Study of the ABC enhancement in the ~ d d → αX 0 reaction

The ~ d d → αX reaction at beam energies close to the η threshold shows very strong structure in the missing mass corresponding to the ABC enhancement. The deuteron tensor analysing power Ayy, and the slope of the vector analysing power Ay with respect to angle, have been measured for this reaction around the forward direction. Both signals are small, and their variations with the α-particle momentum are in broad agreement with a theoretical model in which each pair of nucleons in the projectile and target deuterons undergoes pion production through the NN → dπ reaction. e-mail: [email protected] e-mail: [email protected] The discovery of a sharp enhancement in the missing mass spectrum of the p d → HeX reaction almost forty years ago [1] excited great interest since it seemed to indicate either an enormous pion-pion scattering length (typically 3 fm [2]) or a resonance in that system at only 30 MeV above the 2π threshold. The absence of any effect in the parallel p d → HX case showed that the anomalous behaviour was associated with isospin-zero pion pairs and hence presumably s-wave. However, the weight of evidence is that the isoscalar scattering length is small [3], so that the ABC anomaly cannot be an intrinsic property of the two-pion system but must be associated with the presence of other particles. The characteristics of the ABC phenomenon were independently confirmed in the p d → HeX reaction [4] and exactly the same type of effect was observed in both pn → dX [5] and pp → ppX at low pp excitation energies [6]. The most splendid ABC manifestation is in the d d → HeX reaction in the 1 GeV region, where it dominates a large fraction of the differential cross section, with sharp peaks associated with its production in the forward and backward c.m. hemispheres [7, 8]. Since the masses and widths of these peaks change somewhat with beam energy and production angle, this reinforces the idea that the ABC might be some kind of kinematic enhancement. The simplest model for ABC production in pn → dX involves the excitation of both nucleons into ∆-isobars through one pion exchange where, after the decay of the two ∆’s, the final neutron-proton pair sticks together to produce the observed deuteron [9, 10]. A theoretical description actually looks simpler in the case of d d → HeX, where double-pion production can be achieved by two pairs of beam and target nucleons undergoing a pn → dπ reaction (or similarly for charged pion production), as illustrated in Fig. 1 [11]. Each of the single-pion-production amplitudes can be driven by virtual ∆ excitation, and this allows the large momentum transfer to be shared evenly amongst all the nucleons. If the Fermi momenta in the initial deuterons are neglected, then the dd c.m. system is also that in the np channel, which means that the produced pions have the same energy. The dominantly p-wave nature