Charged-Particle Versus Neutron-Induced Reactions

Abstract

The questions of charge symmetry and charge independence (isospin conservation) and their possible breaking have always been important, also as a field where mass differences between up and down quarks in the nucleons might show up. Examples for such effects are the Nolen-Schiffer anomaly of the Coulomb energy differences of mirror nuclei [1] and the difference between the scattering lengths \(a_{\rm nn},\,a_{\rm pp},\) and \(a_{\rm np}\) of the singlet \(^1S_0\) nucleon-nucleon interaction (for a recent discussion see Ref. [2]). When initiating nuclear reactions with protons and comparing them to their neutron-induced mirror reaction, after correctly subtracting the “trivial” Coulomb part of the interaction, under isospin conservation the remaining observables should be equal. This is of course true for all, i.e. also polarization observables. Therefore, the use of polarized neutrons which normally have to be produced in special nuclear reactions (see Sect. 10.2) is very important. Examples are the intensive study of the three-particle breakup reaction \(^{2}\hbox{H(p,pp)n}\) and comparison with \(^{2}\hbox{H(n,nn)}^{1}\hbox{H},\) as well as of the elastic scatterings \(^{2}\hbox{H(p,p)}^{2}\hbox{H}\) and \(^{2}\hbox{H(n,n)}^{2}\hbox{H}.\) Although neutron-induced reactions are technically more difficult and in general less precise than their proton-induced counterparts they are important because the realistic inclusion of the long-range Coulomb interaction into “numerically exact” Faddeev calculations has been achieved only recently allowing now the realistic study of charged-particle three-body reactions. The comparison between both revealed not only discrepancies between them, but of both with most advanced theories such as meson exchange or EFT Faddeev-type calculations (for recent discussions of these low-energy discrepancies see e.g. Refs. [3, 4]).