The string-junction picture of multiquark states: an update

TL;DR

This paper updates a 40-year-old string-junction model of multiquark states, linking theoretical concepts with experimental evidence, and extends the OZI rule to explain the narrowness of certain exotic hadrons.

Contribution

It introduces a refined string-junction framework for multiquark states, supported by large-N and lattice QCD, and generalizes the OZI rule to include junction dynamics.

Findings

Support for the string-junction model from large-N and lattice QCD.

Extension of the OZI rule to junction-antijunction suppression.

Experimental evidence of narrow multiquark states below baryonic thresholds.

Abstract

We recall and update, both theoretically and phenomenologically, our (nearly) forty-years-old proposal of a string-junction as a necessary complement to the conventional classification of hadrons based just on their quark-antiquark constituents. In that proposal single (though in general metastable) hadronic states are associated with "irreducible" gauge-invariant operators consisting of Wilson lines (visualized as strings of color flux tubes) that may either end on a quark or an antiquark, or annihilate in triplets at a junction $J$ or an anti-junction $\bar{J}$. For the junction-free sector (ordinary $q\, \bar{q}$ mesons and glueballs) the picture is supported by large-$N$ (number of colors) considerations as well as by a lattice strong-coupling expansion. Both imply the famous OZI rule suppressing quark-antiquark annihilation diagrams. For hadrons with $J$ and/or $\bar{J}$ constituents the same expansions support our proposal, including its generalization of the OZI rule to the suppression of $J-\bar{J}$ annihilation diagrams. Such a rule implies that hadrons with junctions are "mesophobic" and thus unusually narrow if they are below threshold for decaying into as many baryons as their total number of junctions (two for a tetraquark, three for a pentaquark). Experimental support for our claim, based on the observation that narrow multiquark states typically lie below (well above) the relevant baryonic (mesonic) thresholds, will be presented.