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Search Results: 1 - 10 of 130337 matches for " V. Metag "
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Neutral Pions and Eta Mesons as Probes of the Hadronic Fireball in Nucleus-Nucleus Collisions around 1A GeV
R. Averbeck,R. Holzmann,V. Metag,R. S. Simon
Physics , 2000, DOI: 10.1103/PhysRevC.67.024903
Abstract: Chemical and thermal freeze-out of the hadronic fireball formed in symmetric collisions of light, intermediate-mass, and heavy nuclei at beam energies between 0.8A GeV and 2.0A GeV are discussed in terms of an equilibrated, isospin-symmetric ideal hadron gas with grand-canonical baryon-number conservation. For each collision system the baryochemical potential mu_B and the chemical freeze-out temperature T_c are deduced from the inclusive neutral pion and eta yields which are augmented by interpolated data on deuteron production. With increasing beam energy mu_B drops from 800 MeV to 650 MeV, while T_c rises from 55 MeV to 90 MeV. For given beam energy mu_B grows with system size, whereas T_c remains constant. The centrality dependence of the freeze-out parameters is weak as exemplified by the system Au+Au at 0.8A GeV. For the highest beam energies the fraction of nucleons excited to resonance states reaches freeze-out values of nearly 15 %, suggesting resonance densities close to normal nuclear density at maximum compression. In contrast to the particle yields, which convey the status at chemical freeze-out, the shapes of the related transverse-mass spectra do reflect thermal freeze-out. The observed thermal freeze-out temperatures T_th are equal to or slightly lower than T_c, indicative of nearly simultaneous chemical and thermal freeze-out.
In-medium Properties of Hadrons
Metag Volker
EPJ Web of Conferences , 2012, DOI: 10.1051/epjconf/20123701025
Abstract: An overview is given over recent results on in-medium properties of hadrons, obtained in experiments with photon, proton and deuteron beams at ANKE, CBELSA/TAPS, Crystal Ball, HADES, and BigRIPS. These experiments focus on identifying spectral inmedium modifications of hadrons, frequently discussed in the context of a partial restoration of chiral symmetry at finite nuclear densities. Three experimental approaches are presented: the measurement of the transparency ratio, the meson line shape analysis, and the search for meson-nucleus bound states. Results for ω, , and η' mesons indicate a broadening in the nuclear medium. Corresponding inelastic in-medium meson-nucleon cross sections have been extracted. Evidence for an in-medium mass shift has not been reported. Further information on the meson-nucleus interaction is derived from a spectroscopy of meson-nucleus bound states. A critical comparison of the results with theoretical predictions is presented.
Medium Modifcations of Mesons in Elementary Reactions and Heavy-Ion Collisions
Volker Metag
Physics , 2007, DOI: 10.1016/j.ppnp.2007.12.041
Abstract: Experimental searches for modifications of vector mesons in the nuclear medium are reviewed. Data on $\rho,\omega$ and $\Phi$ mesons are presented. The results have been obtained in elementary reactions with proton and photon beams as well as in heavy-ion collisions. Compared to the free particle properties, the $\omega$ and $\Phi$ meson are found to drop in mass at normal nuclear matter density by 9-14% and 3.5% whereas their widths are reported to increase by factors of about 16 and 3.6, respectively. For the $\rho$ meson, conflicting results on in-medium mass shifts and broadening have been published. The experimental data are compared to recent model calculations.
In-medium Properties of Hadrons - an Overview
Volker Metag
Physics , 2012,
Abstract: An overview is given over recent results on in-medium properties of hadrons, obtained in experiments with photon, proton and deuteron beams at ANKE, CBELSA/TAPS, Crystal Ball, HADES, and BigRIPS. These experiments focus on identifying spectral in-medium modifications of hadrons, frequently discussed in the context of a partial restoration of chiral symmetry at finite nuclear densities. Three experimental approaches are presented: the measurement of the transparency ratio, the meson line shape analysis, and the search for meson-nucleus bound states. Results for $\omega,\phi, $ and $ \eta^\prime$ mesons indicate a broadening in the nuclear medium. Corresponding inelastic in-medium meson-nucleon cross sections have been extracted. Evidence for an in-medium mass shift has not been reported. Further information on the meson-nucleus interaction is derived from a spectroscopy of meson-nucleus bound states. A critical comparison of the results with theoretical predictions is presented.
Coherent pi0-photoproduction from atomic nuclei
B. Krusche,J. Ahrens,R. Beck,S. Kamalov,V. Metag,R. O. Owens,H. Stroeher
Physics , 2003, DOI: 10.1016/S0370-2693(01)01503-9
Abstract: Differential and total cross sections for coherent pi0-photoproduction from 12C, 40Ca, 93Nb and Pb targets have been measured throughout the region of the Delta(1232)-resonance. The experiments were performed with the TAPS-detector at the Mainz accelerator MAMI. The characteristic proportionality of the cross section to the square of the atomic mass number and to the nuclear mass form factor is clearly demonstrated. The data allow for the first time detailed tests of model predictions for this reaction. The comparison of the data to model predictions shows that the Delta-nucleus interaction saturates: it is described for heavy nuclei with the same potential parameters as for 4He.
