Search For Supernarrow Dibaryons

The two nucleon system contains only a single stable bound state, the deuteron. Many experiments have been performed in the last few decades to search for evidence of other two nucleon bound states or resonances, collectively called dibaryons. The various reactions studied have produced multiple candidates whose masses range from 1900 MeV to 2150 MeV. This experiment was the first search for dibaryons with masses below pion threshold using the reaction γ d → π+D*(D* → γ nn) where the π+, γ, and one neutron were detected. The key feature of the present experiment was the detection of the photon from the decay of the dibaryon without which the data would be completely dominated by the quasi-free pion production from the proton in the deuteron. The data suggested the existence of at least two supernarrow dibaryons. Previous experiments produced narrow isolated peaks, while the present experiment was limited by the resolution of pion detection. The three measured dibaryon masses of 1904+9-14 MeV (90% C.L.), 1926+11-12 MeV (90% C.L.), and 1950 ± 7 MeV agree within uncertainties to those measured by Fil'kov. The total photoproduction cross sections associated with the peaks are 25 ± 13 nb, 23 ± 12 nb, and 14 ± 10 nb