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V. Metag

Publications and source records attributed to V. Metag.

At least 145 records · Page 8Linked to original sources
arXiv2010

Lambda-p femtoscopy in collisions of Ar+KCl at 1.76 AGeV

Results on $Λ$p femtoscopy are reported at the lowest energy so far. At a beam energy of 1.76A~GeV, the reaction Ar+KCl was studied with the High Acceptance Di-Electron Spectrometer (HADES) at SIS18/GSI. A high-statistics and high-purity $Λ$ sample was collected, allowing for the investigation of $Λ$ p correlations at small relative momenta. The experimental correlation function is compared to corresponding model calculations allowing the determination of the space-time extent of the $Λ$p emission source. The $Λ$p source radius is found slightly smaller than the pp correlation radius for a similar collis ion system. The present $Λ$p radius is significantly smaller than that found for Au+Au/Pb+Pb collisio ns in the AGS, SPS and RHIC energy domains, but larger than that observed for electroproduction from He. Taking into account all available data, we find the $Λ$p source radius to increase almost linearly with the number of participants to the power of one-third.

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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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arXiv2010

Radiative pi0 photoproduction on protons in the Delta+(1232) region

The reaction gamma p -> p pi0 gamma' has been measured with the Crystal Ball / TAPS detectors using the energy-tagged photon beam at the electron accelerator facility MAMI-B. Energy and angular differential cross sections for the emitted photon gamma' and angular differential cross sections for the pi0 have been determined with high statistics in the energy range of the Delta+(1232) resonance. Cross sections and the ratio of the cross section to the non-radiative process gamma p -> p pi0 are compared to theoretical reaction models, having the anomalous magnetic moment kappa_Delta+ as free parameter. As the shape of the experimental distributions is not reproduced in detail by the model calculations, currently no extraction of kappa_Delta+ is feasible.

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arXiv2009

Recent results from HADES on electron pair production in relativistic heavy-ion collisions

Systematic investigations of dilepton production are performed at the SIS accelerator of GSI with the HADES spectrometer. The goal of this program is a detailed understanding of di-electron emission from hadronic systems at moderate temperatures and densities. New results obtained in HADES experiments focussing on electron pair production in elementary collisions are reported here. They pave the way to a better understanding of the origin of the so-called excess pairs earlier on observed in heavy-ion collisions by the DLS collaboration and lately confirmed in two measurements of the HADES collaboration using C+C and Ar+KCl collisions. Results of these studies are discussed.

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arXiv2009

Photoproduction of pi^0 eta on protons and the Delta(1700)D_{33} resonance

Total and differential cross sections for the reaction gamma p -> pi^o eta p have been measured with the Crystal Ball/TAPS detector using the tagged photon facility at the MAMI C accelerator in Mainz. In the energy range E_gamma=0.95-1.4 GeV the reaction is dominated by the excitation and sequential decay of the Delta(1700)D33 resonance. Angular distributions measured with high statistics allow us to determine the ratio of hadronic decay widths Γ_{ηΔ}/Γ_{πS11} and the ratio of the helicity amplitudes A_{3/2}/A_{1/2} for this resonance.

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arXiv2009

Origin of the low-mass electron pair excess in light nucleus-nucleus collisions

We report measurements of electron pair production in elementary p+p and d+p reactions at 1.25 GeV/u with the HADES spectrometer. For the first time, the electron pairs were reconstructed for n+p reactions by detecting the proton spectator from the deuteron breakup. We find that the yield of electron pairs with invariant mass Me+e- > 0.15 GeV/c2 is about an order of magnitude larger in n+p reactions as compared to p+p. A comparison to model calculations demonstrates that the production mechanism is not sufficiently described yet. The electron pair spectra measured in C+C reactions are compatible with a superposition of elementary n+p and p+p collisions, leaving little room for additional electron pair sources in such light collision systems.

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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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arXiv2009

Deep sub-threshold $Ξ^-$ production in Ar+KCl reactions at 1.76A GeV

We report first results on a deep sub-threshold production of the doubly strange hyperon $Ξ^-$ in a heavy-ion reaction. At a beam energy of 1.76A GeV the reaction Ar+KCl was studied with the High Acceptance Di-Electron Spectrometer (HADES) at SIS18/GSI. A high-statistics and high-purity $Λ$ sample was collected, allowing for the investigation of the decay channel $Ξ^- \to Λπ^-$. The deduced $Ξ^-/(Λ+Σ^0)$ production ratio of $(5.6 \pm 1.2 ^{+1.8}_{-1.7})\cdot 10^{-3}$ is significantly larger than available model predictions.

