A measurement of the double-polarization observable $E$ for the reaction $\gamma p\to \pi^0 p$ is reported. The data were taken with the CBELSA/TAPS experiment at the ELSA facility in Bonn using the Bonn frozen-spin butanol (C$_4$H$_9$OH) target, which provided longitudinally-polarized protons. Circularly-polarized photons were produced via bremsstrahlung of longitudinally-polarized electrons. The data cover the photon energy range from $E_\gamma =600$~MeV to $E_\gamma =2310$~MeV and nearly the complete angular range. The results are compared to and have been included in recent partial wave analyses.

The reaction $\gamma p \rightarrow p \pi^0 \eta$ has been studied with the CBELSA/TAPS detector at the electron stretcher accelerator ELSA in Bonn for incident photon energies from threshold up to 3.1 GeV. This paper has been motivated by the recently claimed observation of a narrow structure in the M$_{N\eta}$ invariant mass distribution at a mass of 1678 MeV. The existence of this structure cannot be confirmed in the present work. Instead, for E$_{\gamma}$ = 1400 - 1500 MeV and the cut M$_{p\pi^0} \le 1190 $ MeV a statistically significant structure in the M$_{p\eta}$ invariant mass distribution near 1700 MeV is observed with a width of $\Gamma\approx 35$ MeV while the mass resolution is $\sigma_{res}$ = 5 MeV. Increasing the incident photon energy from 1420 to 1540 MeV this structure shifts in mass from $\approx 1700 $ to 1725 MeV; the width increases to about 50 MeV and decreases thereafter. The cross section associated with this structure reaches a maximum of $\approx$ 100 nb around E$_{\gamma} \approx$ 1490 MeV (W $\approx $ 1920 MeV), which coincides with the $p a_0$ threshold. Three scenarios are discussed which might be the origin of this structure in the M$_{p\eta}$ invariant mass distribution. The most likely interpretation is that it is due to a triangular singularity in the $\gamma p \rightarrow p a_0 \rightarrow p \pi^0 \eta$ reaction

The reaction $$\gamma p \rightarrow p \pi ^0 \eta $$ has been studied with the CBELSA/TAPS detector at the electron stretcher accelerator ELSA in Bonn for incident photon energies from threshold up to 3.1 GeV. This paper has been motivated by the recently claimed observation of a narrow structure in the $$M_{N\eta }$$ invariant mass distribution at a mass of $$1678\,\hbox {MeV}/\hbox {c}^2$$ . The existence of this structure cannot be confirmed in the present work. Instead, for $$E_{\gamma }$$ = 1400–1500 MeV and the cut $$M_{p\pi ^0} \le 1190 \,\hbox {MeV}/\hbox {c}^2$$ a statistically significant structure in the $$M_{p\eta }$$ invariant mass distribution near 1700 $$\hbox {MeV}/\hbox {c}^2$$ is observed with a width of $$\varGamma \approx 35$$ $$\hbox {MeV}/\hbox {c}^2$$ while the mass resolution is $$\sigma _{res}$$ = 5 $$\hbox {MeV}/\hbox {c}^2$$ . Increasing the incident photon energy from 1420 to 1540 MeV this structure shifts in mass from $$\approx $$ 1700 $$\hbox {MeV}/\hbox {c}^2$$ to $$\approx $$ 1725 $$\hbox {MeV}/\hbox {c}^2$$ ; the width increases to about 50 $$\hbox {MeV}/\hbox {c}^2$$ and decreases thereafter. The cross section associated with this structure reaches a maximum of $$\approx $$ 100 nb around $$E_{\gamma } \approx $$ 1490 MeV (W $$\approx $$ 1920 MeV), which coincides with the $$p a_0$$ threshold. Three scenarios are discussed which might be the origin of this structure in the $$M_{p\eta }$$ invariant mass distribution. The most likely interpretation is that it is due to a triangular singularity in the $$\gamma p \rightarrow p a_0 \rightarrow p \pi ^0 \eta $$ reaction.

A measurement of the double-polarization observable E for the reaction $$\gamma p\rightarrow \pi ^0 p$$ is reported. The data were taken with the CBELSA/TAPS experiment at the ELSA facility in Bonn using the Bonn frozen-spin butanol (C $$_4$$ H $$_9$$ OH) target, which provided longitudinally-polarized protons. Circularly-polarized photons were produced via bremsstrahlung of longitudinally-polarized electrons. The data cover the photon energy range from $$E_\gamma =600$$ to 2310 MeV and nearly the complete angular range. The results are compared to and have been included in recent partial wave analyses.

