Narrow Results

The ABC effect – an intriguing low-mass enhancement in the ππ invariant mass spectrum – is known from inclusive measurements of two-pion production in nuclear fusion reactions. Exclusive measurements have been carried out at CELSIUS/WASA and WASA@COSY for the fusion reactions leading to d, ³He and ⁴He. They all reveal this effect to be a σ channel phenomenon combined with a resonance-like behavior in the total cross section. The latter points to the formation of an isoscalar dibaryonic resonance, which couples to the pn channel as well as to a ΔΔ intermediate state.

Charmed dibaryon states with the spin-parity $J^{\pi }=0^{+},1^{+}$, and $2^{+}$ are predicted for the two-body $Y_{c}N$ ($={\mathrm{\Lambda }}_c$, ${\mathrm{\Sigma }}_c$, or ${\mathrm{\Sigma }}_c^{\ast }$) systems. We employ the complex scaling method for the coupled channel Hamiltonian with the $Y_{c}N$-CTNN potentials, which were proposed in our previous study. We find four sharp resonance states near the ${\mathrm{\Sigma }}_{c}N$ and ${\mathrm{\Sigma }}_c^{\ast }N$ thresholds. From the analysis of the binding energies of partial channel systems, we conclude that these resonance states are Feshbach resonances. We compare the results with the $Y_{c}N$ resonance states in the heavy quark limit, where the ${\mathrm{\Sigma }}_{c}N$ and ${\mathrm{\Sigma }}_c^{\ast }N$ thresholds are degenerate, and find that they form two pairs of the heavy-quark doublets in agreement with the heavy quark spin symmetry.