Tetraquarks in large-N<sub>c</sub> QCD

Author(s)
Wolfgang Lucha, Dmitri Melikhov, Hagop Sazdjian
Abstract

The generalization of the color gauge group SU(3) to SU(Nc), with Nc taking arbitrarily large values, as had been proposed and developed by 't Hooft, has allowed for a decisive progress in the understanding of many qualitative, as well as quantitative, aspects of QCD in its nonperturbative regime. In particular, the notion of valence quarks receives there a precise meaning. The present work reviews the various aspects of the extension of this approach to the case of tetraquark states, which are a category of the general class of exotic states, also called multiquark states, whose internal valence-quark structure does not match with that of ordinary hadrons, and which have received, in recent years, many experimental confirmations. The primary question of describing, or probing, on theoretical grounds, multiquark states is first examined. The signature of such states inside Feynman diagrams in relation with their singularities is highlighted. The main mechanisms of formation of tetraquark states, provided by the diquark model and the molecular scheme, are considered together with their specific implications. The properties of tetraquark states at large Nc are analyzed through the Feynman diagrams that describe two-meson scattering amplitudes. It turns out that, in that limit, the possible formation of tetraquark states is mainly due to the mutual interactions of their internal mesonic clusters. These essentially arise from the quark-rearrangement, or quark-interchange, mechanism. In coupled-channel meson-meson scattering amplitudes, one may expect the occurrence of two independent tetraquark states, each having privileged couplings with the two mesons of their dominant channel. The question of the energy balance of various schemes in the static limit is also analyzed. The clarification of the mechanisms that are at work in the formation of tetraquarks is the main outcome from the large-Nc approach to this problem. (C) 2021 Elsevier B.V. All rights reserved.

Organisation(s)
Particle Physics
External organisation(s)
Österreichische Akademie der Wissenschaften (ÖAW), Lomonosov Moscow State University (MSU), Joint Institute for Nuclear Research, Université Paris-Saclay, Universität Wien
Journal
Progress in Particle and Nuclear Physics
Volume
120
No. of pages
54
ISSN
0146-6410
DOI
https://doi.org/10.1016/j.ppnp.2021.103867
Publication date
04-2021
Peer reviewed
Yes
Austrian Fields of Science 2012
103034 Particle physics
Keywords
Portal url
https://ucris.univie.ac.at/portal/en/publications/tetraquarks-in-largenc-qcd(cc74dcda-831a-4f3f-a202-71061762ec5c).html