Structure in two-pion production in thedd→αXreaction

Abstract

A model for the dd → αππ reaction, based on two parallel NN → dπ reactions, is extended to incorporate a complete set of input amplitudes. While low-energy cross sections are underestimated, the rich structure observed in the α-particle momentum distributions for 0.8 < T d < 1.9 GeV (the ABC effect) is extraordinarily well reproduced. In addition, a recent measurement of deuteron analyzing powers agrees quite well with our predictions, both in frequency and magnitude of the oscillations.

Study of the ABC enhancement in the reaction

“…The resultant broad central structure at the maximum missing mass of the reaction is a notable feature in all cases where the ABC is seen clearly [4,5,7,8]. By taking the dominant pn → dπ 0 amplitudes into account in the calculation, a good description of the d d → 4 He X 0 spectrum and its angular dependence could be obtained [11]. In order to provide extra tests on this and other theoretical models, we report here upon an experiment to measure the deuteron vector and tensor analysing powers in d d → 4 He X 0 in a small angular region around the forward direction.…”

“…The model also predicts an enhancement when the pion momenta are antiparallel, though this is somewhat suppressed by the much smaller "sticking factor" when the two recoil deuterons are produced back to back [11]. The resultant broad central structure at the maximum missing mass of the reaction is a notable feature in all cases where the ABC is seen clearly [4,5,7,8].…”

“…A theoretical description actually looks simpler in the case of d d → 4 He X 0 , where double-pion production can be achieved by two pairs of beam and target nucleons undergoing a pn → dπ 0 reaction (or similarly for charged pion production), as illustrated in Fig. 1 [11].…”

“…The shape of the latter is well reproduced by the double-∆ model of ref. [11] where the theoretical calculation has been integrated over |θ x | ≤ 50 mrad, |θ y | ≤ 20 mrad. The predictions in the central bump are the most uncertain since, in this region, the dd → α "sticking" factor is required for high relative momenta.…”

“…[11], only the dominant NN → dπ partial wave amplitude was considered and this gives zero for both the vector and tensor analysing powers. By including all the significant amplitudes of the C500 solution of ref.…”