arXiv subjects

K. Essig

Publications and source records attributed to K. Essig.

9 recordsLinked to original sources

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

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

Effect of a sweeping conductive wire on electrons stored in the Penning trap between the KATRIN spectrometers

The KATRIN experiment is going to search for the mass of the electron antineutrino down to 0.2 eV/c^2. In order to reach this sensitivity the background rate has to be understood and minimised to 0.01 counts per second. One of the background sources is the unavoidable Penning trap for electrons due to the combination of the electric and magnetic fields between the pre- and the main spectrometer at KATRIN. In this article we will show that by sweeping a conducting wire periodically through such a particle trap stored particles can be removed, an ongoing discharge in the trap can be stopped, and the count rate measured with a detector looking at the trap is reduced.

physics.ins-det↗

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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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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