Division of Computer Physics

Topics

• Title: Decays of nonconventional mesons.
  Project coordinator: Francesco Giacosa
  

  Matter is made out of small atoms. An atom has a nucleus in the middle and electrons which fly around it. The nucleus is made of protons and neutrons. That is not the end yet: a proton and a neutron are made of three even smaller objects, called quarks. Quarks and electrons are elementary particles: to the best of our knowledge, they cannot be further divided. Particle physicists try to answer the following questions concerning quarks: how do they interact with each other? What is the weight of a quark? Quarks are like small lego-blocks, which can combine in different ways. Which combinations are possible and which not? We know some of the answers but not all of them. Quarks interact with each other by exchanging particles called gluons. As the name tells us, the interaction is very strong, actually so strong that we cannot see an isolated quark (a property called confinement). We know that a quark and an antiquark can combine to form a bound state: many particles of these type, called mesons, have been found. Another possibility is to put three quarks together: in this way we construct a baryon, such as a proton. More precisely, a quark is a 'colored' object: it can be red, green, or blue. Of course, these are not real colors, yet they offer a useful mathematical analogy, which also gives the name to the theory describing quarks and gluons: Quantum Chromodynamics (QCD). However, contrary to quarks, mesons and baryons are white, i.e. the color charges of quarks inside them neutralize: one has a quark with color and an antiquark with anticolor inside mesons and an equal admixture of red, green, and blue quarks in baryons. Indeed, this is a basic property of all existing bound objects made of quarks which goes at the heart of confinement: these objects are white. Yet, there can be other possibilities: can one build particles which are made of two quarks and two antiquarks? This is a so-called four-quark state, which also needs to be white. Very recent experiments (LHC-b at CERN and BES-III) show that these objects do exist in nature. In our work, we concentrate on the existence of four-quark states and on the way they decay in smaller and conventional quark-antiquark objects. Note, we deal here with very small decay times, of the order of: 10^-22 sec! In this short lifetime the fuzziness of quantum mechanics plays an important role and is studied in detail in our investigations. But there is more: gluons themselves, the force carriers, interact with each other. This is a very peculiar property of gluons. Can gluons form white bound states? Computer simulations tell us 'yes'. These hypothetical states are called 'glueballs', that is, balls of gluons. They also represent a possibility which goes beyond the quark-antiquark picture. Up to now the experiments have not found glueballs, but the research goes on. As also the glueballs live very short (also 10^-22 sec), we need to know precisely how they decay in order to find them. In the proposed work, we will calculate many possible decays of glueballs, in order to facilitate their experimental discovery (in particular, the future PANDA experiment at FAIR in Darmstadt will focus on the experimental search of glueballs). In conclusion, the theoretical investigation of novel possibilities for forming bound states of quarks and gluons aims to help our understanding on how elementary particles making the worlds around us work.

  
  

  Our research project involves:
  
  1) Development and use of Quantum Field Theory to study the decays of non-conventional mesons, such as glueballs.
  
  2) Understanding of the fundaments of decays within Quantum Mechanics and Quantum Field Theory.
  
  Webpage: http://th.physik.uni-frankfurt.de/~giacosa/

  
  

• Title: Entanglement in trapped few-particle systems
  Research team: Anna Okopinska, Przemysław Kościk, Arkadiusz Kuroś
  

  Entanglement is nowadays considered as a key quantity for understanding the properties of composite quantum systems. The phenomenon of entanglement gives rise to nonclassical correlations between different parts of many-body systems which can be quantified by calculating von Neumann or R´enyi entropies of the reduced density matrix of a subsystem.
  
  The study of entanglement is important in view of possible applications in quantum computation. Among the most promising candidates for scalable devices which would simulate complex quantum systems, intractable on classical computers, are quantum systems composed of a few interacting particles trapped in external potentials. Recently, it has become possible to fabricate systems which are effectively described by a few-body Schr\"odinger equation in experiments with ultracold gases or ion traps and as superconducting quantum dots. This gave an impetus for theoretical studies of the entanglement content of their few-body models.
  

  Our research project involves:
  
  1) Investigating the entanglement properties of bound and resonant states of various few-body systems, including ultracold atoms with short range interactions, systems with particles interacting through the long-range inverse power-law interaction, and natural systems such as helium-like ions.
  
  2) Developing computationally-efficient approaches that are capable of tackling many-body systems.
  
  3) Discussing entanglement in exactly solvable models.
  
  Publications of the research team:
  
  http://www.ujk.edu.pl/strony/Anna.Okopinska/publications.html
  https://www.researchgate.net/profile/Przemyslaw_Koscik
  https://www.researchgate.net/profile/Arkadiusz_Kuros3/

  
  

• Title: Correlations in ultra-relativistic nuclear collisions
  Research team: Wojciech Broniowski, Adam Olszewski
  

  One of the prime goals of the Large Hadron Collider (LHC) is the investigation of matter at extemely high densities and temperatures, achieved in nuclear collisions at the highest energies available on Earth. Our present knowledge of these collisions distinguishes three phases in the dynamical evolution of the formed system: 1) eqrly emission of partons and formation of dense and hot initial state, 2) intermediate collective expansion (quark-gluon plasma), and finally 3) formation of hadrons, which are then measured in detectors.
  
  From the point of view of the physics of strong interations, the most interesting are the first two phases, with the first one determining the initial condition for the following collective dynamics. Relevant information on the early phase may be obtained from investigations of correlations of various physical quantities, measured in the so-called event-by-event fluctuation studies. This is because such fluctuations are sensitive to correlations. Investigations of various correlations have been studied for many years, starting with the elementary proton-proton collisions, however, recently completely novel results were obtained at the LHC, trigering a new stage in these studies.
  
  Our present explorations focus on:
  
  1) Theoretical analysis of the recently measured so-called longitudinal correlations (i.e. in the rapidity variable), formed in the earliest stages of ultra-relativistic nuclear collisions.
  
  2) Modelling of the system based on effective approaches to quantum chromodynamics (e.g, application of flucuating strings, which are generated in elementary nucleon-nucleon collisions or their partonic components).
  
  3) Numerical simulations of, i.a., multiplicity correlations, forward-backward transverse-momentum fluctuations, forward-backward correlations of the principal axes and eccentricities in the initial distributions, based on newly-proposed statistical measures.
  
  Papers: link to the publication list