ω photoproduction off hydrogen and deuterium has been studied with the tagged photon beam of the ELSA accelerator in Bonn for photon energies up to 2.0 GeV. The ω meson has been identified via the ω → π$^{0}$ γ → γγγ decay mode, using the combined setup of the Crystal Barrel/TAPS detector systems. Both inclusive and exclusive analyses have been carried out. Differential and total cross-sections have been derived for ω mesons produced off free protons and off protons and neutrons bound in deuterium. The cross-section for the production off the bound neutron is found to be a factor of ≈ 1.3 larger than the one off the bound proton in the incident beam energy region 1.2 GeV < E$_{γ}$ < 1.6 GeV. For higher incident beam energies this factor goes down to ≈ 1.1 at 2.0 GeV. The cross-sections of this work have been used as normalization for transparency ratio measurements.
Differential cross-sections of $\omega$ mesons produced off the free proton versus $\cos(\theta^\omega_{\mathrm{c.m.}})$ and versus the momentum transfer to the nucleon, $t$, for incident photon energy $E_\gamma$ = 1.125-1.150 GeV.
Differential cross-sections of $\omega$ mesons produced off the free proton versus $\cos(\theta^\omega_{\mathrm{c.m.}})$ and versus the momentum transfer to the nucleon, $t$, for incident photon energy $E_\gamma$ = 1.150-1.175 GeV.
Differential cross-sections of $\omega$ mesons produced off the free proton versus $\cos(\theta^\omega_{\mathrm{c.m.}})$ and versus the momentum transfer to the nucleon, $t$, for incident photon energy $E_\gamma$ = 1.175-1.200 GeV.
The polarization observable I^s, a feature exclusive to the acoplanar kinematics of multi-meson final states produced via linearly polarized photons, has been measured for the first time. Results for the reaction g p -> p pi0 eta are presented for incoming photon energies between 970 MeV and 1650 MeV along with the beam asymmetry I^c. The comparably large asymmetries demonstrate a high sensitivity of I^s to the dynamics of the reaction. Fits using Bonn-Gatchina partial wave analysis demonstrate that the new polarization observables carry significant information on the contributing partial waves.
Measured beam asymmetry I_S as a function of the angle between the reaction plane and the plane of the two final state particles with the the proton as the recoiling particle for the cm energy range 1642 to 1770 MeV.
Measured beam asymmetry I_S as a function of the angle between the reaction plane and the plane of the two final state particles with the the proton as the recoiling particle for the cm energy range 1770 to 1898 MeV.
Measured beam asymmetry I_S as a function of the angle between the reaction plane and the plane of the two final state particles with the the proton as the recoiling particle for the cm energy range 1898 to 1994 MeV.
Measurements of the pn -> p_spec dK^+K^- reaction, where p_spec is a spectator proton, have been undertaken at the Cooler Synchrotron COSY-Juelich by detecting a fast deuteron in coincidence with a K^+K^- pair in the ANKE facility. Although the proton beam energy was fixed, the moving target neutron allowed values of the non-resonant quasi-free pn -> dK^+K^- total cross section to be deduced up to an excess energy epsilon ~ 100 MeV. Evidence is found for the effects of K^-d and KKbar final state interactions. The comparison of these data with those of pp -> ppK^+K^- and pp -> dK^+Kbar shows that all the total cross sections are very similar in magnitude.
Total cross section for the non-PHI component of the P N --> DEUT K+ K- reaction as a function of the excess energy with respect to the DEUT K+ K- threshold.
Photoproduction of $\eta$ mesons off $^{12}$C, $^{40}$Ca, $^{93}$Nb, and $^{nat}$Pb nuclei has been measured with a tagged photon beam with energies between 0.6 and 2.2 GeV. The experiment was performed at the Bonn ELSA accelerator with the combined setup of the Crystal Barrel and TAPS calorimeters. It aimed at the in-medium properties of the S$_{11}$(1535) nucleon resonance and the study of the absorption properties of nuclear matter for $\eta$ mesons. Careful consideration was given to contributions from $\eta\pi$ final states and secondary production mechanisms of $\eta$-mesons e.g. from inelastic $\pi N$ reactions of intermediate pions. The analysis of the mass number scaling shows that the nuclear absorption cross section $\sigma_{N\eta}$ for $\eta$ mesons is constant over a wide range of the $\eta$ momentum. The comparison of the excitation functions to data off the deuteron and to calculations in the framework of a BUU-model show no unexplained in-medium modifications of the S$_{11}$(1535).
Inclusive energy distribution for incident photon energy 0.650 to 0.835 GeV.
Inclusive energy distribution for incident photon energy 0.835 to 1.050 GeV.
Inclusive energy distribution for incident photon energy 1.050 to 1.550 GeV.
Quasi-free photoproduction of eta-mesons off nucleons bound in the deuteron has been measured with the CBELSA/TAPS detector for incident photon energies up to 2.5 GeV at the Bonn ELSA accelerator. The eta-mesons have been detected in coincidence with recoil protons and recoil neutrons, which allows a detailed comparison of the quasi-free n(gamma,eta)n and p(gamma,eta)p reactions. The excitation function for eta-production off the neutron shows a pronounced bump-like structure at W=1.68 GeV (E_g ~ 1 GeV), which is absent for the proton.
Measured value of the quasi-free eta cross section off protons and neutrons as a function of incident photon energy.
Ratio of the measured quasi-free neutron to proton cross sections as a function of incident photon energy.
Measured angular distribution for an incident photon energy of 0.700 GeV.
