Coherent photoproduction of the neutral-pion and eta-meson on the deuteron at incident energies below 1.15 GeV

Coherent photoproduction of the neutral-pion and eta-meson on the deuteron, gamma d -> pi(0)eta d, was experimentally studied at incident photon energies ranging from the reaction threshold to 1.15 GeV. The total cross section demonstrates a rapid rise below 1 GeV. The data are underestimated by the existing theoretical calculations based on quasifree pi(0)eta photoproduction on the nucleon followed by deuteron coalescence. At the same time, the data are rather well reproduced by the calculations taking into account the final-state interaction. We also measured for the first time the differential cross sections: the pi(0)eta invariant-mass distribution d sigma/dM(pi eta), the pi(0)d invariant-mass distribution d sigma/dM pi d, the eta d invariant-mass distribution d sigma/dM(eta d), and the distribution over the deuteron emission angle d sigma/d Omega(d) in the overall center-of-mass frame. The measured cross section d sigma/d Omega(d) does not exhibit strongly backward-peaking behavior predicted by the calculations. At all incident energies, an increase in d sigma/dM(eta d) near the eta d threshold is observed, which indicates a bound or virtual eta d state resulting from a strong attraction between eta and a deuteron. The possibilities of using coherent pi(0)eta photoproduction on a nucleus to study the eta-nuclear interaction are also discussed.