Polarization phenomena in inelastic meson photoproduction on a nucleus whose excited final state undergoes fast gamma decay

Incoherent Neutral Pion Photoproduction onC12

“…The importance of the incoherent (γ, π 0 ) process from nuclear targets has been appreciated for some time [1,2,3]. However, although nuclear π 0 photoproduction has been studied at various facilities for over 30 years, no results for the population of discrete residual nuclear states have been obtained because the accuracy of the angle and the energy determination of the photons from the π 0 → 2γ decay needed to resolve states in the residual nucleus has not been achieved.…”

“…The predictions highlight the sensitivity of the incoherent reaction to the character of the nuclear transition involved and to specific ∆-nucleus processes such as ∆N interactions, which have a much smaller effect on other observables such as the coherent cross section. The other calculation [2] is less sophisticated. It uses the plane wave impulse approximation and makes a rough estimate of the effect of the π 0 -nucleus final-state interaction.…”

“…The required π 0 -γ angular correlation was taken from Ref. [2] where it is given in terms of the angle α between the decay photon and the 12 C recoil direction which has the distribution 15 8 sin 2 (2α). The combined π 0 -γ detection efficiency so obtained varies from ∼20 to 30% over the π 0 angular range 30-160 • but is smaller outside this range due to holes in the CB at forward and backward angles.…”

“…The validity of the π 0 -γ angular correlation obtained in Ref. [2], which was used to calculate the overall CB efficiency, was checked by plotting the experimental distribution of the angle α. Figure 4 shows the data for incident energies E γ =235±10 MeV.…”

“…In fact the calculation of Ref. [2] suggests that the spin-dependent terms will also provide a contribution to the incoherent excitation of the 4.4-MeV state at the few percent level and that this contribution will have a cos 2 (2α) distribution. Such a contribution may account for some of the remaining discrepancy between the data and the prediction in Fig.…”