Investigation of the anomaly in eta-photoproduction off the neutron

Quasi-free photoproduction of eta-mesons off the neutron and off the proton has been studied using a deuterium target and bremsstrahlung photons produced by MAMI-C with incident energies up to 1.5 GeV. The eta-mesons were detected in coincidence with the recoil nucleons thus a fully exclusive measurement was performed. Preliminary results show a bump-like structure in the excitation function for the neutron close to W $\approx$ 1675 MeV which is not seen for the proton. Considering the experimental resolution and using a Breit-Wigner fit the width of this structure was approximated below 50 MeV.Comment: oral contribution to the workshop on physics of the excited nucleon - NSTAR 2009, Beijing, April 200

Interference phenomena in the JP=1/2−J^P=1/2^--wave in η\eta photoproduction

The recent precise experimental results for the photoproduction of $\eta$-mesons off the neutron measured with the Crystal Ball/TAPS calorimeter at the MAMI accelerator have been investigated in detail in the framework of the Bonn-Gatchina coupled channel model. The main result is that the narrow structure observed in the excitation function of $\gamma n \rightarrow n\eta$ can be reproduced fully with a particular interference pattern in the $J^P=1/2^-$ partial wave. Introduction of the narrow resonance $N(1685)$ with the properties reported in earlier publications deteriorates the quality of the fit.Comment: 10 pages, 13 figures, accepted for publication in EPJ

Proton Zemach radius from measurements of the hyperfine splitting of hydrogen and muonic hydrogen

While measurements of the hyperfine structure of hydrogen-like atoms are traditionally regarded as test of bound-state QED, we assume that theoretical QED predictions are accurate and discuss the information about the electromagnetic structure of protons that could be extracted from the experimental values of the ground state hyperfine splitting in hydrogen and muonic hydrogen. Using recent theoretical results on the proton polarizability effects and the experimental hydrogen hyperfine splitting we obtain for the Zemach radius of the proton the value 1.040(16) fm. We compare it to the various theoretical estimates the uncertainty of which is shown to be larger that 0.016 fm. This point of view gives quite convincing arguments in support of projects to measure the hyperfine splitting of muonic hydrogen.Comment: Submitted to Phys. Rev.

The {\eta}'-carbon potential at low meson momenta

The production of $\eta^\prime$ mesons in coincidence with forward-going protons has been studied in photon-induced reactions on $^{12}$C and on a liquid hydrogen (LH$_2$) target for incoming photon energies of 1.3-2.6 GeV at the electron accelerator ELSA. The $\eta^\prime$ mesons have been identified via the $\eta^\prime\rightarrow \pi^0 \pi^0\eta \rightarrow 6 \gamma$ decay registered with the CBELSA/TAPS detector system. Coincident protons have been identified in the MiniTAPS BaF$_2$ array at polar angles of $2^{\circ} \le \theta _{p} \le 11^{\circ}$. Under these kinematic constraints the $\eta^\prime$ mesons are produced with relatively low kinetic energy ($\approx$ 150 MeV) since the coincident protons take over most of the momentum of the incident-photon beam. For the C-target this allows the determination of the real part of the $\eta^\prime$-carbon potential at low meson momenta by comparing with collision model calculations of the $\eta^\prime$ kinetic energy distribution and excitation function. Fitting the latter data for $\eta^\prime$ mesons going backwards in the center-of-mass system yields a potential depth of V = $-$(44 $\pm$ 16(stat)$\pm$15(syst)) MeV, consistent with earlier determinations of the potential depth in inclusive measurements for average $\eta^\prime$ momenta of $\approx$ 1.1 GeV/$c$. Within the experimental uncertainties, there is no indication of a momentum dependence of the $\eta^\prime$-carbon potential. The LH$_2$ data, taken as a reference to check the data analysis and the model calculations, provide differential and integral cross sections in good agreement with previous results for $\eta^\prime$ photoproduction off the free proton.Comment: 9 pages, 13 figures. arXiv admin note: text overlap with arXiv:1608.0607

Photoproduction of π0\pi^0-pairs off protons and off neutrons

Total cross sections, angular distributions, and invariant-mass distributions have been measured for the photoproduction of $\pi^0\pi^0$ pairs off free protons and off nucleons bound in the deuteron. The experiments were performed at the MAMI accelerator facility in Mainz using the Glasgow photon tagging spectrometer and the Crystal Ball/TAPS detector. The accelerator delivered electron beams of 1508 and 1557~MeV, which produced bremsstrahlung in thin radiator foils. The tagged photon beam covered energies up to 1400~MeV. The data from the free proton target are in good agreement with previous measurements and were only used to test the analysis procedures. The results for differential cross sections (angular distributions and invariant-mass distributions) for free and quasi-free protons are almost identical in shape, but differ in absolute magnitude up to 15\%. Thus, moderate final-state interaction effects are present. The data for quasi-free neutrons are similar to the proton data in the second resonance region (final state invariant masses up to $\approx$1550~MeV), where both reactions are dominated by the $N(1520)3/2^-\rightarrow \Delta(1232)3/2^+\pi$ decay. At higher energies, angular and invariant-mass distributions are different. A simple analysis of the shapes of the invariant-mass distributions in the third resonance region is consistent with strong contributions of an $N^{\star}\rightarrow N\sigma$ decay for the proton, while the reaction is dominated by a sequential decay via a $\Delta\pi$ intermediate state for the neutron. The data are compared to predictions from the Two-Pion-MAID model and the Bonn-Gatchina coupled channel analysis.Comment: accepted for publication in Eur. Phys. J.