The reaction $$\gamma p \rightarrow p \pi ^0 \eta $$ has been studied with the CBELSA/TAPS detector at the electron stretcher accelerator ELSA in Bonn for incident photon energies from threshold up to 3.1 GeV. This paper has been motivated by the recently claimed observation of a narrow structure in the $$M_{N\eta }$$ invariant mass distribution at a mass of $$1678\,\hbox {MeV}/\hbox {c}^2$$ . The existence of this structure cannot be confirmed in the present work. Instead, for $$E_{\gamma }$$ = 1400–1500 MeV and the cut $$M_{p\pi ^0} \le 1190 \,\hbox {MeV}/\hbox {c}^2$$ a statistically significant structure in the $$M_{p\eta }$$ invariant mass distribution near 1700 $$\hbox {MeV}/\hbox {c}^2$$ is observed with a width of $$\varGamma \approx 35$$ $$\hbox {MeV}/\hbox {c}^2$$ while the mass resolution is $$\sigma _{res}$$ = 5 $$\hbox {MeV}/\hbox {c}^2$$ . Increasing the incident photon energy from 1420 to 1540 MeV this structure shifts in mass from $$\approx $$ 1700 $$\hbox {MeV}/\hbox {c}^2$$ to $$\approx $$ 1725 $$\hbox {MeV}/\hbox {c}^2$$ ; the width increases to about 50 $$\hbox {MeV}/\hbox {c}^2$$ and decreases thereafter. The cross section associated with this structure reaches a maximum of $$\approx $$ 100 nb around $$E_{\gamma } \approx $$ 1490 MeV (W $$\approx $$ 1920 MeV), which coincides with the $$p a_0$$ threshold. Three scenarios are discussed which might be the origin of this structure in the $$M_{p\eta }$$ invariant mass distribution. The most likely interpretation is that it is due to a triangular singularity in the $$\gamma p \rightarrow p a_0 \rightarrow p \pi ^0 \eta $$ reaction.

The reaction γ p → p π ^0 η has been studied with the CBELSA/TAPS detector at the electron stretcher accelerator ELSA in Bonn for incident photon energies from threshold up to 3.1 GeV. This paper has been motivated by the recently claimed observation of a narrow structure in the M_Nη invariant mass distribution at a mass of 1678 MeV/c^2 . The existence of this structure cannot be confirmed in the present work. Instead, for E_γ = 1400–1500 MeV and the cut M_pπ ^0≤ 1190 MeV/c^2 a statistically significant structure in the M_pη invariant mass distribution near 1700 MeV/c^2 is observed with a width of ≈ 35 MeV/c^2 while the mass resolution is σ _res = 5 MeV/c^2 . Increasing the incident photon energy from 1420 to 1540 MeV this structure shifts in mass from ≈ 1700 MeV/c^2 to ≈ 1725 MeV/c^2 ; the width increases to about 50 MeV/c^2 and decreases thereafter. The cross section associated with this structure reaches a maximum of ≈ 100 nb around E_γ≈ 1490 MeV (W ≈ 1920 MeV), which coincides with the p a_0 threshold. Three scenarios are discussed which might be the origin of this structure in the M_pη invariant mass distribution. The most likely interpretation is that it is due to a triangular singularity in the γ p → p a_0 → p π ^0 η reaction.

. The production of η^' 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 η^' mesons have been identified via the η^'→π^0π^0η→ 6 γ decay registered with the CBELSA/TAPS detector system. Coincident protons have been identified in the MiniTAPS BaF 2 array at polar angles of 2^∘≤θ_p≤ 11^∘ . Under these kinematic constraints the η^' mesons are produced with relatively low kinetic energy ( ≈ 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 η^' -carbon potential at low meson momenta by comparing with collision model calculations of the η^' kinetic energy distribution and excitation function. Fitting the latter data for η^' mesons going backwards in the center-of-mass system yields a potential depth of V = -(44 ± 16(stat) ± 15(syst)) MeV, consistent with earlier determinations of the potential depth in inclusive measurements for average η^' momenta of ≈ 1.1 GeV/ c . Within the experimental uncertainties, there is no indication of a momentum dependence of the η^' -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 η^' photoproduction off the free proton.