The exclusive reactions $\gamma p \to K^{*0} \Sigma^+(1189)$ and $\gamma p \to K^{0} \pi^{0}\Sigma^+(1189)$, leading to the p 4$\pi^{0}$ final state, have been measured with a tagged photon beam for incident energies from threshold up to 2.5 GeV. The experiment has been performed at the tagged photon facility of the ELSA accelerator (Bonn). The Crystal Barrel and TAPS detectors were combined to a photon detector system of almost 4$\pi$ geometrical acceptance. Differential and total cross sections are reported. At energies close to the threshold, a flat angular distribution has been observed for the reaction $\gamma p\to K^{0} \pi^{0}\Sigma^+$ suggesting dominant s-channel production. $\Sigma^*(1385)$ and higher lying hyperon states have been observed. An enhancement in the forward direction in the angular distributions of the reaction $\gamma p \to K^{*0}\Sigma^+$ indicates a $t$-channel exchange contribution to the reaction mechanism. The experimental data are in reasonable agreement with recent theoretical predictions.
Differential cross section D(SIG)/DCOS(THETA(SIG+)) for the GAMMA P --> K0 PI0 SIGMA+ reaction for beam energies 1500-1700 and 1700-1850 MeV.
Differential cross section D(SIG)/DCOS(THETA(SIG+)) for the GAMMA P --> K0 PI0 SIGMA+ reaction for beam energies 1850-2000 and 2000-2150 MeV.
Differential cross section D(SIG)/DCOS(THETA(SIG+)) for the GAMMA P --> K0 PI0 SIGMA+ reaction for beam energies 2150-2300 and 2300-2500 MeV.
In photoproduction experiments, a large number of final states yielding various resonance contributions are accessible. To extract resonance parameters via partial-wave analysis not only the measurement of differential cross-sections is necessary, but also the determination of polarization observables. At the electron accelerator ELSA (Bonn) the coherent bremsstrahlung method was used to generate a linearly polarized photon beam. Using the CBELSA/TAPS detector setup, the beam asymmetry Σ in the reaction γp → pπ 0 η was determined as a function of various masses and angles for photon energies between 970MeV and 1650MeV.
Measured beam asymmetry as a function of the cosine of the scattered proton angle for beam energies 0.970 to 1.200 GeV.
Measured beam asymmetry as a function of the cosine of the scattered proton angle for beam energies 1.200 to 1.450 GeV.
Measured beam asymmetry as a function of the cosine of the scattered proton angle for beam energies 1.450 to 1.650 GeV.
The differential and total cross sections for kaon pair production in the pp->ppK+K- reaction have been measured at three beam energies of 2.65, 2.70, and 2.83 GeV using the ANKE magnetic spectrometer at the COSY-Juelich accelerator. These near-threshold data are separated into pairs arising from the decay of the phi-meson and the remainder. For the non-phi selection, the ratio of the differential cross sections in terms of the K-p and K+p invariant masses is strongly peaked towards low masses. This effect can be described quantitatively by using a simple ansatz for the K-p final state interaction, where it is seen that the data are sensitive to the magnitude of an effective K-p scattering length. When allowance is made for a small number of phi events where the K- rescatters from the proton, the phi region is equally well described at all three energies. A very similar phenomenon is discovered in the ratio of the cross sections as functions of the K-pp and K+pp invariant masses and the identical final state interaction model is also very successful here. The world data on the energy dependence of the non-phi total cross section is also reproduced, except possibly for the results closest to threshold.
Total cross sections for the reaction P P --> P P K+ K- reaction in the phi poor and phi rich regions.
Differential cross section as a function of the invariant mass of the K+ K- pair.
Differential cross section as a function of the invariant mass of the K+ P and K- P pair and their ratio.
The pp -> p K+ Y0 reaction has been studied for hyperon masses m(Y0)<1540 MeV/c2 at COSY-J\'ulich by using a 3.65 GeV/c circulating proton beam incident on an internal hydrogen target. Final states comprising two protons, one positively charged kaon and one negatively charged pion have been identified with the ANKE spectrometer. Such configurations are sensitive to the production of the ground state Lambda and Sigma0 hyperons as well as the Sigma0(1385) and Lambda(1405) resonances. Applying invariant- and missing-mass techniques, the two overlapping excited states can be separated unambiguously. The shape and position of the Lambda(1405) distribution, reconstructed cleanly from its Sigma0 pion0 decay, are similar to those found in other production modes and there is no obvious mass shift. This finding constitutes a challenging test for models that predict Lambda(1405) to be a two-state resonance.
Cross section for SIGMA(1385)0 production.
Cross section for LAMBDA(1405) production.
The beam asymmetry, $\Sigma$, was measured at ELSA in the reaction $\vec \gamma p \to \eta p$ using linearly polarised tagged photon beams, produced by coherent bremsstrahlung off a diamond. The crystal was oriented to provide polarised photons in the energy range $E_\gamma = 800$ to 1400 MeV with the maximum polarisation of $P_\gamma = 49$ % obtained at 1305 MeV. Both dominant decay modes of the $\eta$ into two photons and $3\pi^0$ were used to extract the beam asymmetry from the azimuthal modulation of the cross section. The measurements cover the angular range $\Theta_\text{cm}\simeq 50$ -- 150 degrees. Large asymmetries up to 80 % are observed, in agreement with a previous measurement. The eta-MAID model and the Bonn--Gatchina partial wave analysis describe the measurements, but the required partial waves differ significantly.
Photon asymmetry for eta photoproduction at incident photon energy of 850 MeV.
Photon asymmetry for eta photoproduction at incident photon energy of 950 MeV.
Photon asymmetry for eta photoproduction at incident photon energy of 1050 MeV.
Forum