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Register a new userThe role of pion exchange in $eta$ meson photo production on the deuteron
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Incoherent $eta$ photoproduction on the deuteron is studied with the main emphasis on the role of final state interactions. In addition to the previously studied mechanisms of $NN$ and $eta N$ rescatterings, the role of an intermediate pion exchange is considered in detail, where first a pion is photoproduced on one nucleon and then rescatters into an $eta$ meson on the other, the spectator nucleon. It is found, that the role of this pion mediated contribution is comparable in size to that of $eta N$ rescattering. Results for total and semi-inclusive differential cross sections and associated polarization observables are presented. In particular polarization observables show a significant sensititvity to final state interactions.
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Chiral effective field theory (ChEFT) is a modern framework to analyze the properties of few-nucleon systems at low energies. It is based on the most general effective Lagrangian for pions and nucleons consistent with the chiral symmetry of QCD. For energies below the pion-production threshold it is possible to eliminate the pionic degrees of freedom and derive nuclear potentials and nuclear current operators solely in terms of the nucleonic degrees of freedom. This is very important because, despite a lot of experience gained in the past, the consistency between two-nucleon forces, many-nucleon forces and the corresponding current operators has not been achieved yet. In this presentation we consider the recently derived long-range two-pion exchange (TPE) contributions to the nuclear current operator which appear at next-to leading order of the chiral expansion. These operators do not contain any free parameters. We study their role in the deuteron photodisintegration reaction and compare our predictions with experimental data. The bound and scattering states are calculated using five different chiral N2LO nucleon-nucleon (NN) potentials which allows to estimate the theoretical uncertainty at a given order in the chiral expansion. For some observables the results are very close to the reference predictions based on the AV18 NN potential and the current operator (partly) consistent with this force.
We perform an updated coupled-channel analysis of eta-meson production including all recent photoproduction data on the proton. The dip observed in the differential cross sections at c.m. energies W=1.68 GeV is explained by destructive interference between the $S_{11}(1535)$ and $S_{11}(1560)$ states. The effect from $P_{11}(1710)$ is found to be small but still important to reproduce the correct shape of the differential cross section. For the $pi^- N to eta N$ scattering we suggest a reaction mechanism in terms of the $S_{11}(1535)$, $S_{11}(1560)$, and $P_{11}(1710)$ states. Our conclusion on the importance of the $S_{11}(1535)$, $S_{11}(1560)$, and $P_{11}(1710)$ resonances in the eta-production reactions is in line with our previous results. No strong indication for a narrow state with a width of 15 MeV and the mass of 1680 MeV is found in the analysis. $eta N$ scattering length is extracted and discussed.
The two-step model with pi-, rho- and omega- exchanges taken into account is applied to investigate the reactions pn -> d eta and pn -> d eta-prime. The existing experimental data on the reaction pn -> d eta are analysed and predictions for the cross section of the reaction pn -> d eta-prime are presented. It is found that pi- as well as rho exchange yield significant contributions in both reactions. The effect of the final state interaction is also studied.
Small angle electron scattering with intense electron beams opens up the possibility of performing almost real photon induced reactions with thin, polarized hydrogen and few body targets, allowing for the detection of low energy charged particles.This promises to be much more effective than conventional photon tagging techniques. For photo-pion reactions some fundamental new possibilities include: tests of charge symmetry in the N-N system by measurement of the neutron-neutron scattering length $a_{nn}$ in the $gamma D rightarrow pi^{+} nn$ reaction; tests of isospin breaking due to the mass difference of the up and down quarks; measurements with polarized targets are sensitive to $pi$N phase shifts and will test the validity of the Fermi-Watson (final state interaction) theorem. All of these experiments will test the accuracy and energy region of validity of chiral effective theories.
The production of $eta$ mesons in the reactions $ppto ppeta$ and $pnto pneta $ at threshold-near energies is analyzed within a covariant effective meson-nucleon theory. The description of cross section and angular distributions of the available data in this kinematical region in the $pp$ channel is accomplished by including meson currents and nucleon currents with the resonances $S_{11}(1650)$, $P_{11}(1710)$ and $P_{13}(1720)$. Predictions for the $pn$ channel are given. The di-electron production from subsequent $eta$ Dalitz decay $eta to gamma gamma^* togamma e^+e^-$ is also calculated and numerical results are presented for intermediate energy and kinematics of possible experiments with HADES, CLAS and KEK-PS.
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