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/$c^2$. 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 $ MeV/$c^2$ a statistically significant structure in the M$_{p\eta}$ invariant mass distribution near 1700 MeV/$c^2$ is observed with a width of $\Gamma\approx 35$ MeV/$c^2$ while the mass resolution is $\sigma_{res}$ = 5 MeV/$c^2$. Increasing the incident photon energy from 1420 to 1540 MeV this structure shifts in mass from $\approx$ 1700MeV/c$^2$ to $\approx$ 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 $\approx$ 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
Differential cross section for $\pi^0$ in the $\gamma p$ for the incident photon energy range of $E_\gamma$ = 1400-1500 MeV.
Differential cross section for $\eta$ in the $\gamma p$ for the incident photon energy range of $E_\gamma$ = 1400-1500 MeV.
Differential cross section for $p$ in the $\gamma p$ for the incident photon energy range of $E_\gamma$ = 1400-1500 MeV.
A measurement is presented of inelastic photo- and electroproduction of J/psi mesons in ep scattering at HERA. The data were recorded with the H1 detector in the period from 2004 to 2007. Single and double differential cross sections are determined and the helicity distributions of the J/psi mesons are analysed. The results are compared to theoretical predictions in the colour singlet model and in the framework of non-relativistic QCD. Calculations in the colour singlet model using a k_T factorisation ansatz are able to give a good description of the data, while colour singlet model calculations to next-to-leading order in collinear factorisation underestimate the data.
Measured differential photoproduction cross section as a function of the squared transverse momentum of the J/PSI.
Measured differential photoproduction cross section as a function of the elasticity of the J/PSI.
Measured photoproduction cross section as a function of the photon-proton centre of mass energy W.
Diffractive electroproduction of rho and phi mesons is measured at HERA with the H1 detector in the elastic and proton dissociative channels. The data correspond to an integrated luminosity of 51 pb^-1. About 10500 rho and 2000 phi events are analysed in the kinematic range of squared photon virtuality 2.5 < Q^2 < 60 GeV^2, photon-proton centre of mass energy 35 < W < 180 GeV and squared four-momentum transfer to the proton |t| < 3 GeV^2. The total, longitudinal and transverse cross sections are measured as a function of Q^2, W and |t|. The measurements show a transition to a dominantly "hard" behaviour, typical of high gluon densities and small q\bar{q} dipoles, for Q^2 larger than 10 to 20 GeV^2. They support flavour independence of the diffractive exchange, expressed in terms of the scaling variable (Q^2 + M_V^2)/4, and proton vertex factorisation. The spin density matrix elements are measured as a function of kinematic variables. The ratio of the longitudinal to transverse cross sections, the ratio of the helicity amplitudes and their relative phases are extracted. Several of these measurements have not been performed before and bring new information on the dynamics of diffraction in a QCD framework. The measurements are discussed in the context of models using generalised parton distributions or universal dipole cross sections.
Q**2 dependence of the GAMMA* P elastic RHO0 meson production at mean W There is an additional overall normalization uncertainty of 3.9 PCT.
Q**2 dependence of the GAMMA* P cross section for proton dissociative RHO0 meson production at mean W There is an additional overall normalization uncertainty of 4.6 PCT.
Q**2 dependence of the GAMMA* P elastic PHI meson production at mean W There is an additional overall normalization uncertainty of 4.7 PCT.
Total and differential cross sections for $\eta$ and $\eta ^\prime$ photoproduction off the proton have been determined with the CBELSA/TAPS detector for photon energies between 0.85 and 2.55 GeV. The $\eta$ mesons are detected in their two neutral decay modes, $\eta\to\gamma\gamma$ and $\eta\to 3\pi^0\to 6\gamma$, and for the first time, cover the full angular range in $\rm cos \theta_{cm}$ of the $\eta$ meson. These new $\eta$ photoproduction data are consistent with the earlier CB-ELSA results. The $\eta ^\prime$ mesons are observed in their neutral decay to $\pi^0\pi^0\eta\to 6\gamma$ and also extend the coverage in angular range.
