Inclusive pion double charge exchange inHe4at intermediate energies

Abstract

A systematic experimental study of inclusive pion double charge exchange in 4 He has been undertaken. The reaction 4 He(π + , π − )4p was observed at incident energies 120, 150, 180, 240 and 270 MeV; the 4 He(π − , π + )4n reaction was observed at incident energies 180 and 240 MeV. At each incident energy, the doubly differential cross section was measured at three to five outgoing pion laboratory angles between 25 • and 130 • . At each angle, cross sections were measured over the range of outgoing pion energies from 10 MeV up to the kinematic limit for the reaction in which the final state consists of the oppositely charged pion plus four free nucleons.The spectra of outgoing pions are strikingly different from those observed for the inclusive double charge exchange reaction in heavier nuclei, but resemble those observed in the (π − , π + ) reaction in 3 He. The forward-angle spectra in the 3 He and 4 He reactions exhibit a prominent peak at high outgoing pion energies. Interpretation of the peaks in 3 He ( 4 He) as a three-(four-)nucleon resonance is ruled out by kinematic analysis. The results of a calculation, wherein the double charge exchange reaction is assumed to proceed as two sequential single charge exchange interactions, suggest that the high-energy peak is naturally explained by this double scattering mechanism.Non-static treatment of the πN interactions and the inclusion of nuclear binding effects appear to be important in reproducing the shape of the energy spectra at forward angles.Pion double charge exchange (DCX), a nuclear reaction in which two units of charge are exchanged between a pion and a nucleus, is an important tool with which to investigate the details of the pion-nucleus interactions as well as correlations between nucleons within the nucleus. DCX is unique among pion-nuclear scattering reactions, as it must involve at least two nucleons in order to conserve charge. Thus, the physiognomy of the DCX process revealed by the experimental cross section depends on the coordinates of two particles in the wavefunction of the nucleus. As a probe of the pion-nucleus interaction, DCX may be used to isolate contributions from different types of reaction mechanisms. Other pion-nucleus reactions appear to be dominated by single scattering mechanisms, but DCX must proceed through less simple mechanisms such as double scattering, as shown in Fig. 1(a).An experimental investigation of DCX in 4 He at intermediate energies has been performed in order to study the multiple scattering mechanisms of the π-nucleus reaction directly. The choice of 4 He as a nuclear target allows an investigation of DCX in an extensively studied few-body system for which microscopic treatment of the dynamics is possible in principle. At the same time, the 4 He nucleus has a large density which may allow some effects of the nuclear medium to be investigated. The final state is also relatively simple, consisting of a pion and either four protons or four neutrons, resulting from, respectively, the reactions 4 He(π + , π − )...

Inclusive pion double charge exchange in lightp-shell nuclei

“…This feature was most pronounced at an incident energy of 240 MeV, but persisted at incident energies down to 150 MeV in 4 He [7] and 120 MeV in 3 He [4].…”

“…2. The experimental apparatus and procedure have been discussed in detail by Wood et al [3], Yuly et al [4], and Kinney et al [7] and will be treated only briefly here.…”

“…Indeed, one observes a smooth progression in the shape of the spectra as A increases. [6,7], and in 3 He at all energies down to 120 MeV [4].…”

“…In this paper we report measurements of the doubly differential cross section for inclusive 2 DCX in the light p-shell nuclei 6,7 Li, 9 Be, and 12 C. These observations will serve to trace the evolution of the double peaked structure into a nearly featureless spectrum as the complexity of the target nucleus is increased. A second objective of the present work was to investigate, by comparing the (π + , π − ) reactions in 6 Li and 7 Li, the effect on the DCX cross section of a single extra neutron in the target nucleus.…”

“…Subsequently, an extensive series of measurements of inclusive double charge exchange cross sections was carried out at the Los Alamos Meson Physics Facility [3,4,5,6,7,8,9]. The (π − , π + ) reaction was studied in 3 He, and the (π ± , π ∓ ) reactions in nine nuclei: 4 He, 6,7 Li, 9 Be, 12 C, 16 O, 40 Ca, 103 Rh, and 208 Pb. Doubly differential cross sections were measured for each target over the outgoing pion energy range from 10 Mev up to the kinematic limit, at three to five angles for incident pion energies in the range from 120 to 270 MeV.…”