From CELSIUS to COSY: on the observation of a dibaryon resonance

Abstract

Using the high-quality beam of storage rings in combination with a pellet target and the hermetic WASA detector covering practically the full solid angle the two-pion production in nucleon-nucleon collisions has been systematically studied by exclusive and kinematically complete measurements -first at CELSIUS and subsequently at COSY. These measurements resulted in a detailed understanding of the two-pion production mechanism by t-channel meson exchange. The investigation of the ABC effect in double-pionic fusion reactions lead the trace to the observation of a narrow dibaryon resonance with I(J P ) = 0(3 + ) about 80 MeV below the nominal mass of the conventional ∆∆ system. New neutron-proton scattering data, taken with polarized beam at COSY, produce a pole in the coupled 3 D 3 -3 G 3 partial waves at (2380±10 − i 40±5) MeV establishing thus the first observation of a genuine s-channel dibaryon resonance.

First measurement of coherent double neutral-pion photoproduction on the deuteron at incident energies below 0.9 GeV

“…The resonance may be attributed to an isoscalar ∆∆ quasi-bound state, D 03 , predicted by Dyson and Xuong [4]. In addition to the π 0 π 0 d and π + π − d final states, evidence for the d * (2380) resonance has been confirmed by the WASA-at-COSY collaboration in the π 0 π − pp [5], π 0 π 0 pn [6], and π + π − pn [7] final states. The SAID partial wave analysis, which incorporates the analyzing power for the quasi-elastic np → np scattering measured by the WASA-at-COSY collaboration, also supports the existence of the d * (2380) resonance with quantum numbers I(J π ) = 0(3 + ) [8,9].…”

On the history of dibaryons and their final observation

“…Recently also the non-fusion two-pion production channels pn → ppπ 0 π − [29], pn → pnπ 0 π 0 [30], pn → pnπ + π − [360], which are partially isoscalar, have been investigated. The WASA results (solid circles) together with previous data (open symbols) as well as a recent data point (solid triangle) from HADES [366] are shown for the total cross sections in Fig.…”

Recent Results from WASA-at-COSY

“…has definte isospin I = 0. Next, also the non-fusion two-pion production channels pn → ppπ 0 π − [25], pn → pnπ 0 π 0 [26], pn → pnπ + π − [27] have been investigated. Since these reactions are only partially isoscalar, the contributions of the conventional processes relative to the expected isoscalar d * contribution are here much larger.…”