  A systematic experimental study of inclusive pion double charge exchange in
4He has been undertaken. The reaction 4He(pi+,pi-)4p was observed at incident
energies 120, 150, 180, 240 and 270 MeV; the 4He(pi-,pi+)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 deg and 130 deg. 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 (pi-,pi+) reaction in 3He. The forward-angle
spectra in the 3He and 4He reactions exhibit a prominent peak at high outgoing
pion energies. Interpretation of the peaks in 3He (4He) 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 pi-N interactions and the inclusion of nuclear
binding effects appear to be important in reproducing the shape of the energy
spectra at forward angles.
