Possible Dibaryon Resonance around 2.23GeV/c2ind(

Abstract

The measured d(y 9 p)X cross section shows a rapid variation around a photon energy k = 390 MeV, not explained up to now by pion or nucleon rescattering processes. It is suggested that this and other related results could be explained by the existence of a dibaryonic resonance at a mass of 2.23 GeV/c 2 . 13.60.Rj, 25.20.+y One of the unanswered questions at the frontier between nuclear and particle physics is related to the possible existence of nonstrange dibaryon resonances. Such resonances are predicted by the existing models of strong interactions. 1 So far, the only available data which might show some evidence for such states come from N-N scattering experiments. 2 We report in this paper two experiments that have looked for dibaryon resonances in the JViV7r channel. They were performed with use of the bremsstrahlung photon beam of the linac at Saclay. In the first experiment, the reaction d(y,pif)p was investigated by detecting the emitted pion in coincidence with one of the two protons. The momenta of the particles were also measured so as to determine completely the kinematics of the reaction. The great advantage of this method lies in the possibility of reducing the contribution of the dominant process, namely the quasifree photoproduction, by considering different momenta P R of the recoiling system. Similarly to previous works, 3 we detected the pion with the "400"-MeV/ c magnetic spectrometer, but in order to achieve good resolution for P R (AP R ^ 10 MeV/c) and also to eliminate the large corrections brought in by the use of a range telescope, we detected the proton with the "700"-MeV/c magnetic spectrometer. 4 We kept constant the momentum P R of the undetected proton (P R = 150 MeV/c), the invariant mass Q of the pion-nucleon system (Q = 1245 MeV/c 2 ) and the angle co of the pion relative to the incident photon in the pion-nucleon rest frame (co = 105°). We varied only the angle 6 R of the recoiling proton and hence the photon energy k, since these are kinematically related. In order to compare our new data with the previous results 3 obtained for various kinematical conditions (1225<Q< 1250 MeV/c 2 , 130<P^<160 MeV/c, 90°<a><105°), we have plotted on the same figure (Fig. 1) and as a function of k the measured counting rates divided by the predic-tion of the first-order quasifree model (impulse approximation). The new set of data thus confirms the rapid variation of the cross section around k = 400 MeV.The results of a calculation 5 that takes into account the quasifree photoproduction process and the pion and proton rescattering up to the second order are also shown in Fig. 1. Although this model can successfully account for previous experimental results, 4 it fails to reproduce the observed bump (x 2 = 43 for y = 11 degrees of freedom). At the present time this discrepancy is very difficult to explain in terms of contributions from rescattering processes, such as the A-JV scattering. 6 It should be pointed out here that this peculiar behavior of the energy distribution 1. 5 L .0.5 Yd-ppTT ...

On the history of dibaryons and their final observation

“…Another intriguing structure in inclusive photo-induced pion production on the deuteron was reported from Saclay measurements, where a bumpy structure was observed in the energy dependence of the cross section around E γ = 400 MeV, which was associated with a dibaryon of mass 2230 MeV [133]. Subsequent exclusive high-resolution measurements of the γd → ppπ − reaction carried out at Bonn demonstrated that the ppπ − invariant mass distribution is flat showing no evidence for any dibaryon in the mass range 2160 -2320 MeV [134] -in agreement with corresponding pion-induced measurements carried out at Saclay [135] and SIN [136].…”

Laser-Electron-Gamma-Source. Progress report, July 1986

“…3and clearly bears no resemblance to the data. This exercise illustrates the sensitivity of polarization data to the presence of high-spin dibaryons.There are other bibaryon candidates that have been observed in y+d reactions, such as a 40 MeV wide peak reported by the Saclay group in the d(Y»p)X cross section at 2.23 GeV (Ey " 390 MeV},9 and the recent observation from Bonn of a very narrow «5 MeV) peak in the pp invariant mass distribution from d(y,pp}iT at 2.01 GeV 10. (If the latter were formed as an intermediate state in photodisintegration, it would appear at a photon energy of 143 MeV.)…”