Narrow Results

The pion optical potential generated by the hypothetical πNN-coupled NN-decoupled dibaryon resonance d′(2065) is calculated to the lowest order in nuclear matter density. The contribution to the pion optical potential is found to be within the empirical errors, so the d′(2065) existence currently does not contradict to the observed properties of the π^--nucleus bound states. Future progress in the pionic X-ray spectroscopy can reveal contributions of πNN resonances to energy levels and widths of the pionic atoms.

A model for nuclei described directly in terms of quarks has been developed in both relativistic and non-relativistic forms. It describes nuclear binding energy and structure for small nuclei (A=3,4) systematically correctly, including the EMC effect. With one free parameter each for strange and for nonstrange states, it also well describes low energy baryon-nucleon scattering, phase shifts and potentials. It predicts low mass, narrow dibaryon and pentaquark states. To be consistent with reported states, new physics may be required that is not included in any quark model to date.

We review the theoretical and experimental current status of the H dibaryon. We revise the recent experimental results obtained from double-Λ hypernuclei produced by means of Ξ-capture reactions using different target nuclei. Forthcoming experimental projects are briefly summarized. We also review the recent theoretical quark-model studies, some of them constrained by observables of the strangeness -2 two-baryon sector. Ongoing theoretical efforts by means of lattice QCD are also discussed.

Several new findings in the four, five and six quark systems reheat the interest in the field of multiquark states (beyond the trivial $q\bar{q}$ and $qqq$). 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 $\mathrm{\Gamma }$ = 70 MeV and $I(J^P)=0(3^{+})$ has been consistently observed in a wealth of reaction channels, supporting the existence of a resonant dibaryon state - the $d^{\ast }(2380)$. 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 $d^{\ast }(2380)$ is largely unknown. It can contain various ”hidden color” 6q configurations, $\mathrm{\Delta }\mathrm{\Delta }$ 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 $d^{\ast }(2380)$ dibaryon, but does not settle the issue. The $d^{\ast }$ 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.