Evidence for excitation of two resonance states in the isovector two-baryon system with a mass of 2.2 GeV/c2

Abstract

We report on measurements of the differential cross section dσ/d and the first measurement of the analyzing power A y in the (1232) excitation energy region of the reaction pp → {pp} s π 0 where {pp} s is a 1 S 0 proton pair. The experiment has been performed with the ANKE spectrometer at COSY-Jülich. The data reveal a peak in the energy dependence of the forward {pp} s differential cross section, a minimum at zero degrees of its angular distribution, and a large analyzing power. The results present a direct manifestation of two two-baryon resonance-like states with J P = 2 − and 0 − and an invariant mass of 2.2 GeV/c 2 .

On the history of dibaryons and their final observation

“…In a very recent series of measurements carried out at the ANKE detector at COSY the pp → ppπ 0 reaction has been investigated under the condition that the emitted proton pair is in relative S-wave, i.e. in the 1 S 0 state [419]. That way the ANKE measurements are complementary to the reaction pp → dπ + , where the nucleons within the deuteron are in relative 3 S 1 state -neglecting the small D-wave admixture in the deuteron.…”

The legacy of the experimental hadron physics programme at COSY

“…This was shown more clearly in an amplitude analysis that took into account new ANKE measurements of the analyzing power. This provided evidence for resonance structure in both s-and d-wave π 0 production [135].…”

“…It is immediately obvious from looking at the four figures that the ANKE data are reasonably well described by the model at 600 MeV per nucleon but the agreement is less satisfactory at 1. 135 GeV, where the model should be more reliable. The deviations in the differential cross section are on the 10-15% level, as they are also for the deuteron tensor analyzing powers and the spin correlations.…”

On the possibility of revealing the transition of a baryon pair state to a six-quark confinement state

“…The only important positive result of numerous experiments was establishment of three resonance states 1 D 2 , 3 F 3 , 3 P 2 (see [13,14] and refs. therein), and recently 3 P 0 [15] in the pp interactions. The position of the corresponding poles of the S matrix in the complex energy plane close to the ∆N branching line and the width comparable with that of the ∆(1232) resonance led to a common interpretation of the resonances as the conventional hadron states in the ∆N channel but not the quark dibaryons.…”

“…In the fixed-target experiments, the laboratory beam energy corresponds to T min lab ≈ 1.1-2.5 GeV. It means in particular that the resonances observed in the pp collision at 0.7 GeV [15] and in the pn collision at 1.14 GeV [16,17] are most likely to be of a meson-baryon nature, the more so that criteria (2) of the central collisions are not satisfied there.…”