QCD thermodynamics with isospin density and magnetic fields

• Applicant: Professor Dr. Gergely Endrödi
• Subject Area: Nuclear and Elementary Particle Physics, Quantum Mechanics, Relativity, Fields
• Term: from 2015 to 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 272761853

Quantum Chromodynamics (QCD) is the theory of the strong interactions, which describe how composite objects like the proton or the neutron (hadrons) are built up from elementary particles - quarks and gluons. QCD exhibits at least two very different phases: a low-temperature phase with hadronic degrees of freedom, and a high-temperature phase, where quarks are deconfined and the so-called quark-gluon plasma is formed. The determination of the properties of these phases contributes significantly to our understanding of the evolution of the universe in its very early stage, of the behavior of matter in the dense inner core of neutron stars, and of the description of heavy-ion collision experiments. In these systems, relevant phenomena occur at nonzero temperature, particle density and/or background electromagnetic fields. At asymptotically high values of these parameters, perturbation theory can be applied to investigate the thermodynamics of QCD. However, to study the transition between the hadronic and plasma phases, and determine the phase diagram of QCD, non-perturbative methods are necessary.The best available approach to QCD at nonzero temperature and nonzero background magnetic field is by means of numerical Monte-Carlo simulations of the lattice discretized theory. In fact, most of our knowledge about the zero-density region of the phase diagram comes from lattice QCD simulations. On the other hand, the lattice implementation of nonzero densities is notoriously difficult due to the complex action problem (the so-called sign problem), allowing only special cases of the nonzero density situation (like isospin density) to be studied with direct Monte-Carlo methods on the lattice.In the present project, QCD thermodynamics in the presence of isospin densities and background magnetic fields will be studied, via lattice simulations. One part of the project is devoted to the determination of the QCD phase diagram at nonzero isospin density. The relevant effects that will be studied are pion condensation at low temperature and high isospin density - a phenomenon significant for dense neutron stars - and the emergence of a critical endpoint at intermediate densities - a concept most interesting for heavy-ion collisions. In addition, new observables will be investigated to determine the QCD phase diagram in the presence of background magnetic fields and isospin densities. Finally, a novel method will be developed to measure the hadronic contribution to the anomalous magnetic moment of the muon.

• DFG Programme: Independent Junior Research Groups