Special Issue "Tetra- and Pentaquarks “Phenomenology of Multiquark States: Is There Approximate Symmetry between Diquarks and Antiquarks?”"

A special issue of Symmetry (ISSN 2073-8994). This special issue belongs to the section "Physics".

Deadline for manuscript submissions: closed (31 August 2023) | Viewed by 1690

Special Issue Editor

Dr. Alexandr Berezhnoy
E-Mail Website
Guest Editor
Skobeltsyn Institute of Nuclear Physics, Moscow State University, 119991 Moscow, Russia
Interests: heavy quarks; QCD

Special Issue Information

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 χc1(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 Tcc+ 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 χc1(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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Published Papers (1 paper)

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Fully Heavy Tetraquark Spectroscopy in the Relativistic Quark Model
Symmetry 2022, 14(12), 2504; https://doi.org/10.3390/sym14122504 - 26 Nov 2022
Cited by 10 | Viewed by 1393
Masses of the ground and excited (1P, 2S, 1D, 2P, 3S) states of the fully heavy tetraquarks, composed of charm (c) and bottom (b) quarks and antiquarks, are calculated in the diquark–antidiquark picture within the relativistic quark model based [...] Read more.
Masses of the ground and excited (1P, 2S, 1D, 2P, 3S) states of the fully heavy tetraquarks, composed of charm (c) and bottom (b) quarks and antiquarks, are calculated in the diquark–antidiquark picture within the relativistic quark model based on the quasipotential approach and quantum chromodynamics. The quasipotentials of the quark–quark and diquark–antidiquark interactions are constructed similarly to the previous consideration of mesons and baryons. Relativistic effects are consistently taken into account. A tetraquark is considered a bound state of a diquark and an antidiquark. The finite size of the diquark is taken into account, using the form factors of the diquark–gluon interaction. It is shown that most of the investigated states of tetraquarks lie above the decay thresholds into a meson pair; as a result, they can be observed only as broad resonances. The narrow state X(6900) recently discovered in the di-J/ψ production spectrum by the LHCb, CMS and ATLAS Collaborations corresponds to an excited state of the fully charmed tetraquark. Other recently discovered exotic heavy resonances, X(6200), X(6400), X(6600), X(7200), and X(7300), can also be interpreted as the different excitations of the fully charmed tetraquark. Full article
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