Photoproduction ofeta-pi pairs off nucleons and deuterons

Abstract

Quasi-free photoproduction of $\pi \eta$-pairs has been investigated from threshold up to incident photon energies of 1.4 GeV, respectively up to photon-nucleon invariant masses up to 1.9 GeV. Total cross sections, angular distributions, invariant-mass distributions of the $\pi \eta$ and meson-nucleon pairs, and beam-helicity asymmetries have been measured for the reactions $\gamma p\to p{\pi }^0\eta$, $\gamma n\to n{\pi }^0\eta$, $\gamma p\to n{\pi }^{+}\eta$, and $\gamma n\to p{\pi }^{-}\eta$ from nucleons bound inside the deuteron. For the $\gamma p$ initial state data for free protons have also been analyzed. Finally, the total cross sections for quasi-free production of ${\pi }^0\eta$ pairs from nucleons bound in ${}^3$He nuclei have been investigated in view of final state interaction (FSI) effects. The experiments were performed at the tagged photon beam facility of the Mainz MAMI accelerator using an almost $4\pi$ covering electromagnetic calorimeter composed of the Crystal Ball and TAPS detectors. The shapes of all differential cross section data and the asymmetries are very similar for protons and neutrons and agree with the conjecture that the reactions are dominated by the sequential ${\Delta }^{\star }3/2^{-}\to \eta \Delta (1232)\to \pi \eta N$ decay chain, mainly with $\Delta (1700)3/2^{-}$ and $\Delta (1940)3/2^{-}$. The ratios of the magnitude of the total cross sections also agree with this assumption. However, the absolute magnitudes of the cross sections are reduced by FSI effects with respect to free proton data.

arXiv commentaccepted for publication in Eur. Phys. J. A

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• First measurement of the polarization observable $E$ and helicity-dependent cross sections in single ${\pi }^0$ photoproduction from quasi-free nucleons
• Experimental study of the $\gamma p\to K^0{\Sigma }^{+}$, $\gamma n\to K^0\Lambda$, and $\gamma n\to K^0{\Sigma }^0$ reactions at the Mainz Microtron
• Study of the $\gamma p\to {\pi }^0\eta p$ reaction with the A2 setup at MAMI
• Observation of Narrow $N^{+}(1685)$ and $N^0(1685)$ Resonances in $\gamma N\to \eta \pi N$ Reactions
• Observation of a structure in the M${}_{p\eta }$ invariant mass distribution near 1700 MeV/$c^2$ in the $\gamma \mathbf{p}\to \mathbf{p}{\pi }^0\eta$ reaction
• Helicity-dependent cross sections for the photoproduction of ${\pi }^0$ pairs from nucleons
• First measurement of helicity-dependent cross sections in pi0-eta photoproduction from quasi-free nucleons
• Coherent photoproduction of the neutral pion and eta meson on the deuteron at incident energies below 1.15 GeV
• Measurement of the beam-helicity asymmetry in photoproduction of ${\pi }^0\eta$ pairs on carbon, aluminum, and lead
• Search for the $N$(1685) in $\eta \pi$-Photoproduction