Dear Colleagues,

Quantum chromodynamics (QCD), the theoretical basis of hadronic physics, is a gauge theory based on the fundamental property of the strong interaction of a local color symmetry. Due to another fundamental property, color confinement, only color singlet hadrons can be observed in nature. As predicted by QCD, and as has been experimentally proven many times, such colorless objects can have quantum numbers of quark–antiquark states (ordinary mesons) and of three quark states (ordinary baryons). However, the QCD does not exclude the possibility of the existence of more complex colorless states consisting of several quarks and antiquarks (as well as pure gluonic states and hybrid quark–gluon states).

It is worth noting that many phenomenological models describing the properties of multiquark systems are based on the fact that the antidiquark can be in a color-triplet state, and thus, an approximate symmetry between the antidiquark and the quark is possible. This symmetry can result in tetraquarks corresponding to some ordinary three-quark baryons, in which one of the baryon quarks is replaced by an antidiquark, or even to a pentaquark, in which two quarks from a baryon are replaced by antidiquarks. The fact that Regge trajectories for light mesons and light baryons have the same slope argues in favor of such approximate quark–diquark duality.

However, researchers have been unable to find well-established multiquark states for a long time. The situation changed with the discovery of quarkonium χ_c1(3872), which demonstrates properties that do not fit well with the concept of a particle consisting of a charm quark and a charm antiquark only. In the wake of this discovery, others were made. Currently, about two dozen multiquark states with heavy quarks are known. Among them are several charged mesons with a hidden charm, several baryons with a hidden charm, tetraquarks containing four different flavors, and a quadruply heavy state showing itself as a structure in the J/ψ/J/ψ mass spectrum. The latest discovery of the LHCb Collaboration marks a new milestone in the study of exotic states: a doubly charmed tetraquark T_cc⁺ has been experimentally observed.The new state turned out to be extremely narrow, with a width comparable to that of J/ψ and with a mass strikingly close to the mass of χ_c1(3872). The latter fact may indicate a deep but not-yet-understood connection between χc1(3872) and the doubly charm tetraquark. Despite the extensive experimental data and the abundance of theoretical work, the nature of exotic states is not fully understood. Even the most basic characteristics of such particles have not been clarified. At present, it is impossible to assert with complete certainty that exotic states are compact multiquark objects, and the diquark–antiquark duality concept can be applied to describe them. The opposite point of view, namely, that exotic particles are molecules, has just as many arguments in its favor. Therefore, new efforts from theorists are required. This Special Issue of Symmetry provides researchers with the opportunity to express new insights into exotic states and thereby contribute to progress in understanding their physics.

Dr. Alexandr Berezhnoy
Guest Editor

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