Measurement of Charged Pion Photoproduction at ELPH

We investigated the charged pion photoproduction on the deuteron in the energy region Eγ = 0.65− 1.1 GeV at Research Center for Electron Photon Science (ELPH), Tohoku University. The photon absorption by a nucleus mainly takes place on a single nucleon at energies above the first resonance region. The rest nucleons are called as the spectator and this process is called as the quasi-free process. The pion is emitted in the subsequent de-excitation process of the nucleon resonance excited through the absorption of the photon. The deuteron is the loosely bound system of the proton and the neutron and the photoreaction on it is considered to be dominated by the quasi-free process. However, significant contribution of the two nucleon absorption process in the π+π− photoproduction on the deuteron was measured in the energy region from 0.6 to 1.1 GeV[1, 2]. The importance of the nucleon resonances and the intermediate and final state interaction was suggested by the theoretical studies[3, 4]. For the detailed study of the intermediate states, precise experimental data, not only the total cross section but also the differential cross sections are required. The ABC effect [5] is a nuclear fusion reaction which is known to be related to the excitation of the two nucleon resonances (ΔΔ) in the intermediate state and their interaction. It is distinguished by a structure in the mass distribution of isoscalar pion pair[6, 7, 8]. One of our aim is the search for the ABC like effect in the photon induced double pion production reaction on the deuteron in the analysis of the similar final state with ABC effect (γd → π+π−d) and the explicit ΔΔ excitation (γd → ΔΔ → π+π−pn). The reactions d(γ, π+π−d) and d(γ, π+π−p)n were measured using the Neutral Kaon Spectrometer 2 (NKS2) at ELPH. The NKS2 is the magnetic spectrometer with the geometrical acceptance roughly 1/4 of the whole solid angle, dedicated for the multi-particle tracking. By refining the calibration of the detectors and the tracking procedures, the increased mass reconstruction resolution enabled us the efficient identification of these reactions with three charged particles in the final state. Simulation studies have been carried out for the extraction of the contributions of the intermediate states. We report on the current status of the analysis for the derivation of the cross sections of these reactions with the extraction of the intermediate states.