Photoproduction of η -mesons off nuclei for Eγ ⩽ 2.2 GeV

Abstract

Photoproduction of η mesons off 12 C, 40 Ca, 93 Nb, and nat Pb nuclei has been measured with a tagged photon beam with energies between 0.6 and 2.2 GeV. The experiment was performed at the Bonn ELSA accelerator with the combined setup of the Crystal Barrel and TAPS calorimeters. It aimed at the in-medium properties of the S11(1535) nucleon resonance and the study of the absorption properties of nuclear matter for η mesons. Careful consideration was given to contributions from ηπ final states and secondary production mechanisms of η-mesons e.g. from inelastic πN reactions of intermediate pions. The analysis of the mass number scaling shows that the nuclear absorption cross section σNη for η mesons is constant over a wide range of the η momentum. The comparison of the excitation functions to data off the deuteron and to calculations in the framework of a BUU-model show no unexplained in-medium modifications of the S11(1535).

Production of η and η′ mesons on nucleons and nuclei

“…In general, the agreement between the different measurements is good (see comparisons in [152]). Typical results for total inclusive cross sections for heavy nuclei from the most recent measurement [152], covering the widest energy range, are summarized in Fig. 22 (angular and energy-dependent differential cross sections are also given in [152]).…”

“…The situation is completely different for η mesons. Almost independent of their kinetic energy, over a wide range from a few 10 MeV to 1 GeV, the scaling coefficient is close to 2/3 [151,152]. The absorption is large, which means that the meson -nucleon interaction is strong.…”

“…Inclusive photoproduction of η mesons from nuclei has been measured for the deuteron [43,51,52,53,55], for 3 He [45], and for heavier nuclei such as 12 C, 40 Ca, 63 Cu, 93 Nb, nat Pb [151,152,158,159] at MAMI, at KEK, in Tohoku, and at ELSA. In general, the agreement between the different measurements is good (see comparisons in [152]). Typical results for total inclusive cross sections for heavy nuclei from the most recent measurement [152], covering the widest energy range, are summarized in Fig.…”

“…Typical results for total inclusive cross sections for heavy nuclei from the most recent measurement [152], covering the widest energy range, are summarized in Fig. 22 (angular and energy-dependent differential cross sections are also given in [152]). The experimental results are compared to calculations in the framework of the Giessen version (see [160] for a recent review) of the Boltzmann-Uehling-Uhlenbeck (BUU) transport model, which incorporates all relevant elementary production cross sections and then traces the space-time evolution of an ensemble of interacting particles N, N , ∆, π, η, ... in nuclear matter from the moment of their creation to their absorption or their escape through the outer boundaries of the nucleus.…”

“…Furthermore, the pions may be lost due to FSI in the production nucleus. However, one could argue that, for the measurement of the absorption properties of η mesons in nuclear matter, such effects do not matter much because the absorption probability of an η depends only on its kinetic energy and not on its initial production Figure 22: Total cross sections for η photoproduction off heavy nuclei from [152]. Left hand side: fully inclusive η production (γA → Xη, no restriction on X, which can be a multiparticle final state, possibly also including further mesons such as pions).…”