arXiv subjects

D. Menze

Publications and source records attributed to D. Menze.

At least 19 recordsLinked to original sources

arXiv2014

Measurement of polarisation observables in $K^0_sΣ^+$ photoproduction off the proton

The reaction $γ\, p \rightarrow K^0_S\,Σ^+$ is studied in the photon energy range from threshold. Linearly polarised photon beams from coherent bremsstrahlung enabled the first measurement of photon beam asymmetries in this reaction up to $E_γ= 2250$ MeV. In addition, the recoil hyperon polarisation was determined through the asymmetry in the weak decay $Σ^+ \rightarrow p π^0$ up to $E_γ= 1650$ MeV. The data are compared to partial wave analyses, and the possible impact on the interpretation of a recently observed cusp-like structure near the $K^*$ thresholds is discussed.

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arXiv2012

Transparency ratio in γA -> η' A' and the in-medium η' width

The photoproduction of η'-mesons off different nuclei has been measured with the CBELSA/TAPS detector system for incident photon energies between 1500 - 2200 MeV. The transparency ratio has been deduced and compared to theoretical calculations describing the propagation of η'-mesons in nuclei. The comparison indicates a width of the η' -meson of the order of Γ= 15-25 MeV at ρ=ρ_0 for an average momentum p_{η'} = 1050 MeV/c, at which the η'-meson is produced in the nuclear rest frame. The inelastic η' N cross section is estimated to be 3 - 10 mb. Parameterizing the photoproduction cross section of η'-mesons by σ(A) = σ_0 A^α, a value of α= 0.84\pm0.03 has been deduced.

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arXiv2011

Anomaly in the K^0_S Sigma^+ photoproduction cross section off the proton at the K* threshold

The $γ+ p \rightarrow K^0 + Σ^+$ photoproduction reaction is investigated in the energy region from threshold to $E_γ= 2250$\,MeV. The differential cross section exhibits increasing forward-peaking with energy, but only up to the $K^*$ threshold. Beyond, it suddenly returns to a flat distribution with the forward cross section dropping by a factor of four. In the total cross section a pronounced structure is observed between the $K^*Λ$ and $K^*Σ$ thresholds. It is speculated whether this signals the turnover of the reaction mechanism from t-channel exchange below the $K^*$ production threshold to an s-channel mechanism associated with the formation of a dynamically generated $K^*$-hyperon intermediate state.

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arXiv2011

Photoproduction of Neutral Pions off Protons

Photoproduction of neutral pions has been studied with the CBELSA/TAPS detector in the reaction $γp\to pπ^0$ for photon energies between 0.85 and 2.50 GeV. The $π^0$ mesons are observed in their dominant neutral decay mode: $π^0\toγγ$. For the first time, the differential cross sections cover the very forward region, $θ_{\rm c.m.}<60^\circ$. A partial-wave analysis of these data within the Bonn-Gatchina framework observes the high-mass resonances $G_{17}$(2190), $D_{13}$(2080), and $D_{15}$(2070).

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arXiv2011

Quasi-free photoproduction of eta-mesons off the deuteron

Precise data for quasi-free photoproduction of $η$ mesons off the deuteron have been measured at the Bonn ELSA accelerator with the combined Crystal Barrel/TAPS detector for incident photon energies up to 2.5 GeV. The $η$-mesons have been detected in coincidence with recoil protons and neutrons. Possible nuclear effects like Fermi motion and re-scattering can be studied via a comparison of the quasi-free reaction off the bound proton to $η$-production off the free proton. No significant effects beyond the folding of the free cross section with the momentum distribution of the bound protons have been found. These Fermi motion effects can be removed by an analysis using the invariant mass of the $η$-nucleon pairs reconstructed from the final state four-momenta of the particles. The total cross section for quasi-free $η$-photoproduction off the neutron reveals even without correction for Fermi motion a pronounced bump-like structure around 1 GeV of incident photon energy, which is not observed for the proton. This structure is even narrower in the invariant mass spectrum of the $η$-neutron pairs. Position and width of the peak in the invariant mass spectrum are $W\approx 1665$ MeV and FWHM $Γ\approx 25$ MeV. The data are compared to the results of different models.

