The coherent reaction, gamma d -> pi 0 eta d was studied with the BGOOD experiment at ELSA from threshold to a centre-of-mass energy of 3200 MeV for forward deuteron angles. A full kinematic reconstruction was made, with final state deuterons identified in the Forward Spectrometer and pi 0 and eta decays in the central BGO Rugby Ball. The strength of the differential cross section exceeds what can be described by phenomenological models of coherent photoproduction due to the large momentum transfer to the deuteron. The origin of this enhancement is unresolved, whether this is due, for example, to intermediate dibaryon formation or pion re-scattering terms. A toy model of quasi-free O and N(1535) photoproduction with final state interactions is discussed, which gives a qualitative description of the differential cross section.

The differential cross section for γ p→ K^+Λ(1520) was measured from threshold to a centre-of-mass energy of 2090 MeV at forward angles at the BGOOD experiment. The high statistical precision and resolution in centre-of-mass energy and angle allows a detailed characterisation of this low-momentum transfer kinematic region. The data agree with a previous LEPS measurement and support effective Lagrangian models that indicate that the contact term dominates the cross section near threshold.

AbstractThe differential cross section for the quasi-free photoproduction reaction $$\gamma n\rightarrow K^0\Sigma ^0$$ γ n → K 0 Σ 0 was measured at BGOOD at ELSA from threshold to a centre-of-mass energy of $$2400\,\hbox {MeV}$$ 2400 MeV . Close to threshold the results are consistent with existing data and are in agreement with partial wave analysis solutions over the full measured energy range, with a large coupling to the $$\Delta (1900)1/2^-$$ Δ ( 1900 ) 1 / 2 - evident. This is the first dataset covering the $$K^*$$ K ∗ threshold region, where there are model predictions of dynamically generated vector meson-baryon resonance contributions.

The differential cross section for the quasi-free photoproduction reaction $$\gamma n\rightarrow K^0\Sigma ^0$$ γ n → K 0 Σ 0 was measured at BGOOD at ELSA from threshold to a centre-of-mass energy of $$2400\,\hbox {MeV}$$ 2400 MeV . Close to threshold the results are consistent with existing data and are in agreement with partial wave analysis solutions over the full measured energy range, with a large coupling to the $$\Delta (1900)1/2^-$$ Δ ( 1900 ) 1 / 2 - evident. This is the first dataset covering the $$K^*$$ K ∗ threshold region, where there are model predictions of dynamically generated vector meson-baryon resonance contributions.

AbstractThe target asymmetry T, recoil asymmetry P, and beam-target double polarization observable H were determined in exclusive $$\pi ^0$$ π 0 and $$\eta $$ η photoproduction off quasi-free protons and, for the first time, off quasi-free neutrons. The experiment was performed at the electron stretcher accelerator ELSA in Bonn, Germany, with the Crystal Barrel/TAPS detector setup, using a linearly polarized photon beam and a transversely polarized deuterated butanol target. Effects from the Fermi motion of the nucleons within deuterium were removed by a full kinematic reconstruction of the final state invariant mass. A comparison of the data obtained on the proton and on the neutron provides new insight into the isospin structure of the electromagnetic excitation of the nucleon. Earlier measurements of polarization observables in the $$\gamma p \rightarrow \pi ^0 p$$ γ p → π 0 p and $$\gamma p \rightarrow \eta p$$ γ p → η p reactions are confirmed. The data obtained on the neutron are of particular relevance for clarifying the origin of the narrow structure in the $$\eta n$$ η n system at $$W = 1.68\ \textrm{GeV}$$ W = 1.68 GeV . A comparison with recent partial wave analyses favors the interpretation of this structure as arising from interference of the $$S_{11}(1535)$$ S 11 ( 1535 ) and $$S_{11}(1650)$$ S 11 ( 1650 ) resonances within the $$S_{11}$$ S 11 -partial wave.

The target asymmetry T, recoil asymmetry P, and beam-target double polarization observable H were determined in exclusive π0 and η photoproduction off quasi-free protons and, for the first time, off quasi-free neutrons. The experiment was performed at the electron stretcher accelerator ELSA in Bonn, Germany, with the Crystal Barrel/TAPS detector setup, using a linearly polarized photon beam and a transversely polarized deuterated butanol target. Effects from the Fermi motion of the nucleons within deuterium were removed by a full kinematic reconstruction of the final state invariant mass. A comparison of the data obtained on the proton and on the neutron provides new insight into the isospin structure of the electromagnetic excitation of the nucleon. Earlier measurements of polarization observables in the γp→π0p and γp→ηp reactions are confirmed. The data obtained on the neutron are of particular relevance for clarifying the origin of the narrow structure in the ηn system at W=1.68GeV. A comparison with recent partial wave analyses favors the interpretation of this structure as arising from interference of the S11(1535) and S11(1650) resonances within the S11-partial wave.

The BGOOD photoproduction experiment accesses forward meson angles and low momentum exchange kinematics in the uds sector, which may be sensitive to molecular-like hadron structure. Recent results are presented, including strangeness photoproduction at forward meson angles, and pi(0) pi(0) coherent photoproduction off the deuteron.

The differential cross section for the quasi-free photoproduction reaction gamma(n)-> K-0 Sigma(0) was measured at BGOOD at ELSA from threshold to a centre-of-mass energy of 2400 MeV. Close to threshold the results are consistent with existing data and are in agreement with partial wave analysis solutions over the full measured energy range, with a large coupling to the Delta(1900)1/2(-) evident. This is the first dataset covering the K* threshold region, where there are model predictions of dynamically generated vector meson-baryon resonance contributions.

