Chiral symmetry, confinement and the mass generation of hadrons

Quantum-Chromo-Dynamics (QCD), a fundamental theory of strong interactions of quarks and gluons, that is responsible for existence and structure of hadrons, is realised in the confining mode with spontaneous breaking of chiral symmetry. We do not know yet the answer to a key question about mass generation of hadrons. Are confinement and dynamical breaking of chiral symmetry interconnected and how do they contribute to the hadron mass? We want to carefully study this question. For this purpose we can use lattice QCD to check to what extent the hadron mass is connected to the quark condensate of the vacuum and whether hadrons can still exist in a world without breaking of chiral symmetry in a vacuum. If yes, what happens with their mass and what symmetries do they have in this regime? For this purpose we will study within the dynamical lattice simulations with the overlap Dirac operator, that is manifestly chirally symmetric, the influence of the low-lying and higher-lying eigenmodes of the Dirac operator on the existence and symmetry properties of hadrons.

Quantum-Chromodynamics (QCD) is a fundamental theory of strong interactions of quarks and gluons, the building blocks of particles like proton and neutron. Individual quarks and gluons are never observed in our world, a property of QCD that is called confinement. The quarks and gluons are confined inside observable particles like proton and neutron. Masses of the u and d quarks are much smaller than masses of hadrons such as proton and neutron. We can approximately neglect these u and d masses. This property is called chiral symmetry. Masses of hadrons are generated due to both confinement and spontaneous (dynamical) breaking of chiral symmetry. We have observed a survival of hadrons upon artificial restoration of chiral symmetry in computer simulations and discovered a new symmetry of hadrons in this regime that is larger than the chiral symmetry. This new symmetry was also observed in a computer simulation of a very hot QCD. This larger symmetry would imply that actually there are no free quarks and gluons at the very high temperatures and the QCD matter has a more intricate nature than the quark-gluon plasma. It should shed the light on the nature of the elementary objects in the hot QCD.