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arXiv2009

Beam-Helicity Asymmetries in Double Pion Photoproduction off the Proton

Beam-helicity asymmetries have been measured at the MAMI accelerator in Mainz in the three isospin channels $\vecγp\to π^{+}π^0n$, $\vecγp\to π^{0}π^0p$ and $\vecγp\to π^{+}π^{-}p$ . The circularly polarized photons, produced from bremsstrahlung of longitudinally polarized electrons, were tagged with the Glasgow magnetic spectrometer. Charged pions and the decay photons of $π^0$ mesons were detected in a $~4π$ electromagnetic calorimeter which combined the Crystal Ball detector with the TAPS detector. The precisely measured asymmetries are very sensitive to details of the production processes and are thus key observables in the modeling of the reaction dynamics.

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arXiv2009

Future perspectives at SIS-100 with HADES-at-FAIR

Currently, the HADES spectrometer undergoes un upgrade program to be prepared for measurements at the upcoming SIS-100 synchrotron at FAIR. We describe the current status of the HADES di-electron measurements at the SIS-18 and our future plans for SIS-100.

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arXiv2009

Charged pion production in C+C and Ar+KCl collisions measured with HADES

Results of a study of charged pion production in 12C+12C collisions at incident beam energies of 1A GeV and 2A GeV, and 40Ar+natKCl at 1.76AGeV, using the spectrometer HADES at GSI, are presented. We have performed a measurement of the transverse momentum distributions of pi+- mesons covering a fairly large rapidity interval, in case of the C+C collision system for the first time. The yields, transverse mass and angular distributions are compared with a transport model as well as with existing data from other experiments.

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arXiv2009

Measurement of the Slope Parameter $α$ for the $η\to 3π^0$ decay with the Crystal Ball detector at the Mainz Microtron (MAMI-C)

The dynamics of the $η\to 3π^0$ decay have been studied with the Crystal Ball multiphoton spectrometer and the TAPS calorimeter. Bremsstrahlung photons produced by the 1.5-GeV electron beam of the Mainz microtron MAMI-C and tagged by the Glasgow photon spectrometer were used for $η$-meson production. The analysis of $3 \times 10^6$ $γp \to ηp \to 3π^0 p \to 6γp$ events yields the value $α=-0.032\pm0.003$ for the $η\to 3π^0$ slope parameter, which agrees with the majority of recent experimental results and has the smallest uncertainty. The $π^0π^0$ invariant-mass spectrum was investigated for the occurrence of a cusplike structure in the vicinity of the $π^+π^-$ threshold. The observed effect is small and does not affect our measured value for the slope parameter.

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arXiv2009

Measurement of charged pions in 12C + 12C collisions at 1A GeV and 2A GeV with HADES

We present the results of a study of charged pion production in 12C + 12C collisions at incident beam energies of 1A GeV and 2A GeV using the HADES spectrometer at GSI. The main emphasis of the HADES program is on the dielectron signal from the early phase of the collision. Here, however, we discuss the data with respect to the emission of charged hadrons, specifically the production of pi+- mesons, which are related to neutral pions representing a dominant contribution to the dielectron yield. We have performed the first large-angular range measurement of the distribution of pi+- mesons for the 12C + 12C collision system covering a fairly large rapidity interval. The pion yields, transverse-mass and angular distributions are compared with calculations done within a transport model, as well as with existing data from other experiments. The anisotropy of pion production is systematically analyzed.

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arXiv2009

The High-Acceptance Dielectron Spectrometer HADES

HADES is a versatile magnetic spectrometer aimed at studying dielectron production in pion, proton and heavy-ion induced collisions. Its main features include a ring imaging gas Cherenkov detector for electron-hadron discrimination, a tracking system consisting of a set of 6 superconducting coils producing a toroidal field and drift chambers and a multiplicity and electron trigger array for additional electron-hadron discrimination and event characterization. A two-stage trigger system enhances events containing electrons. The physics program is focused on the investigation of hadron properties in nuclei and in the hot and dense hadronic matter. The detector system is characterized by an 85% azimuthal coverage over a polar angle interval from 18 to 85 degree, a single electron efficiency of 50% and a vector meson mass resolution of 2.5%. Identification of pions, kaons and protons is achieved combining time-of-flight and energy loss measurements over a large momentum range. This paper describes the main features and the performance of the detector system.

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arXiv2009

Phi decay: a relevant source for K- production at SIS energies?

We present phase space distributions and multiplicities of K+, K- and phi mesons produced in Ar+KCl reactions at a kinetic beam energy of 1.756 AGeV and measured with the HADES spectrometer. The inverse slope parameters and yields of kaons supplement the systematics of previous measurements. The percentage of K- mesons coming fromphi decay is found to be 18+- 7%.

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arXiv2009

Determination of the Dalitz plot parameter alpha for the decay eta->3pi^0 with the Crystal Ball at MAMI-B

A precise measurement of the Dalitz plot parameter, alpha, for the eta->3pi^0 decay is presented. The experiment was performed with the Crystal Ball and TAPS large acceptance photon detectors at the tagged photon beam facility of the MAMI-B electron accelerator in Mainz. High statistics of 1.8*10^6 eta->3pi^0 events were obtained, giving the result alpha = -0.032 +/- 0.002(stat) +/- 0.002(syst).

hep-ex↗
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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