Clues about the Nature of Multiquark States

We present a comprehensive study of the decay width of multiquark states containing different color singlet components in a coupled-channel approach. We show how the decay width can provide in-depth information about the nature of a coupled-channel resonance. An unexpected behavior of the decay width of a multiquark state could be pointing to a relevant role of the coupled-channel dynamics, aiming at the channel responsible for the formation of the resonance. The symmetrical situation between meson- and baryon-like multiquarks is highlighted. Our study accounts for the existence of narrow resonances with large phase spaces. In the case of resonances far from their detection channel, it is the mass difference with the formation channel that determines their decay width. The larger the binding, the larger the decay width, even though the phase space to the detection channel gets reduced. The trends noticed cast doubts on the molecular assignment of some multiquark candidates. Finally, we wonder about the existence and properties of multiquark partners in other flavor sectors.

Dynamical Approach to Decays of XYZ States

We review the existing results on the exotic XYZ states and their decays obtained within the confined covariant quark model. This dynamical approach is based on a non-local Lagrangian of hadrons with quarks, has built-in quark confinement, and is suited well for the description of different multiquark states, including the four quark ones. We focus our analysis on the various decay modes of five exotic states, X ( 3872 ) , Z c ( 3900 ) , Y ( 4260 ) , Z b ( 10610 ) , and Z b ′ ( 10650 ) , aiming to clarify their internal quark structures. By considering mostly branching fractions and decay widths using the molecular-type or the tetraquark-type interpolating currents, conclusions about the nature of these particles are drawn: the molecular structure is favored for Z c ( 3900 ) , Z b ( 10610 ) , and Z b ′ ( 10650 ) and the tetraquark for X ( 3872 ) and Y ( 4260 ) .

Production of exotic charmonium in heavy ion collisions

We present a short review of exotic charmonium production in heavy ion collisions. We discuss predictions for the production cross section of several of these states in ultra-peripheral Pb-Pb collisions at the LHC. The experimental study of these processes is feasible and can be used to yield valuable information about the structure of multiquark states. We also address X(3872) production in central Pb-Pb collisions. In particular, we discuss the suppression of X(3872) during the hadron gas phase. Finally, we comment on the very recent CMS data on the X(3872) yield in Pb-Pb collisions. the very recent CMS data on the X(3872) yield in Pb-Pb collisions.

Multiquark states in the Thomas–Fermi quark model and on the lattice

We describe work being done at Baylor University investigating the possibility of new states of mesonic matter containing two or more quark–antiquark pairs. To put things in context, we begin by describing the lattice approach to hadronic physics. We point out there is a need for a quark model which can give an overall view of the quark interaction landscape. A new application of the Thomas–Fermi (TF) statistical quark model is described, similar to a previous application to baryons. The main usefulness of this model will be to detect systematic energy trends in the composition of the various particles. It could be a key to identifying families of bound states, rather than individual cases. Numerical results based upon a set of parameters derived from a phenomenological model of tetraquarks are given.

Multiquark States

Why do we see certain types of strongly interacting elementary particles and not others? This question was posed more than 50 years ago in the context of the quark model. M. Gell-Mann and G. Zweig proposed that the known mesons were [Formula: see text] and baryons qqq, with the quarks known at the time, u (up), d (down), and s (strange), having charges of 2/3, −1/3, and −1/3, respectively. Mesons and baryons would then have integral charges. Mesons such as [Formula: see text] and baryons such as [Formula: see text] would also have integral charges. Why weren't they seen? They have now been seen, but only with additional heavy quarks and under conditions that tell us a lot about the strong interactions and how they manifest themselves. This review describes recent progress in our understanding of such exotic mesons and baryons.

Shedding Light on Hexaquarks

Several new findings in the four, five and six quark systems reheat the interest in the field of multiquark states (beyond the trivial [Formula: see text] and [Formula: see text]). A lot of progress has recently been made in the 6q sector, on both the theoretical and experimental side. A resonance like structure observed in double-pionic fusion to the deuteron, at M = 2.38 GeV with [Formula: see text] = 70 MeV and [Formula: see text] has been consistently observed in a wealth of reaction channels, supporting the existence of a resonant dibaryon state - the [Formula: see text]. These studies include measurement of all the principle strong decay channels in pn collisions in the quasifree mode by the WASA-at-COSY and HADES collaborations. The internal structure of the [Formula: see text] is largely unknown. It can contain various ”hidden color” 6q configurations, [Formula: see text] molecular states with angular momentum L = 0,2,4,6 as well as meson-assisted dressed dibaryon structures. The large set of experimental data obtained to date gives some constraints on the internal structure of the [Formula: see text] dibaryon, but does not settle the issue. The [Formula: see text] is the only multiquark state which can be produced copiously at current facilities, offering unique access to information beyond its basic quantum numbers, particularly its physical size and internal structure.