Differential cross section for ETA production at incident photon energy 0.850 to 0.900 GeV.
Differential cross section for ETA production at incident photon energy 0.900 to 0.950 GeV.
Differential cross section for ETA production at incident photon energy 0.950 to 1.000 GeV.
A measurement of elastic deeply virtual Compton scattering gamma* p -> gamma p using e^+ p and e^- p collision data recorded with the H1 detector at HERA is presented. The analysed data sample corresponds to an integrated luminosity of 306 pb^-1, almost equally shared between both beam charges. The cross section is measured as a function of the virtuality Q^2 of the exchanged photon and the centre-of-mass energy W of the gamma* p system in the kinematic domain 6.5 < Q^2 < 80 GeV^2, 30 < W < 140 GeV and |t| < 1 GeV^2, where t denotes the squared momentum transfer at the proton vertex. The cross section is determined differentially in t for different Q^2 and W values and exponential t-slope parameters are derived. Using e^+ p and e^- p data samples, a beam charge asymmetry is extracted for the first time in the low Bjorken x kinematic domain. The observed asymmetry is attributed to the interference between Bethe-Heitler and deeply virtual Compton scattering processes. Experimental results are discussed in the context of two different models, one based on generalised parton distributions and one based on the dipole approach.
The DVCS cross section as a function of Q**2.
The DVCS cross section as a function of W.
The DVCS cross section as a function of W for three different Q**2 regions.
Inclusive non-diffractive photoproduction of rho(770)^0, K^*(892)^0 and phi(1020) mesons is investigated with the H1 detector in ep collisions at HERA. The corresponding average \gamma p centre-of-mass energy is 210 GeV. The mesons are measured in the transverse momentum range 0.5<p_T<7 GeV and the rapidity range |y_{lab}|<1. Differential cross sections are presented as a function of transverse momentum and rapidity, and are compared to the predictions of hadroproduction models.
Inclusive non-diffractive photoproduction cross sections in the specified kinematic region.
Ratios of cross sections.
Inclusive non-diffractive photoproduction invariant differential cross sections as a function of PT in the specified kinematic range.
The first measurement of diffractive scattering of quasi-real photons with large momentum transfer gamma p -> gamma Y, where Y is the proton dissociative system, is made using the H1 detector at HERA. The measurement is performed for initial photon virtualities Q^2 < 0.01 GeV^2. Cross sections are measured as a function of W, the incident photon-proton entre of mass energy, and t, the square of the four-momentum transferred at the proton vertex, in the range 175 < W < 247 GeV and 4<|t|<36 GeV^2. The W dependence is well described by a model based on perturbative QCD using a leading logarithmic approximation of the BFKL evolution. The measured |t| dependence is harder than that predicted by the model and those observed in exclusive vector meson production.
Cross section for the process E+ P --> E+ GAMMA DD as a function of W.
Cross section for the process GAMMA P --> GAMMA DD as a function of W.
Cross section for the process E+ P --> E+ GAMMA DD as a function of T.
The reaction $\gamma p\to p\pi^0\eta$ has been studied with the CBELSA detector at the tagged photon beam of the Bonn electron stretcher facility. The reaction shows contributions from $\Delta^+(1232)\eta$, $N(1535)^+\pi^0$ and $pa_0(980)$ as intermediate states. A partial wave analysis suggests that the reaction proceeds via formation of six $\Delta$ resonances, $\Delta(1600)P_{33}$, $\Delta(1920)P_{33}$, $\Delta(1700)D_{33}$, $\Delta(1940)D_{33}$, $\Delta(1905)F_{35}$, $\Delta(2360)D_{33}$, and two nucleon resonances $N(1880)P_{11}$ and $N(2200)P_{13}$, for which pole positions and decay branching ratios are given.
Total cross section for GAMMA P --> P PI0 ETA.
Differential cross sections as a function of the angles of the individual final state particles for the W range 1.7 to 1.9 GeV.. Errors shown are statistical only.
Differential cross sections as a function of the angles of the individual final state particles for the W range 1.9 to 2.1 GeV.. Errors shown are statistical only.
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.
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