The isospin structure of photoproduction of pi-eta pairs from the nucleon in the threshold region

Photoproduction of $\pi\eta$-pairs from nucleons has been investigated from threshold up to incident photon energies of $\approx$~1.4~GeV. The quasi-free reactions $\gamma p\rightarrow p\pi^0\eta$, $\gamma n\rightarrow n\pi^0\eta$, $\gamma p\rightarrow n\pi^+\eta$, and $\gamma n\rightarrow p\pi^-\eta$ were for the first time measured from nucleons bound in the deuteron. The corresponding reactions from a free-proton target were also studied to investigate final-state interaction effects (for neutral pions the free-proton results could be compared to previous measurements; the $\gamma p\rightarrow n\pi^+\eta$ reaction was measured for the first time). For the $\pi^0\eta$ final state coherent production via the $\gamma d\rightarrow d\pi^0\eta$ reaction was also investigated. The experiments were performed at the tagged photon beam of the Mainz MAMI accelerator using an almost $4\pi$ coverage electromagnetic calorimeter composed of the Crystal Ball and TAPS detectors. The total cross sections for the four different final states obey the relation $\sigma(p\pi^0\eta)$ $\approx$ $\sigma(n\pi^0\eta)$ $\approx$ $2\sigma(p\pi^-\eta)$ $\approx$ $2\sigma(n\pi^+\eta)$ as expected for a dominant contribution from a $\Delta^{\star}\rightarrow\eta\Delta(1232)\rightarrow\pi\eta N$ reaction chain, which is also supported by the shapes of the invariant-mass distributions of nucleon-meson and $\pi$-$\eta$ pairs. The experimental results are compared to the predictions from an isobar reaction model.Comment: accepted for publication in Phys. Lett.

Photoproduction of pi0-mesons off neutrons in the nucleon resonance region

Precise angular distributions have been measured for the first time for the photoproduction of $\pi^{0}$-mesons off neutrons bound in the deuteron. The effects from nuclear Fermi motion have been eliminated by a complete kinematic reconstruction of the final state. The influence of final-state-interaction effects has been estimated by a comparison of the reaction cross section for quasi-free protons bound in the deuteron to the results for free protons and then applied as a correction to the quasi-free neutron data. The experiment was performed at the tagged photon facility of the Mainz Microtron MAMI with the Crystal Ball and TAPS detector setup for incident photon energies between $0.45$~GeV and $1.4$~GeV. The results are compared to the predictions from reaction models and partial-wave analyses based on data from other isospin channels. The model predictions show large discrepancies among each other and the present data will provide much tighter constraints. This is demonstrated by the results of a new analysis in the framework of the Bonn-Gatchina coupled-channel analysis which included the present data.Comment: accepted for publication in Phys; Rev. Let

Photoproduction of π0-pairs off protons and off neutrons

Total cross sections, angular distributions, and invariant-mass distributions have been measured for the photoproduction of π0π0 pairs off free protons and off nucleons bound in the deuteron. The experiments were performed at the MAMI accelerator facility in Mainz using the Glasgow photon tagging spectrometer and the Crystal Ball/TAPS detector. The accelerator delivered electron beams of 1508 and 1557MeV, which produced bremsstrahlung in thin radiator foils. The tagged photon beam covered energies up to 1400MeV. The data from the free proton target are in good agreement with previous measurements and were only used to test the analysis procedures. The results for differential cross sections (angular distributions and invariant-mass distributions) for free and quasi-free protons are almost identical in shape, but differ in absolute magnitude up to 15%. Thus, moderate final-state interaction effects are present. The data for quasi-free neutrons are similar to the proton data in the second resonance region (final-state invariant masses up to ≈1550 MeV), where both reactions are dominated by the N(1520)3/2−→Δ(1232)3/2+π decay. At higher energies, angular and invariant-mass distributions are different. A simple analysis of the shapes of the invariant-mass distributions in the third resonance region is consistent with strong contributions of an N⋆→Nσ decay for the proton, while the reaction is dominated by a sequential decay via a Δπ intermediate state for the neutron. The data are compared to predictions from the Two-Pion-MAID model and the Bonn-Gatchina coupled-channel analysis

T and F asymmetries in π0 photoproduction on the proton

The γp→π0p reaction was studied at laboratory photon energies from 425 to 1445 MeV with a transversely polarized target and a longitudinally polarized beam. The beam-target asymmetry F was measured for the first time and new high precision data for the target asymmetry T were obtained. The experiment was performed at the photon tagging facility of the Mainz Microtron (MAMI) using the Crystal Ball and TAPS photon spectrometers. The polarized cross sections were expanded in terms of associated Legendre functions and compared to recent predictions from several partial-wave analyses. The impact of the new data on our understanding of the underlying partial-wave amplitudes and baryon resonance contributions is discussed