Experimental search for nonstrange narrow isovector dibaryons

Missing mass spectra from ${}_{}{}^3{}_{}{}^{}\mathrm{He}$(p,d)X at $T_{\mathrm{p}=0.75\mathrm{}}$ GeV (${\theta }_{\mathrm{lab}=22\mathrm{}}$°, 32°, and 40°) and $T_{\mathrm{p}=0.925\mathrm{}}$ GeV (${\theta }_{\mathrm{lab}=30\mathrm{}}$° and 40°) have been measured. Missing mass spectra from p${(}^3$He,d)X at $T_{{\mathrm{}}^3\mathrm{He}=2.7\mathrm{}}$ GeV and ${\theta }_{\mathrm{lab}=18\mathrm{}}$° have also been measured. The experiments have been carried out with a high missing mass resolution in order to detect possible narrow structures associated with B=2, T=1 quantum numbers. Such structures have been seen, with the following masses and widths: $M_x$=2.240±0.005, (${\Gamma }_{1/2}$≊0.016±0.003) GeV, $M_x$=2.192±0.003, (${\Gamma }_{1/2}$≊0.025±0.006) GeV, and $M_x$=2.121±0.003, (${\Gamma }_{1/2}$≊0.025±0.002) GeV. A broad structure with mass close to $M_x$≊2.17 GeV, the mass of free N+Δ, and a width close to ${\Gamma }_{1/2}$≊0.1 GeV is observed. The observed peaks might be related to six-quark states.