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

Abstract

Coherent photoproduction of the neutral pion and eta meson on the deuteron, γd→π 0 ηd, has been 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 quasi-free π 0 η 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 have also measured for the first time the differential cross sections: the π 0 η invariant-mass distribution dσ/dMπη, the π 0 d invariant-mass distribution dσ/dM πd , the ηd invariant-mass distribution dσ/dM ηd , and the distribution over the deuteron emission angle dσ/dΩ d in the overall center-of-mass frame. The measured cross section dσ/dΩ d does not exhibit strongly backward-peaking behavior predicted by the calculations. At all incident energies, an increase in dσ/dM ηd near the ηd threshold is observed, which indicates a bound or virtual ηd state resulting from a strong attraction between η and a deuteron. The possibilities of using coherent π 0 η photoproduction on a nucleus to study the η-nuclear interaction are also discussed.

Theoretical study of the $γd \to π^0ηd$ reaction

“…We shall see in the present work that these shifts are tied to the dynamics of the γp → π 0 ηp reaction, something that is also found using the model of Ref [13] at the level of the impulse approximation in Ref. [24].…”

“…As to using the results of the reaction to claim a possible ηd bound state, as claimed in Refs. [23,24], it is a difficult task given the intrinsic uncertainties of the conventional mechanisms disclosed by our calculations. The striking experimental shape of the angular distribution will require further thoughts along other mechanisms not envisaged by us, and any other theoretical calculation so far, that help share the momentum transfer, which is extremely large for forward deuteron angles.…”

“…[23] and further mass and angular distributions have been recently reported in Ref. [24]. The experimental data are compared with theoretical results from Refs.…”

“…In view of this, a pure empirical model was suggested in Ref. [24] based on the sum of the two amplitudes depicted in Fig. 3.…”

“…One of the aims of the work of Refs. [23,24] was to show that the data demanded the presence of a bound ηd state or maybe a virtual state, and the shift of the peaks of π 0 d, ηd invariant mass distributions with respect to the phase space distributions were considered in Ref. [23] as a signal of the presence of this bound state.…”