This paper reports on a measurement of the double-polarization observable G in \( \pi^0\) photoproduction off the proton using the CBELSA/TAPS experiment at the ELSA accelerator in Bonn. The observable G is determined from reactions of linearly polarized photons with longitudinally polarized protons. The polarized photons are produced by bremsstrahlung off a diamond radiator of well-defined orientation. A frozen spin butanol target provides the polarized protons. The data cover the photon energy range from 617 to 1325 MeV and a wide angular range. The experimental results for G are compared to predictions by the Bonn-Gatchina (BnGa), Jülich-Bonn (JüBo), MAID and SAID partial wave analyses. Implications of the new data for the pion photoproduction multipoles are discussed.

Results from measurements of the photoproduction of \(\eta\) mesons from quasifree protons and neutrons are summarized. The experiments were performed with the CBELSA/TAPS detector at the electron accelerator ELSA in Bonn using the \(\eta\rightarrow 3\pi^{0}\rightarrow 6\gamma\) decay. A liquid deuterium target was used for the measurement of total cross sections and angular distributions. The results confirm earlier measurements from Bonn and the MAMI facility in Mainz about the existence of a narrow structure in the excitation function of \(\gamma n\rightarrow n\eta\). The current angular distributions show a forward-backward asymmetry, which was previously not seen, but was predicted by model calculations including an additional narrow \(P_{11}\) state. Furthermore, data obtained with a longitudinally polarized, deuterated butanol target and a circularly polarized photon beam were analyzed to determine the double polarization observable E. Both data sets together were also used to extract the helicity-dependent cross sections \(\sigma_{1/2}\) and \(\sigma_{3/2}\). The narrow structure in the excitation function of \(\gamma n\rightarrow n\eta\) appears associated with the helicity-1/2 component of the reaction.

The photoproduction of ω and η^' mesons off carbon and niobium nuclei has been measured as a function of the meson momentum for incident photon energies of 1.2-2.9 GeV at the electron accelerator ELSA. The mesons have been identified via the ω→π^0 γ→ 3 γ and η^'→π^0 π^0η→ 6 γ decays, respectively, registered with the CBELSA/TAPS detector system. From the measured meson momentum distributions the momentum dependence of the transparency ratio has been determined for both mesons. Within a Glauber analysis the in-medium ω and η^' widths and the corresponding absorption cross sections have been deduced as a function of the meson momentum. The results are compared to recent theoretical predictions for the in-medium ω width and η^'-N absorption cross sections. The energy dependence of the imaginary part of the ω- and η^'-nucleus optical potential has been extracted. The finer binning of the present data compared to the existing data allows a more reliable extrapolation towards the production threshold. The modulus of the imaginary part of the η^' nucleus potential is found to be about three times smaller than recently determined values of the real part of the η^'-nucleus potential, which makes the η^' meson a suitable candidate for the search for meson-nucleus bound states. For the ω meson, the modulus of the imaginary part near threshold is comparable to the modulus of the real part of the potential. As a consequence, only broad structures can be expected which makes the observation of ω mesic states very difficult experimentally.

V. Sokhoyan a,∗, E. Gutz a,b, H. van Pee a, A.V. Anisovich a,c, J.C.S. Bacelar d, B. Bantes e, O. Bartholomy a, D. Bayadilov a,c, R. Beck a, Y. Beloglazov c, R. Castelijns d, V. Crede f, H. Dutz e, D. Elsner e, R. Ewald e, F. Frommberger e, M. Fuchs a, Ch. Funke a, R. Gregor b, A. Gridnev c, W. Hillert e, Ph. Hoffmeister a, I. Horn a, I. Jaegle g, J. Junkersfeld a, H. Kalinowsky a, S. Kammer e, V. Kleber e, Frank Klein e, Friedrich Klein e, E. Klempt a, M. Kotulla b,g, B. Krusche g, M. Lang a, H. Löhner d, I. Lopatin c, S. Lugert b, T. Mertens g, J.G. Messchendorp d, V. Metag b, M. Nanova b, V.A. Nikonov a,c, D. Novinski c, R. Novotny b, M. Ostrick e, L. Pant b, M. Pfeiffer b, D. Piontek a, A. Roy b, A.V. Sarantsev a,c, Ch. Schmidt a, H. Schmieden e, S. Shende d, A. Süle e, V.V. Sumachev c, T. Szczepanek a, A. Thiel a, U. Thoma a, D. Trnka b, R. Varma b, D. Walther a, Ch. Wendel a, A. Wilson a,f

Photoproduction of two neutral pions off the proton is studied using linearly polarized photons, and the polarization observables I-s and I-c are measured for the first time. These two observables are unique to multi-meson final states; they characterize correlations between the linear photon polarization and the direction of outgoing single particles in photoproduction of three-body final states. The I-s and I-c distributions suggest that, in the 1.8 to 2.0 GeV mass region, the N(1520) 3/2(-)pi intermediate state is reached with reaction dynamics consistent with a dominant J(P) = 3/2(+) wave. These data are included in the Bonn-Gatchina (BnGa) partial wave analysis which is based on a large variety of data; the analysis confirms a significant contribution from the reaction chain gamma p -> N(1900)3/2(+) -> N(1520)3/2(-)pi(0) -> p pi(0)pi(0). (C) 2015 The Authors. Published by Elsevier B.V.

