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In charm and beauty sectors a manyfold of exotic multiquark states of tetra- or pentaquark structure have been observed. Since they generally reside close to decay thresholds, they appear as narrow resonances due to the small available phasespace and the fact that the decay products are hadronically stable. In the unflavored sector, however, such multiquark states will appear as broad resonances in general, since the decay products themselves constitute already very broad resonances. This complicates enormeously their unambiguous detection.
In this contribution the focus is on dibaryonic multiquark states, which may exist as loosely bound molecular systems or as deeply bound hexaquarks. The history of such objects has been paved with reams of pros and cons, until recently the d*(2380) has been detected - a quite narrow dibaryonic state with I(JP) = 0(3+) and deeply bound relative to the ΔΔ threshold [1,2].
Though this is the only unambigously identified dibaryon resonance so far, in view of the many recently discovered threshold states in the flavored sectors also the longstanding discussion about further unflavored dibaryon resonances near thresholds appears in a new light. There are a number of well-known states with definite spin-parity at the ΔN threshold. Since they appear as broad overlapping resonances in elastic scattering and single-pion production, sophisticated partial-wave analyses had to be utilized for their identification.
Very recently also evidence for dibaryonic states near the N*(1440)N threshold have been reported [3]. WASA-at-COSY data for the isoscalar single-pion production show that its cross section does not grow above threshold as expected conventionally, but rather exhibits a Lorerentzian shape suggesting isoscalar states with JP = 1+ and 1-.
Interestingly, a sophisticated NN-interaction model with intermediate dibaryon formation can account for all these states leading thus to a quantitative description of the corresponding experimental NN-phase-shifts in the range from 0 up to 1 GeV[4].
For the flavored sector many model predictions including LQCD [5] have been published with partly controversial results. Experimentally the search for the H-dibaryon is still going on. Very recently a deeply bound H-dibaryon, the socalled sexaquark has been proposed - being possibly even a dark matter candidate [6]. Possibly it could be searched for at the future K_long beamline of JLab.
[1] for a review see, e.g., H.Clement and T. Skorodko, Chin. Phys. C 45 (2021) 022001 and references therein
[2] M.Bashkanov and H. Clement, Nucl. Phys. A 1037 (2023) 122698
[3] H. Clement et al., Phys. Rev. C 106 (2022) 065204
[4] V. I. Kukulin et al., Chin. Phys. C 46 (2022) 114116
[5] HAL QCD, Phys. Rev. Lett. 127 (2021) 072003
[6] G. R. Farrar and N. Wintergerst, JHEP 12 (2023) 099 and references therein
