Chiral restoration in excited hadrons

Quantum-Chromo-Dynamics (QCD) is a fundamental theory of interactions responsible for the structure of the strongly interacting particles - hadrons - as well as for the nuclear force. The elementary objects of the theory are quarks and gluons, the building blocks of all hadrons. Masses of the lightest u and d quarks are very small and to a good approximation can be neglected. In this case the theory has a symmetry called chiral symmetry. This symmetry is spontaneously broken, which is the most important phenomenon that drives the physics and properties of the lowest-lying hadrons. The other crucial property of QCD is confinement. Confinement means that quarks and gluons cannot be observed as free particles. The ultimate goal of QCD is to explain both confinement, spontaneous breaking of chiral symmetry and their possible interrelations. In the high-lying hadrons confinement is a crucial property. Then the question arises, what is the role of chiral symmetry and its spontaneous breaking in the high-lying hadrons? It has been recently suggested that the chiral symmetry gets approximately restored here. This idea has received certain theoretical and phenomenological support. If correct, it allows one to relate the properties of the highly-excited hadrons with the symmetry of the fundamental theory. We want to carefully study this phenomenon in order to understand it microscopically and to identify mechanisms of chiral symmetry breaking and its restoration in the excited hadrons and its interrelation with confinement.

Hadrons, such as proton and neutron and their excited states are made of quarks and gluons. A fundamental theory that describes interaction of quarks and gluons - Quantum Chromodynamics (QCD) - is responsible for structure, masses and other characteristics of hadrons. There are two crucially important properties of QCD that are responsible for all observables - confinement and spontaneous (dynamical) breaking of chiral symmetry. It is understood that it is dynamical breaking of chiral symmetry that is the most important phenomenon that influences masses of the ground state of hadrons. A main output of the project is a conclusion that in the highly excited hadrons most probably it is not so. This means that the mass generation mechanisms for the ground state hadrons and the highly excited hadrons are essentially different. Most of the mass of the highly excited hadrons comes from the manifestly chiral-invariant dynamics. Such conclusion is based on systematic spectroscopic patterns and decay properties of hadrons that are observed experimentally. A microscopic mechanism that could underline the proposed phenomenon has been worked out. Performed research has a direct impact on present and future experimental programs at leading world laboratories studying excited states of hadrons. If the mass of highly excited hadrons can be formed from the chiral invariant dynamics then it is quite possible that at low temperatures and high densities there could exist a new phase of the matter - chirally symmetric phase with confinement. It was believed that such a phase was impossible. Given a microscopic mechanism leading for the chiral restoration in highly excited hadrons it was possible to demonstrate that it is not so and that actually a chirally symmetric but confining matter could exist at low temperatures and large densities. It radically changes our view of the QCD phase diagram. Such a phase could influence dynamics of neutron stars and other astrophysical phenomena. This conclusion is of key importance for future experimental programs at all world heavy ion facilities.