Abstract
QCD-motivated models for hadrons predict an assortment of "exotic" hadrons that have structures that are more complex than the quark-antiquark mesons and three-quark baryons of the original quark-parton model. These include pentaquark baryons, the six-quark H-dibaryon, and tetraquark, hybrid and glueball mesons. Despite extensive experimental searches, no unambiguous candidates for any of these exotic configurations have been identified. On the other hand, a number of meson states, one that seems to be a proton-antiproton bound state, and others that contain either charmed-anticharmed quark pairs or bottom-antibottom quark pairs, have been recently discovered that neither fit into the quark-antiquark meson picture nor match the expected properties of the QCD-inspired exotics. Here I briefly review results from a recent search for the H-dibaryon, and discuss some properties of the newly discovered states --the proton-antiproton state and the so-called XYZ mesons-- and compare them with expectations for conventional quark-antiquark mesons and the predicted QCD-exotic states.
Highlights
Pentaquarks and H-dibaryonsThe Belle experiment recently reported results of a search for production of an H-dibaryon with mass near 2mΛ in inclusive Υ(1S) and Υ(2S) decays [12]
The recent BESIII findings, taken together with previous experimental results, establishes a concentration of charmoniumlike states crowding the DD ∗ and D∗D ∗ mass threshold regions and bottomoniumlike isospin triplets near the BB∗ and B∗B∗ thresholds, which are suggestive of molecule-like structures [118,119,120,121,122,123,124,125,126,127,128,129]
A tetraquark interpretation of the charged bottomoniumlike Zb(10610) and Zb(10650) states is advocated by Ali and collaborators [172, 173]
Summary
The Belle experiment recently reported results of a search for production of an H-dibaryon with mass near 2mΛ in inclusive Υ(1S) and Υ(2S) decays [12]. It was subsequently pointed out that the BW form used by BESII should be modified to include the effect of final-state-interactions on the shape of the ppmass spectrum [19, 20] When this was done, it was found that the effects of FSI are not sufficient to explain the observed structure and the peak mass of the BW term shifted downward, from 1859 MeV to 1831 ± 7 MeV while the range of allowed widths increased to Γ < 153 MeV. A subsequent BESII study of J/ψ → γπ+π−η decays found a distinct peak at 1834±7 MeV and width 68 ± 21 MeV as shown in panel (c) of Fig. 3 [22], in good agreement with the mass and width results from the FSI-corrected fit to the ppmass spectrum. BESIII is actively looking at various other radiative J/ψ decay channels for evidence for or against other signs of resonance behaviour near 1835 MeV [28]
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