The Crystal Barrel is an electromagnetic calorimeter consisting of 1380 CsI(Tl) scintillators, and is currently installed at the CBELSA/TAPS experiment where it is used to detect decay products from photoproduction of mesons. The readout of the Crystal Barrel has been upgraded in order to integrate the detector into the first level of the trigger and to increase its sensitivity for neutral final states. The new readout uses avalanche photodiodes in the front-end and a dual back-end with branches optimized for energy and time measurement, respectively. An FPGA-based cluster finder processes the whole hit pattern within less than 100 ns. The important downside of APDs -- the temperature dependence of their gain -- is handled with a temperature stabilization and a compensating bias voltage supply. Additionally, a light pulser system allows the APDs' gains to be measured during beamtimes.

$K^+\Lambda(1405)$ photoproduction has been studied at the BGOOD experiment via the all neutral decay, $\Lambda(1405)\rightarrow\Sigma^0\pi^0$. The unique BGOOD experimental setup allows both the cross section and $\Lambda(1405)$ invariant mass distribution (line shape) to be measured over a broad $K^+$ polar angle range, extending to extreme forward $K^+$ angles unattainable at previous experiments. Evidence is provided for the role of a triangle singularity driven by the $N^*(2030)$ resonance, which appears to contribute significantly to $K^+\Lambda(1405)$ photoproduction. This is observed in both the angular distributions and the integrated cross section which was determined with unprecedented energy resolution. The measured line shape is also in agreement with the previous results of CLAS and ANKE, and is consistent with two poles derived in $\chi$PT based models.

The coherent reaction, γd→π0π0d was studied with the BGOOD experiment at ELSA from threshold to a centre-of-mass energy of 2850MeV. A full kinematic reconstruction was made, with final state deuterons identified in the forward spectrometer and π0 decays in the central BGO Rugby Ball. The strength of the differential cross section exceeds what can be described by models of coherent photoproduction and instead supports the three isoscalar dibaryon candidates reported by the ELPH collaboration at 2.38, 2.47 and 2.63GeV/c2. A low mass enhancement in the π0π0 invariant mass is also observed at the d⁎(2380) centre-of-mass energy which is consistent with the ABC effect. At higher centre-of-mass energies, a narrow peak in the π0d invariant mass at 2114MeV/c2 with a width of 20MeV/c2 supports a sequential two-dibaryon decay mechanism.

$$\gamma p \rightarrow K^{+} \Lambda $$ differential cross sections and recoil polarisation data from threshold for extremely forward angles are presented. The measurements were performed at the BGOOD experiment at ELSA, utilising the high angular and momentum resolution forward spectrometer for charged particle identification. The high statistics and forward angle acceptance enables the extraction of the cross section as the minimum momentum transfer to the recoiling hyperon is approached.

The γp→K+Σ0 differential cross section at extremely forward angles was measured at the BGOOD experiment. A three-quarter drop in strength over a narrow range in energy and a strong dependence on the polar angle of the K+ in the centre-of-mass of the reaction is observed at a centre-of-mass energy of 1900 MeV. Residing close to multiple open and hidden strangeness thresholds, the structure appears consistent with meson-baryon threshold effects which may contribute to the reaction mechanism.

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

G. Scheluchin, ∗ S. Alef, P. Bauer, R. Beck, A. Braghieri, P. Cole, R. D. Salvo, D. Elsner, A. Fantini, 6 O. Freyermuth, F. Ghio, 8 A. Gridnev, D. Hammann, † J. Hannappel, T. Jude, K. Kohl, N. Kozlenko, A. Lapik, P. L. Sandri, V. Lisin, G. Mandaglio, 13 R. Messi, 7, † D. Moricciani, A. Mushkarenkov, V. Nedorezov, D. Novinsky, P. Pedroni, A. Polonski, B.-E. Reitz, † M. Romaniuk, 14 H. Schmieden, V. Sumachev, ‡ and V. Tarakanov (BGOOD Collaboration) Rheinische Friedrich-Wilhelms-Universität Bonn, Physikalisches Institut, Nußallee 12, 53115 Bonn, Germany Helmholtz-Institut für Strahlenund Kernphysik, Universität Bonn, Nußallee 14-16, 53115 Bonn Germany INFN sezione di Pavia, Via Agostino Bassi, 6 27100 Pavia, Italy Lamar University, Department of Physics, Beaumont, Texas, 77710, USA INFN Roma “Tor Vergata”, Rome, Italy Università di Roma “Tor Vergata”, Via della Ricerca Scientifica 1, 00133 Rome, Italy INFN sezione di Roma La Sapienza, P.le Aldo Moro 2, 00185 Rome, Italy Istituto Superiore di Sanita, Viale Regina Elena 299, 00161 Rome, Italy Petersburg Nuclear Physics Institute, Gatchina, Leningrad District, 188300, Russia Russian Academy of Sciences Institute for Nuclear Research, prospekt 60-letiya Oktyabrya 7a, Moscow 117312, Russia INFN Laboratori Nazionali di Frascati, Via E. Fermi 40, 00044 Frascati, Italy INFN sezione Catania, 95129 Catania, Italy Universita degli Studi di Messina, Via Consolato del Mare 41, 98121 Messina, Italy Institute for Nuclear Research of NASU, 03028, Kyiv, Ukraine (Dated: August 30, 2021)

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.