Double-Pionic Fusion of Nuclear Systems and the “ABC” Effect: Approaching a Puzzle by Exclusive and Kinematically Complete Measurements

Abstract

The ABC effect-a puzzling low-mass enhancement in the pipi invariant mass spectrum, first observed by Abashian, Booth, and Crowe-is well known from inclusive measurements of two-pion production in nuclear fusion reactions. Here we report on the first exclusive and kinematically complete measurements of the most basic double-pionic fusion reaction pn-->dpi;{0}pi;{0} at beam energies of 1.03 and 1.35 GeV. The measurements, which have been carried out at CELSIUS-WASA, reveal the ABC effect to be a (pipi)_{I=L=0} channel phenomenon associated with both a resonancelike energy dependence in the integral cross section and the formation of a DeltaDelta system in the intermediate state. A corresponding simple s-channel resonance ansatz provides a surprisingly good description of the data.

QCD sum rule study of thed*(2380)

“…Recently, the WASA detector at COSY confirmed their early observations of the d * (2380) [1][2][3][4]6]: to explore the nature of the ABC effect [7], they measured the polarized np scattering, and their partial-wave analysis exhibited a resonance pole at (2380 ± 10 − i40 ± 5) MeV with quantum numbers I(J P ) = 0(3 + ) [5]. The d * (2380) may be a ∆∆ dibaryon or a six-quark state, first predicted on the basis of SU (6) symmetry by Dyson and Xuang in 1964 [8].…”

Di-pion and di-electron production in NN reactions with HADES at 1.25GeV incident beam energy.

“…The systematic studies of the π 0 π 0 production [5] by the WASA collaboration indeed revealed a resonance-like feature in the total cross section in n-p reactions at energies below the ∆∆ excitation associated with an unexpected enhancement at low ππ invariant mass, this is called the ABC effect. The effect appears in the di-pion isospin 0 channel and cannot be described by channels like ∆∆ or N*(1440).…”

“…The effect appears in the di-pion isospin 0 channel and cannot be described by channels like ∆∆ or N*(1440). To explain the experimental results, a di-baryon resonance (d*) has been proposed with the width of 70 MeV and a pole mass of 2380 MeV [5]. The excitation of such a six-quark object should also be visible in the reaction pn → dπ + π − , which is an admixture of isospin 0 and 1 states.…”

“…The data are now being compared to the predictions of ref. [5], including d* dibaryon resonance, ∆∆ excitation and N*(1440). The preliminary results show a good description of the total yield.…”

“…More generally, these complementary data provide quantitative information on hadronic interactions, as well as resonance excitations and resonance properties. Recently a resonant structure in the cross section of the reaction pn → dπ 0 π 0 was observed by the WASA collaboration [5] and interpreted as a dibaryon resonance at M = 2380 MeV/c 2 . It is therefore important to check that the other pion production channels in the same energy range are consistent with this interpretation.…”