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arXiv2011

Photoproduction of $ω$ mesons on nuclei near the production threshold

The photoproduction of $ω$ mesons on LH_2, C and Nb has been studied for incident photon energies from 900 to 1300 MeV using the CB/TAPS detector at ELSA. The $ω$ line shape does not show any significant difference between the LH_2 and the Nb targets. The data are compared with results of transport calculations that predict a sensitivity of the $ω$ line shape to in-medium modifications near the production threshold on a free nucleon of $E_γ^{lab}$ = 1109 MeV.}

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arXiv2010

Photoproduction of $η'$-mesons off the deuteron

Quasi-free photoproduction of $η'$ mesons off nucleons bound in the deuteron has been measured with the combined Crystal Barrel - TAPS detector. The experiment was done at a tagged photon beam of the ELSA electron accelerator in Bonn for incident photon energies from the production threshold up to 2.5 GeV. The $η'$-mesons have been detected in coincidence with recoil protons and recoil neutrons. The quasi-free proton data are in good agreement with the results for free protons, indicating that nuclear effects have no significant impact. The coincidence with recoil neutrons provides the first data for the $γn \rightarrow nη'$ reaction. In addition, also first estimates for coherent $η'$-production off the deuteron have been obtained. In agreement with model predictions, the total cross section for this channel is found to be very small, at most at the level of a few nb. The data are compared to model calculations taking into account contributions from nucleon resonances and $t$-channel exchanges.

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arXiv2010

Measurement of the reaction $γp \rightarrow K^ + Σ^{\pm}π^{\mp}$ at photon energies up to 2.6 GeV

The reactions $γp \rightarrow K^{+}Σ^{\pm}π^{\mp}$ were studied with the SAPHIR detector using a tagged photon beam at the electron stretcher facility ELSA in Bonn. The decays $Σ^{-} \rightarrow nπ^{-}$ and $Σ^{+} \rightarrow nπ^{+}, pπ^0$ were fully reconstructed. Reaction cross sections were measured as a function of the photon energy from threshold up to $2.6\,$GeV with considerably improved statistics compared to a previous bubble chamber measurement. The cross sections rise monotonously with increasing photon energy. The two-particle mass distributions of $Σ^{\pm}π^{\mp}$ and $K^+π^-$ show substantial production of resonant states.

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arXiv2010

Measurement of the reaction $γp \TO K^ + Λ(1520)$ at photon energies up to 2.65 GeV

The reaction $γp \TO K^+Λ(1520)$ was measured in the energy range from threshold to 2.65 GeV with the SAPHIR detector at the electron stretcher facility ELSA in Bonn. The $Λ(1520)$ production cross section was analyzed in the decay modes $pK^-$, $n \bar{K}^0$, $Σ^{\pm}π^{\mp}$, and $Λπ^+π^-$ as a function of the photon energy and the squared four-momentum transfer $t$. While the cross sections for the inclusive reactions rise steadily with energy, the cross section of the process $γp \TO K^+Λ(1520)$ peaks at a photon energy of about 2.0 GeV, falls off exponentially with $t$, and shows a slope flattening with increasing photon energy. The angular distributions in the $t$-channel helicity system indicate neither a $K$ nor a $K^\star$ exchange dominance. The interpretation of the $Λ(1520)$ as a $Σ(1385)π$ molecule is not supported.

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arXiv2010

In-medium $ω$ mass from the $γ+ Nb \to π^{0}γ+ X$ reaction

Data on the photoproduction of $ω$ mesons on nuclei have been re-analyzed in a search for in-medium modifications. The data were taken with the Crystal Barrel(CB)/TAPS detector system at the ELSA accelerator facility in Bonn. First results from the analysis of the data set were published by D. Trnka et al. in Phys. Rev. Lett 94 (2005) 192303 \cite{david}, claiming a lowering of the $ω$ mass in the nuclear medium by 14$%$ at normal nuclear matter density. The extracted $ω$ line shape was found to be sensitive to the background subtraction. For this reason a re-analysis of the same data set has been initiated and a new method has been developed to reduce the background and to determine the shape and absolute magnitude of the background directly from the data. Details of the re-analysis and of the background determination are described. The $ω$ signal on the $Nb$ target, extracted in the re-analysis, does not show a deviation from the corresponding line shape on a $LH_2$ target, measured as reference. The earlier claim of an in-medium mass shift is thus not confirmed. The sensitivity of the $ω$ line shape to different in-medium modification scenarios is discussed.