Photoproduction off protons of the pπ 0 η three-body final state was studied with the Crystal Barrel/TAPS detector, at the electron stretcher accelerator ELSA in Bonn, for incident energies from the π 0 η production threshold up to 2.5 GeV. Differential cross sections and the total cross sections are presented. The use of linearly polarized photons gives access to the polarization observables Σ, I s , and I c , the latter two characterize beam asymmetries in case of three-body final states. Δ(1232)η, N(1535)1/2− π, and pa 0(980) are the dominant isobars contributing to the reaction. The partial wave analysis confirms the existence of some nucleon and Δ resonances, for which so far only fair evidence was reported. A large number of decay modes of known nucleon and Δ resonances is presented. It is shown that detailed investigations of decay branching ratios may provide a key to unravelling the structure of nucleon and Δ resonances.

Data on the polarization observables T, P, and H for the reaction gamma p -> p pi(0) are reported. Compared to earlier data from other experiments, our data are more precise and extend the covered range in energy and angle substantially. The results were extracted from azimuthal asymmetries measured using a transversely polarized target and linearly polarized photons. The data were taken at the Bonn electron stretcher accelerator ELSA with the CBELSA/TAPS detector. Within the Bonn-Gatchina partial wave analysis, the new polarization data lead to a significant narrowing of the error band for the multipoles for neutral-pion photoproduction. (C) 2015 The Authors. Published by Elsevier B.V.

Data on the polarization observables T, P, and H for the reaction γp→pπ0 are reported. Compared to earlier data from other experiments, our data are more precise and extend the covered range in energy and angle substantially. The results were extracted from azimuthal asymmetries measured using a transversely polarized target and linearly polarized photons. The data were taken at the Bonn electron stretcher accelerator ELSA with the CBELSA/TAPS detector. Within the Bonn-Gatchina partial wave analysis, the new polarization data lead to a significant narrowing of the error band for the multipoles for neutral-pion photoproduction.

Received 22 April 2015DOI:https://doi.org/10.1103/PhysRevLett.114.199903© 2015 American Physical Society

The first measurements of the beam-target-helicity-asymmetries E and G in the photoproduction of omega-mesons off protons at the CBELSA/TAPS experiment are reported. E (G) was measured using circularly (linearly) polarised photons and a longitudinally polarised target. E was measured over the photon energy range from close to threshold (E-gamma = 1108 MeV) to E-gamma = 2300 MeV and G at a single energy interval of 1108 < E-gamma < 1300 MeV. Both measurements cover the full solid angle. The observables E and G are highly sensitive to the contribution of baryon resonances, with E acting as a helicity filter in the s-channel. The new results indicate significant s-channel resonance contributions together with contributions from t-channel exchange processes. A partial wave analysis reveals strong contributions from the partial waves with spin-parity J(P) = 3/2(+), 5/2(+), and 3/2(-). (C) 2015 The Authors. Published by Elsevier B.V.

Photoproduction of two neutral pions off the proton is studied using linearly polarized photons, and the polarization observables Is and Ic are measured for the first time. These two observables are unique to multi-meson final states; they characterize correlations between the linear photon polarization and the direction of outgoing single particles in photoproduction of three-body final states. The Is and Ic distributions suggest that, in the 1.8 to 2.0 GeV mass region, the N(1520)3/2−π intermediate state is reached with reaction dynamics consistent with a dominant JP=3/2+ wave. These data are included in the Bonn–Gatchina (BnGa) partial wave analysis which is based on a large variety of data; the analysis confirms a significant contribution from the reaction chain γp→N(1900)3/2+→N(1520)3/2−π0→pπ0π0. Keywords: Baryon spectroscopy, Double meson photoproduction, Polarization observables

6 reports decay properties of the N (1710)1/2 + and N (1895)1/2 -resonances into the πN (1440)1/2 + final state.These decays were not observed.Branching ratios and transition probabilities for this decay mode are compatible with zero or ill-defined in the respective analysis.