arXiv subjects

S. Schadmand

Publications and source records attributed to S. Schadmand.

83 records · Page 5Linked to original sources

arXiv2004

Photoproduction of eta-mesic 3He

The photoproduction of eta-mesic 3He has been investigated using the TAPS calorimeter at the Mainz Microtron accelerator facility MAMI. The total inclusive cross section for the reaction gamma+3He->eta+X has been measured for photon energies from threshold to 820 MeV. The total and angular differential coherent eta cross sections have been extracted up to energies of 745 MeV. A resonance-like structure just above the eta production threshold with an isotropic angular distribution suggests the existence of a resonant quasi-bound state. This is supported by studies of a competing decay channel of such a quasi-bound eta-mesic nucleus into pi^0+p+X. A binding energy of (-4.4+-4.2) MeV and a width of (25.6+-6.1) MeV is deduced for the quasi-bound eta-mesic state in 3He.

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arXiv2004

Photoproduction of pi0-mesons from nuclei

Photoproduction of neutral pions from nuclei (carbon, calcium, niobium, lead) has been studied for incident photon energies from 200 MeV to 800 MeV with the TAPS detector using the Glasgow photon tagging spectrometer at the Mainz MAMI accelerator. Data were obtained for the inclusive photoproduction of neutral pions and the partial channels of quasifree single pi0, double pi0, and pi0pi+/- photoproduction. They have been analyzed in terms of the in-medium behavior of nucleon resonances and the pion - nucleus interaction. They are compared to earlier measurements from the deuteron and to the predictions of a Boltzmann-Uehling-Uhlenbeck (BUU) transport model for photon induced pion production from nuclei.

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arXiv2003

Double $π^0$ Photoproduction off the Proton at Threshold

The reaction $γp \to π^0 π^0 p$ has been measured using the TAPS BaF$_2$ calorimeter at the tagged photon facility of the Mainz Microtron accelerator. Chiral perturbation theory (ChPT) predicts that close to threshold this channel is significantly enhanced compared to double pion final states with charged pions. In contrast to other reaction channels, the lower order tree terms are strongly suppressed in 2$π^0$ photoproduction. The consequence is the dominance of pion loops in the 2$π^0$ channel close to threshold - a result that opens new prospects for the test of ChPT and in particular its inherent loop terms. The present measurement is the first which is sensitive enough for a conclusive comparison with the ChPT calculation and is in agreement with its prediction. The data also show good agreement with a calculation in the unitary chiral approach.

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arXiv2003

Study of Non-Strange Baryon Resonances with Meson Photoproduction

Photoproduction of mesons is an excellent tool for the study of nucleon resonances. Complementary to pion induced reactions, photoproduction on the free proton contributes to the determination of the basic properties of nucleon resonances like excitation energy, decay widths, spin, and the coupling to the photon. Photoproduction from light nuclei, in particular from the deuteron, reveals the isospin structure of the electromagnetic excitation of the nucleon. During the last few years, progress in this field has been substantial. New accelerator facilities combined with state-of-the-art detector technologies have pushed the experiments to unprecedented sensitivity and precision. The experimental progress has been accompanied by new developments for the reaction models, necessary to extract the properties of the nucleon states, and for modern hadron models which try to connect these properties to QCD. The emphasis of this review lies on the experimental side and focuses on experiments aiming at precise studies of the low-lying nucleon resonances.

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arXiv2002

The reaction $γp \to π^\circ γ^\prime p$ and the magnetic dipole moment of the $Δ^+(1232)$ resonance

The reaction $γp \to π^\circ γ^\prime p$ has been measured with the TAPS calorimeter at the Mainz Microtron accelerator facility MAMI for energies between $\sqrt{s}$ = 1221--1331 MeV. Cross sections differential in angle and energy have been determined for all particles in the final state in three bins of the excitation energy. This reaction channel provides access to the magnetic dipole moment of the $Δ^{+}(1232)$ resonance and, for the first time, a value of $μ_{Δ^+} = (2.7_{-1.3}^{+1.0}(stat.) \pm 1.5 (syst.) \pm 3(theo.)) μ_N$ has been extracted.

