YITP long-term workshop

New Frontiers in QCD 2018
- Confinement, Phase Transition, Hadrons, and Hadron Interactions -

May 28 - June 29, 2018
Yukawa Institute for Theoretical Physics, Kyoto University

NFQCD 2018 & YKIS 2018b


Over half a century has passed since quantum chromodynamics (QCD) was proposed through introduction of spontaneous chiral symmetry breaking, quark model, and color degrees of freedom. QCD was established as a basic theory of strong interaction by understanding of asymptotic freedom, color confinement and so on in the 1970s, and later developments make it possible to provide precise predictions on physical quantities such as proton structure functions and hadron masses. However, we have not yet obtained the satisfactory understanding of the properties of ordinary nuclei, the vacuum structure at various temperatures and densities such as the initial universe and celestial objects, the nature of phase transition, the quark-gluon structure in excited hadrons. These subjects remain as a rich and undeveloped field.

One of the current subjects is the QCD phase transition at finite temperature and density. In heavy ion collision experiments at the top energies of the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory and the Large Hadron Collider (LHC) at CERN, formation of the quark gluon plasma (QGP) at high temperature has been confirmed. These high temperature matter is made at almost zero baryon density, where the phase transition is known to be crossover according to the lattice QCD simulation. At lower energies, on the other hand, the baryons in the incoming nuclei lose energy and remain at mid-rapidities, and create high temperature / high density matter. The QCD phase transition at low temperature and high density may be the first order according to the QCD effective models. In this case, it is expected that the QCD critical point where the fluctuations of some physical quantities diverge. Heavy-ion collision experiments in such energy range are carried out at the Beam Energy Scanning (BES) program at RHIC, FAIR (Anti-proton and Ion Research Facility) to be constructed in Germany, and NICA (Nuclotron- based Ion Collider fA scheme) to be constructed in Russia. Furthermore, it is planned to conduct heavy ion collision experiments at J-PARC in Japan.

In order to understand finite density QCD, it is essential to utilize various theoretical methods and establish new theories as well as experimental research. In lattice QCD, there is a sign problem at finite baryon chemical potential, and it is difficult to directly discuss the properties of high density matter. Therefore, along with the development of techniques for avoiding and weakening the sign problem, we need to improve the many-body problem method starting from the nuclear force and/or the effective QCD model, and to analyze heavy ion data by using dynamical models such as transport models and fluid dynamics. We need the equation of state at high density (high baryon density) also to understand compact astronomical phenomena.

There are two major phase transition mechanisms of the QCD phase transition, chiral transition and confinement / deconfinement transition. The lattice QCD at zero baryon density showed that two phase transitions occur at approximately the same temperature, then these two transitions are expected to be correlated. On the other hand, there are several works suggesting the independence of these transitions: The two phase transition temperatures are found to be different in more recent lattice QCD calculations, the string tension between quarks and antiquarks is found to be insensitive to the low energy modes that strongly contribute to the chiral phase transition, and the confinement-deconfinement transition is proposed to have topological phase transition nature. Thus the picture of the confinement-deconfinement phase transition is still ambiguous.

The properties of various hadrons and interaction between hadrons are also at the frontier of quark-hadron physics. Baryons and mesons have been considered to consist of three quarks and quark-antiquark pairs, respectively. After 2003, however, many states that can not be understood by these configurations (exotic hadrons) have been found. For example, Ds0 (2317) is a charm quark and anti-strange quark (c sbar), which is supposed to be an isospin 0 state in ordinary quark models, but it decays into the isospin 1 state (πDs). Also, Zc+ (3900) contains a charm and anti-charm pair (c cbar) but it has a charge, then it is a tetraquark state containing at least four quarks (two quarks and two antiquarks). Many researchers have been interested in understanding properties of such new type hadrons. Experimental research is being conducted in accelerating facilities around the world such as Belle experiment at KEK, Hadron Hall at J-PARC of KEK-JAEA, BaBar experiment at Stanford Linear Accelerator Center (SLAC), CLEO experiment at Cornell University, BES experiment of Beijing electron positron collision type accelerator.

The exotic hadron states appear in the excited state. Recent studies have revealed that one particle excitation of quark and hadron molecular state compete in excited states of hadrons. One particle excitation in the quark model is about ℏ ω = 500 MeV, which is larger than the pion mass and comparable with the K meson mass. Models based on chiral perturbation theory has been successful in describing bound and resonance hadronic-molecule states. In recent years, interactions between hadrons have been investigated using lattice QCD, and reliable information is being given to interactions undetermined by experiments.

We also find significant progress and future prospects in subjects such as nucleon structure, gauge / gravity correspondence, hadrons in the medium, and quark and hadron physics in compact astronomical phenomena. Research on nucleon structure is a central issue of the Electron Ion Collider (EIC) to be constructed. Research based on gauge / gravity correspondence has expanded research subjects throughout the physical field. Quark-hadron physics in compact astronomical phenomena is important for solving heavy neutron star puzzles and also for understanding the binary neutron star mergers.

Subjects, Aims, and Schedule

In NFQCD 2018, the long-term workshop, we focus on physics of confinement, QCD phase transition, high energy heavy ion collisions, exotic hadrons, and hadron interactions. We take over the physics discussed in HHIQCD (2015) and NFQCD (2008, 2010, 2013) workshops.

In YKIS 2018b, we try to overview quark-hadron physics. For this reason, we also discuss nucleon structure, gauge / gravity correspondence, hadrons in the medium, and quark and hadron physics in compact astronomical phenomena, in addition to the workshop subjects.

We hope that research in these fields is accelerated by gathering of researchers from around the world. Also, we expected that new ideas come out from discussions among researchers having different backgrounds.

The YKIS symposium is held in the 3rd week, and workshop is held in the other 4 weeks (1st, 2nd, 4th and 5th). We will discuss mainly "Confinement & Phase transition" and "Hadrons and Hadron Interaction" in the first and last 2 weeks, respectively.

Workshop Subjects (NFQCD 2018)

YKIS 2018b Subjects