Dibaryons: Molecular versus compact hexaquarks *

Abstract

Hexaquarks constitute a natural extension of complex quark systems, just as tetra- and pentaquarks do. To this end, the current status of in both experiment and theory is reviewed. Recent high-precision measurements in the nucleon-nucleon channel and analyses thereof have established as an indisputable resonance in the long-sought dibaryon channel. Important features of this state are its narrow width and deep binding relative to the threshold. Its decay branchings favor theoretical calculations predicting a compact hexaquark nature of this state. We review the current status of experimental and theoretical studies on as well as new physics aspects it may bring in future. In addition, we review the situation at the and thresholds, where evidence for a number of resonances of presumably molecular nature has been found – similar to the situation in charmed and beauty sectors. Finally, we briefly discuss the situation of dibaryon searches in the flavored quark sectors.

Isospin symmetry breaking in double-pion production in the region of ${d^*(2380)}$ and the scalar $σ$ meson

“…6 It is seen from Fig. 5a that the striking difference between the neutral and charged dipion production cross sections in the isoscalar channel, which results from the structure of the σ decay width (13) shown in Fig. 4, is in agreement with the experimental data.…”

“…However, the D 03 state can also contain 3q-3q configurations with colorless three-quark clusters, the main of which is the ∆∆ configuration. In fact, all known six-quark states are located near the di-hadron thresholds and should be strongly coupled to the respective di-hadron molecular-like states [13,33,34]. The nearer the threshold, the stronger the coupling, and the higher the weight of the respective molecular-like state in the total dibaryon wavefunction.…”

“…The above structure of the total σ width leads to a singularity in the π 0 π 0 production cross section at the π + π − threshold, which is absent in the π + π − production cross section. The M ππ spectra resulted from the intermediate σ excitation mechanism with the width parametrization (13) are shown in Fig. 5.…”

“…4 GeV have been obtained (see Refs. [8][9][10][11] and reviews [12,13]). In these experiments, existence of the near-threshold enhancement in the ππ invariant-mass spectrum known since 1960s as the Abashian-Booth-Crowe (ABC) effect [14] in all reactions accompanied by isoscalar dipion production and formation of the bound nuclei in the final state has been confirmed.…”

Search for the charmed baryonium and dibaryon structures via the QCD sum rules

“…The data of the electromagnetic form factors of the state are possible to differentiate whether it is a molecular type or a compact six-quark entity. For the deuteron, its charge radius is 2.1 fm, the proton and the neutron inside the deuteron are on average 4 fm apart from each other and do not overlap [7,8], these data depict a clear physical picture of the hadronic molecule for the deuteron. One could also make the differentiation via the decay branching, in Ref.…”