Surface and volume effects in the photoabsorption of nuclei
B. Krusche,J. Lehr,F. Bloch,M. Kotulla,V. Metag,U. Mosel,S. Schadmand
Physics , 2004, DOI: 10.1140/epja/i2004-10098-4
Abstract: Recent experimental results for meson photoproduction from nuclei obtained with TAPS at MAMI are analyzed in view of the suppression of the second nucleon resonance region in total photoabsorption. The cross sections can be split into a component from the low density surface region of nuclei and a component which scales more like the nuclear volume. The energy dependence of the surface component is similar to the deuteron cross section, it shows a clear signal for the second resonance peak assigned to the excitation of the P11(1440), D13(1520), and S11(1535). The volume component behaves differently, it is lacking the second resonance peak and shows an enhancement at intermediate photon energies.
Studying the $ω$-mass in-medium in $γ+ A\to π^0γ+X$ reactions
J. G. Messchendorp,A. Sibirtsev,W. Cassing,V. Metag,S. Schadmand
Physics , 2001, DOI: 10.1007/s100500170099
Abstract: Simulations based on a coupled-channel transport model have been performed to analyze the feasibility to study the in-medium $\omega$ mass exploiting the process $\gamma+A\to\pi^0\gamma+X$ for C, Ca and Nb nuclei. The distortions due to final-state interactions of the $\pi^0$ and background contributions from the reaction $\gamma+A\to\pi^0\pi^0+X$ are found to be small in the mass range of interest ($0.6
In-medium properties of the D13(1520) nucleon resonance
B. Krusche,J. Ahrens,R. Beck,I. J. D. MacGregor,J. C. McGeorge,V. Metag,H. Stroeher
Physics , 2003, DOI: 10.1103/PhysRevLett.86.4764
Abstract: The in-medium properties of the D13(1520) nucleon resonance were studied via photoproduction of pi0-mesons from nuclei (C, Ca, Nb, Pb) with the TAPS-detector at the Mainz MAMI accelerator. The inclusive (single and multiple pion production) data disagree with model predictions which explain the disappearance of the second resonance bump in total photoabsorption via a medium modification of the D13 --> N rho decay. The exclusive single pi0 production data show no broadening of the resonance structure beyond Fermi smearing. Although due to final state interaction effects pion production tests the nucleus at densities less than the average nuclear density, both results together cast doubt on attempts to explain the vanishing of the second resonance bump for heavy nuclei by a broadening of the D13-resonance in nuclear matter.
Exclusive measurements of quasi-free eta-photoproduction from deuterium
J. Weiss,P. Achenbach,R. Beck,V. Hejny,V. Kleber,M. Kotulla,B. Krusche,V. Kuhr,R. Leukel,V. Metag,V. M. Olmos de Leon,R. O. Owens,F. Rambo,A. Schmidt,U. Siodlaczek,H. Stroeher,F. Wissman,M. Wolf
Physics , 2002,
Abstract: Quasi-free photoproduction of eta-mesons from the deuteron has been measured at the tagged photon facility of the Mainz microtron MAMI with the photon spectrometer TAPS for incident photon energies from the production threshold at 630 MeV up to 820 MeV. In a fully exclusive measurement eta-mesons and recoil nucleons were detected in coincidence. At incident photon energies above the production threshold on the free nucleon, where final state interaction effects are negligible, an almost constant ratio of sigma(n)/sigma(p)=0.66+/-0.10 was found. At lower incident photon energies the ratio rises due to re-scattering effects. The average ratio agrees with the value extracted from a comparison of the inclusive d(gamma,eta)X cross section to the free proton cross section via a participant - spectator model for the deuteron cross section (sigma(n)/sigma(p)=0.67+/-0.07). The angular dependence of the ratio agrees with the expected small deviation of neutron and proton cross section from isotropy due to the influence of the D13(1520) resonance. The energy dependence of the ratio in the excitation energy range of the S11(1535) resonance disfavors model predictions which try to explain this resonance as K-Sigma bound state.
Pluto: A Monte Carlo Simulation Tool for Hadronic Physics
I. Froehlich,L. Cazon Boado,T. Galatyuk,V. Hejny,R. Holzmann,M. Kagarlis,W. Kuehn,J. G. Messchendorp,V. Metag,M. -A. Pleier,W. Przygoda,B. Ramstein,J. Ritman,P. Salabura,J. Stroth,M. Sudol
Physics , 2007,
Abstract: Pluto is a Monte-Carlo event generator designed for hadronic interactions from Pion production threshold to intermediate energies of a few GeV per nucleon, as well as for studies of heavy ion reactions. This report gives an overview of the design of the package, the included models and the user interface.
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