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arXiv2010

Measurement of the Beam Asymmetry $Σ$ in the Forward Direction for pi0 Photoproduction

Photoproduction of neutral pions has been studied with the CBELSA/TAPS detector for photon energies between 0.92 and 1.68~GeV at the electron accelerator ELSA. The beam asymmetry~$Σ$ has been extracted for $115^\circ < θ_{\rm c.m.} < 155^\circ$ of the $π^0$~meson and for $θ_{\rm c.m.} < 60^\circ$. The new beam asymmetry data improve the world database for photon energies above 1.5~GeV and, by covering the very forward region, extend previously published data for the same reaction by our collaboration. The angular dependence of $Σ$ shows overall good agreement with the SAID parameterization.

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arXiv2010

Photoproduction of meson pairs: First measurement of the polarization observable I^s

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.

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arXiv2009

Photoproduction of $η$ and $η\prime$ Mesons off Protons

Total and differential cross sections for $η$ and $η^\prime$ photoproduction off the proton have been determined with the CBELSA/TAPS detector for photon energies between 0.85 and 2.55 GeV. The $η$ mesons are detected in their two neutral decay modes, $η\toγγ$ and $η\to 3π^0\to 6γ$, and for the first time, cover the full angular range in $\rm cos θ_{cm}$ of the $η$ meson. These new $η$ photoproduction data are consistent with the earlier CB-ELSA results. The $η^\prime$ mesons are observed in their neutral decay to $π^0π^0η\to 6γ$ and also extend the coverage in angular range.

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arXiv2008

Photoproduction of eta-mesons off nuclei for Eg < 2.2 GeV

Photoproduction of $η$ 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 $η$ mesons. Careful consideration was given to contributions from $ηπ$ final states and secondary production mechanisms of $η$-mesons e.g. from inelastic $πN$ reactions of intermediate pions. The analysis of the mass number scaling shows that the nuclear absorption cross section $σ_{Nη}$ for $η$ mesons is constant over a wide range of the $η$ 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).

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arXiv2008

Linearly polarised photon beams at ELSA and measurement of the beam asymmetry in pi^0-photoproduction off the proton

At the electron accelerator ELSA a linearly polarised tagged photon beam is produced by coherent bremsstrahlung off a diamond crystal. Orientation and energy range of the linear polarisation can be deliberately chosen by accurate positioning of the crystal with a goniometer. The degree of polarisation is determined by the form of the scattered electron spectrum. Good agreement between experiment and expectations on basis of the experimental conditions is obtained. Polarisation degrees of P = 40% are typically achieved at half of the primary electron energy. The determination of P is confirmed by measuring the beam asymmetry, Σ, in pi^0 photoproduction and a comparison of the results to independent measurements using laser backscattering.

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arXiv2008

Quasi-free photoproduction of eta-mesons of the neutron

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.

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arXiv2008

Modification of the $ω$-Meson Lifetime in Nuclear Matter

The photo production of $ω$ mesons on the nuclei C, Ca, Nb and Pb has been measured using the Crystal Barrel/TAPS detector at the ELSA tagged photon facility in Bonn. The dependence of the $ω$ meson cross section on the nuclear mass number has been compared with three different types of models, a Glauber analysis, a BUU analysis of the Giessen theory group and a calculation by the Valencia theory group. In all three cases, the inelastic $ω$ width is found to be $130-150 \rm{MeV/c^2}$ at normal nuclear matter density for an average 3-momentum of 1.1 GeV/c. In the restframe of the $ω$ meson, this inelastic $ω$ width corresponds to a reduction of the $ω$ lifetime by a factor $\approx 30$. For the first time, the momentum dependent $ω$N cross section has been extracted from the experiment and is in the range of 70 mb.

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arXiv2007

N* and Delta* decays into N pi0 pi0

Decays of baryon resonances in the second and the third resonance region into N pi0 pi0 are studied by photoproduction of two neutral pions off protons. Partial decay widths of N* and Delta* resonances decaying into Delta(1232) pi, N(ππ)_{S}, N(1440)P_{11} pi, and N(1520)D_{13} pi are determined in a partial wave analysis of this data, and data from other reactions. Several partial decay widths were not known before. Interesting decay patterns are observed which are not even qualitatively reproduced by quark model calculations. In the second resonance region, decays into Delta(1232) pi dominate clearly. The N(ππ)_{S}-wave provides a significant contribution to the cross section, especially in the third resonance region. The P_{13}(1720) properties found here are at clear variance to PDG values.

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