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arXiv2002

In-medium modifications of the $ππ$ interaction in photon-induced reactions

Differential cross sections of the reactions $(γ,π^\circπ^\circ)$ and $(γ,π^\circπ^++π^\circπ^-)$ have been measured for several nuclei ($^1$H,$^{12}$C, and $^{\rm nat}$Pb) at an incident-photon energy of $E_γ$=400-460 MeV at the tagged-photon facility at MAMI-B using the TAPS spectrometer. A significant nuclear-mass dependence of the $ππ$ invariant-mass distribution is found in the $π^\circπ^\circ$ channel. This dependence is not observed in the $π^\circπ^{+/-}$ channel and is consistent with an in-medium modification of the $ππ$ interaction in the $I$=$J$=0 channel. The data are compared to $π$-induced measurements and to calculations within a chiral-unitary approach.

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arXiv2001

Suppression of soft nuclear bremsstrahlung in proton-nucleus collisions

Photon energy spectra up to the kinematic limit have been measured in 190 MeV proton reactions with light and heavy nuclei to investigate the influence of the multiple-scattering process on the photon production. Relative to the predictions of models based on a quasi-free production mechanism a strong suppression of bremsstrahlung is observed in the low-energy region of the photon spectrum. We attribute this effect to the interference of photon amplitudes due to multiple scattering of nucleons in the nuclear medium.

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arXiv2001

Hard photon and neutral pion production in cold nuclear matter

The production of hard photons and neutral pions in 190 MeV proton induced reactions on C, Ca, Ni, and W targets has been for the first time concurrently studied. Angular distributions and energy spectra up to the kinematical limit are discussed and the production cross-sections are presented. From the target mass dependence of the cross-sections the propagation of pions through nuclear matter is analyzed and the production mechanisms of hard photons and primordial pions are derived. It is found that the production of subthreshold particles proceeds mainly through first chance nucleon-nucleon collisions. For the most energetic particles the mass scaling evidences the effect of multiple collisions.

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arXiv2001

Studying the $ω$-mass in-medium in $γ+ A\to π^0γ+X$ reactions

Simulations based on a coupled-channel transport model have been performed to analyze the feasibility to study the in-medium $ω$ mass exploiting the process $γ+A\toπ^0γ+X$ for C, Ca and Nb nuclei. The distortions due to final-state interactions of the $π^0$ and background contributions from the reaction $γ+A\toπ^0π^0+X$ are found to be small in the mass range of interest ($0.6<M_{π^0γ}<0.8$ GeV). Furthermore, the effect of the detector resolution on the $π^0γ$-mass determination is discussed.

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arXiv2001

Evidence for Thermal Equilibration in Multifragmentation Reactions probed with Bremsstrahlung Photons

The production of nuclear bremsstrahlung photons (E$_γ>$ 30 MeV) has been studied in inclusive and exclusive measurements in four heavy-ion reactions at 60{\it A} MeV. The measured photon spectra, angular distributions and multiplicities indicate that a significant part of the hard-photons are emitted in secondary nucleon-nucleon collisions from a thermally equilibrated system. The observation of the thermal component in multi-fragment $^{36}$Ar+$^{197}$Au reactions suggests that the breakup of the thermalized source produced in this system occurs on a rather long time-scale.

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arXiv2000

Thermal bremsstrahlung probing the thermodynamical state of multifragmenting systems

Inclusive and exclusive hard-photon (E$_γ>$ 30 MeV) production in five different heavy-ion reactions ($^{36}$Ar+$^{197}$Au, $^{107}$Ag, $^{58}$Ni, $^{12}$C at 60{\it A} MeV and $^{129}$Xe+$^{120}$Sn at 50{\it A} MeV) has been studied coupling the TAPS photon spectrometer with several charged-particle multidetectors covering more than 80% of 4$π$. The measured spectra, slope parameters and source velocities as well as their target-dependence, confirm the existence of thermal bremsstrahlung emission from secondary nucleon-nucleon collisions that accounts for roughly 20% of the total hard-photon yield. The thermal slopes are a direct measure of the temperature of the excited nuclear systems produced during the reaction.

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