict similar deuteron yields. But the elliptic flow of the light nuclei could not be described with the blast-wave model through simply replacing the proton mass with the ones of the light nuclei. Ultimately, the elliptic flow observable might be able to distinguish between statistical production and the coalescence model for the deuterons.

View of Gabriele Inghirami, University of Jyväskylä, Finland

We study what are the characteristics of the force that binds atomic nuclei together and how matter forms. In principle, we have wonderful theories, combined together in the famous “Standard Model”, that describe how Nature works at subatomic scales, but every theory needs to be validated by experiments, to verify if we got everything right or if there is something wrong or unexpected, for example new emergent non trivial effects or even hints of new physics. In this process we face two big challenges: to provide detailed theoretical predictions and to perform accurate measurements. These problems are common to many scientific fields, but they are particularly severe when dealing with atomic nuclei and their constituents, given the very complicated mathematical apparatus of the theory and the tiny characteristic dimensional scale of the phenomena of interest. In our field, i.e. relativistic heavy-ion collisions, atomic nuclei of heavy elements, like gold or lead, are smashed together in powerful accelerators while traveling at almost the speed of light. There are solid evidences that, at a sufficiently high collision energy, for a short time after the collision an exotic state of matter is created, the so called “Quark-Gluon Plasma”, a soup at a temperature around 2000 billions C. degrees (i.e. 100000 times hotter than the core of the Sun), that most likely filled the whole Universe until a few microseconds after the Big-Bang, long before any star or galaxy existed. However, this hot system which forms in heavy ion collisions is extremely small and very quickly dissolves into a rapidly expanding cloud composed by thousands of particles. We are not able to directly observe such a small and volatile system, the best that we can do is to use detectors to capture the emitted particles, analyze their properties and try to reconstruct the state and the evolution of the system from which they originated. The connection between theory and experiments is generically called “phenomenology” and it is based on approximate semi-analytical or, more and more often, numerical models. Usually, the development of these models requires not only a noticeable computational effort, but also a good knowledge of many different aspects of the physical phenomena under investigation. Regarding the latter point, the STSM missions offer a formidable opportunity to acquire, exchange or merge knowledge at the highest level, by visiting the leading experts of the field or by teaming and summing up different expertise. As a concrete example we report our experience. In the first half of the year we were studying the so called “kinetic freeze-out”, i.e. the moment where the particles emitted in a heavy ion collision stop to interact and do not change their momenta distribution (i.e., roughly speaking, their velocities) anymore. These are the same momenta that are measured by the detectors, therefore a full understanding of this step is crucial to draw an accurate picture of the whole system evolution. In many common models, the macroscopic properties of the system at kinetic freeze-out, for example the temperature, are derived by fitting the experimental data to some parameters of an analytic model. In our project, we tried the opposite approach, starting from a microscopic model implemented in a numerical code (UrQMD), which takes into account how the various particles mutually interact with each other, and we computed what the macroscopic properties of the system should be when it becomes kinetically frozen. The comparison of the results between these two different approaches can provide useful insights about the details of the system evolution and help to refine both of them. The STSM visit at the main UrQMD developer and expert, supported by the COST THOR action, allowed an acceleration of the project, a better determination of its scope and a superior quality and robustness of the results, summarized in a scientific article (https://arxiv.org/abs/1909.00643). Therefore, we express our gratitude to COST action THOR, funded by the EU and its citizens, that made all this possible.

View of Miklós Zétényi and Baiyang Zhang, Wigner Research Centre for Physics, Hungary

In low energy heavy-ion collisions, a fireball of strongly interacting particles (hadrons) is created. Many of these hadrons are short-lived instable states, so-called resonances. In order to understand the evolution of a heavy-ion collision and the properties of the fireball, we need a solid understanding of the physics of hadrons – which is certainly an interesting topic in its own right too.

50 years after the discovery of Quantum Chromodynamics (QCD - the theory of strong interactions), our knowledge about the spectrum of hadrons and their interactions is still far from complete. Part of the physics program of the HADES collaboration in the GSI Helmholtzzentrum in Darmstadt is related to the physics of hadrons in vacuum (as opposed to hadron properties in the dense matter formed in heavy-ion collisions). In particular, the recent pion beam experiments of HADES provide a unique opportunity to study the electromagnetic interaction of baryon resonances formed in pion-proton collisions, via their Dalitz decay into a nucleon and an electron-positron pair (also known as dilepton). In this process, electromagnetic interaction of baryons can be investigated in a kinematical region not accessible in other reactions, like photo-, electroproduction, or electron-positron annihilation experiments. According to the Vector Meson Dominance model, neutral vector mesons (in the HADES energy domain most importantly the rho meson) play an important role in the electromagnetic interaction of hadrons. Our group is working on model calculations of the process measured by HADES. We collaborate with members of the HADES group in the comparison of model calculations and experimental results. Our results show how one can reconstruct the polarization state of the intermediate rho meson based on the angular distribution of electrons and positrons in the dilepton production process. The rho meson polarization contains information about the decaying baryon resonance and its electromagnetic interaction which can hopefully be extracted from the experimental data using our model calculations.

View of Miklós Zétényi and Baiyang Zhang, Wigner Research Centre for Physics, Hungary

The macro world around us is a complex composition which includes several structure levels. The fundamental particles on the lowest level are called quarks. They combine to form neutrons and protons which make nuclei. These nuclei surrounded by electrons construct atoms that join together to make molecules and other chemical substances. All different elements interact with each other in different ways by the exchange of particles. Over short distances gluons, mesons, and other types of bosons are the relevant mediators of the forces. Photons and gravitons describe the long-range electromagnetic and gravitational interaction. The most compacted matter on Earth is found in heavy nuclei. It can be studied in heavy-ion collision (HIC) experiments. These high densities are interesting to investigate because exotic forms of matter can be produced and particles, e.g. hyperons, can appear that are not found in ordinary matter. One of the best places to explore matter under extreme conditions with densities higher than the density produced in HIC are compact stars (CSs). In the core of CSs we expect the appearance of substantial amounts of hyperons or even deconfined quark matter accompanied with a phase transition from hadronic to quark matter when the density increases. It has been demonstrated that CS constraints can be used to narrow down the variation of predictions for the onset of this phase transition. In CSs it will occur at very neutron-rich conditions. In contrast, dense matter close to neutron-proton symmetry will be probed in the future at FAIR, NICA and other facilities for HICs. Detailed information on the composition and properties of dense matter are relevant to the goals of the working group 1 in the COST Action “THOR”. During this STSM, three different approaches have been used to calculate the equation of state (EoS) of the two phases of strongly interacting matter and to investigate the location and properties of matter at the phase boundary. We have drawn conclusions on the chemical composition of matter in the deconfinement phase transition inside CSs as well as in isospin-symmetric matter. Two different approaches (LOCVY and RMF-DD2Y) for the hyperonic phase and a nl-NJL model for the quark phase were used in a comparative study to explore the model dependence of the deconfinement properties. We would like to continue this collaboration to include finite temperature effects and to have an anticipation of the QCD phase diagram for future applications.