Exclusive and kinematically complete high-statistics measurements of the double pionic fusion reaction $dd\to {}^4\mathrm{He}{\pi }^0{\pi }^0$ have been performed in the energy range 0.8–1.4 GeV covering thus the region of the Abashian-Booth-Crowe effect, which denotes a pronounced low-mass enhancement in the $\pi \pi$ invariant mass spectrum. The experiments were carried out with the WASA detector setup at the cooler synchrotron at Forshungszentrum Julich GmbH. Similar to the observation in the basic $pn\to d{\pi }^0{\pi }^0$ reaction, the data reveal a correlation between the ABC effect and a resonancelike energy dependence in the total cross section. The maximum occurs at $m=2.37$ GeV $+$ 2$m_N$, i.e., at the same position as in the basic reaction. The observed resonance width $\Gamma \approx 160$ MeV can be understood from broadening due to Fermi motion of the nucleons in initial and final nuclei together with collision damping. Differential cross sections are described equally well by the hypothesis of a $pn$ resonance formation during the reaction process.

• Received 27 June 2012

DOI:https://doi.org/10.1103/PhysRevC.86.032201