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We report on an exclusive and kinematically complete high-statistics measurement of the basic doublepionic fusion reaction pn ! d 0 0 over the full energy region of the ABC effect, a pronounced low-mass enhancement in the -invariant mass spectrum. The measurements, which cover also the transition region to the conventional t-channel ÁÁ process, were performed with the upgraded WASA detector setup at COSY. The data reveal the Abashian-Booth-Crowe effect to be uniquely correlated with a Lorentzian energy dependence in the integral cross section. The observables are consistent with a narrow resonance PRL

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.

Exclusive and kinematically complete high-statistics measurements of quasifree polarized np scattering have been performed in the energy region of the narrow resonance-like structure d * with 2 I(J P ) = 0(3 + ), M ≈ 2380 MeV and Γ ≈ 70 MeV observed recently in the double-pionic fusion channels pn → dπ 0 π 0 and pn → dπ + π − . The experiment was carried out with the WASA detector setup at COSY having a polarized deuteron beam impinged on the hydrogen pellet target and utilizing the quasifree process dp → np + pspectator. This allowed the np analyzing power, Ay, to be measured over a broad angular range. The obtained Ay angular distributions deviate systematically from the current SAID SP07 NN partial-wave solution. Incorporating the new Ay data into the SAID analysis produces a pole in the 3 D3 − 3 G3 waves in support of the d * resonance hypothesis.

Exclusive measurements of the quasi-free np → npπ 0 π 0 reaction have been performed by means of dp collisions at T d = 2.27 GeV using the WASA detector setup at COSY. Total and differential cross sections have been obtained covering the energy region √ s = (2.35-2.46) GeV, which includes the region of the ABC effect and its associated d * (2380) resonance. Adding the d * resonance amplitude to that for the conventional processes leads to a reasonable description of the data. The observed resonance effect in the total cross section is in agreement with the predictions of Fäldt and Wilkin as well with those of Albadajedo and Oset. The ABC effect, i.e. the low-mass enhancement in the π 0 π 0 -invariant mass spectrum, is found to be very modest -if present at all, which might pose a problem to some of its interpretations.

New data on quasifree polarized neutron-proton scattering, in the region of the recently observed d * resonance structure, have been obtained by exclusive and kinematically complete high-statistics measurements with WASA at COSY. This paper details the determination of the beam polarization, 2 checks of the quasifree character of the scattering process, on all obtained Ay angular distributions and on the new partial-wave analysis, which includes the new data producing a resonance pole in the 3 D3-3 G3 coupled partial waves at (2380±10−i40±5) MeV -in accordance with the d * dibaryon resonance hypothesis. The effect of the new partial-wave solution on the description of total and differential cross section data as well as specific combinations of spin-correlation and spin-transfer observables available from COSY-ANKE measurements at T d = 2.27 GeV is discussed.

We calculate the charge distributions of d * (2380). Two different interpretations of the d * are considered for a comparison. One is a compact explanation with coupled ∆∆ + CC two-channel approximation in the chiral constituent quark model. Another is a resonance state of D12π. The remarkable differences of the charge distributions in the two pictures are shown and it is expected that the future experiments may provide a clear test for the different theoretical interpretations.

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