arXiv subjects

M. Schumacher

Publications and source records attributed to M. Schumacher.

32 records · Page 2Linked to original sources

arXiv2003

Threshold enhancement in eta photoproduction from 2H and 4He

The photoproduction of eta-mesons from 2H and 4He has been studied for energies close to the production thresholds. The experiments were carried out with the tagged photon beam of the Mainz MAMI accelerator. The eta-mesons were detected via their two photon decays with the electromagnetic calorimeter TAPS. Total cross sections, angular and momentum distributions of the eta-mesons have been determined for both reactions. The total cross sections in the threshold region show a large enhancement over the predictions of a participant - spectator model, indicating significant final state interaction effects. The results are compared to recent model calculations taking into account nucleon-nucleon and nucleon-eta final state interaction effects on different levels of sophistication.

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arXiv2003

Quasi-free Compton Scattering and the Polarizabilities of the Neutron

Differential cross sections for quasi-free Compton scattering from the proton and neutron bound in the deuteron have been measured using the Glasgow/Mainz tagging spectrometer at the Mainz MAMI accelerator together with the Mainz 48 cm $\oslash$ $\times$ 64 cm NaI(Tl) photon detector and the Göttingen SENECA recoil detector. The data cover photon energies ranging from 200 MeV to 400 MeV at $θ^{LAB}_γ=136.2^\circ$. Liquid deuterium and hydrogen targets allowed direct comparison of free and quasi-free scattering from the proton. The neutron detection efficiency of the SENECA detector was measured via the reaction $p(γ,π^+ n)$. The "free" proton Compton scattering cross sections extracted from the bound proton data are in reasonable agreement with those for the free proton which gives confidence in the method to extract the differential cross section for free scattering from quasi-free data. Differential cross sections on the free neutron have been extracted and the difference of the electromagnetic polarizabilities of the neutron have been obtained to be $α-β= 9.8\pm 3.6(stat){}^{2.1}_1.1(syst)\pm 2.2(model)$ in units $10^{-4}fm^3$. In combination with the polarizability sum $α+β=15.2\pm 0.5$ deduced from photoabsorption data, the neutron electric and magnetic polarizabilities, $α_n=12.5\pm 1.8(stat){}^{+1.1}_{-0.6}\pm 1.1(model)$ and $β_n=2.7\mp 1.8(stat){}^{+0.6}_{-1.1}(syst)\mp 1.1(model)$ are obtained. The backward spin polarizability of the neutron was determined to be $γ^{(n)}_π=(58.6\pm 4.0)\times 10^{-4}fm^4$.

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arXiv2002

Electric polarizabilities of proton and neutron and the relativistic center-of-mass coordinate

We argue that the relativistic correction $δ{\bf R}_{c.m.}$ to the center-of-mass vector can lead to the approximate equality of the proton and neutron electric polarizabilities in the quark model. The explicit form of $δ{\bf R}_{c.m.}$ depends only on the non-relativistic potential between quarks. In particular, this correction is the same for the potential generated by Lorentz-vector and -scalar interactions.

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arXiv2002

Neutron polarizabilities investigated by quasi-free Compton scattering from the deuteron

Measuring Compton scattered photons and recoil neutrons in coincidence, quasi-free Compton scattering by the neutron has been investigated at MAMI (Mainz) at $theta^{lab}_γ=136^o$ in an energy range from 200 to 400 MeV. From the data a polarizability difference of $α_n - β_n = 9.8 \pm 3.6(stat)^{+2.1}_{-1.1}(syst)\pm 2.2(model)$ in units of $10^{-4}fm^3$ has been determined. In combination with the polarizability sum $α_n+β_n= 15.2\pm 0.5$ deduced from photo absorption data, the neutron electric and magnetic polarizabilities, $α_n=12.5\pm 1.8(stat)^{+1.1}_{-0.6}(syst)\pm 1.1(model)$ and $β_n = 2.7\mp 1.8(stat)^{+0.6}_{-1.1}(syst)\mp 1.1(model)$, are obtained.

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arXiv2001

Backward spin polarizability $γ_π$ of the proton

Using the Mainz $48 cm Ø\times 64 cm$ NaI(Tl) detector and the segmented Göttingen recoil detector SENECA in coincidence, Compton scattering by the proton at $θ^{lab}_γ= 136^\circ$ has been measured at MAMI (Mainz) in the energy range from 200 to 470 MeV. The new data confirm the previous observation that there is a systematic discrepancy between MAMI and LEGS (Brookhaven) data leading to different spin polarizabilities $γ_π= - 38.7 \pm 1.8$ and $-27.2 \pm 3.1$ $(\times 10^{-4}fm^4)$, respectively.

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arXiv2001

Compton Scattering by the Proton using a Large-Acceptance Arrangement

Compton scattering by the proton has been measured using the tagged-photon facility at MAMI (Mainz) and the large-acceptance arrangement LARA. The new data are interpreted in terms of dispersion theory based on the SAID-SM99K parameterization of photo-meson amplitudes. It is found that two-pion exchange in the t-channel is needed for a description of the data in the second resonance region. The data are well represented if this channel is modeled by a single pole with mass parameter m(sigma)=600 MeV. The asymptotic part of the spin dependent amplitude is found to be well represented by pi-0-exchange in the t-channel. A backward spin-polarizability of gamma(pi)=(-37.1+-0.6(stat+syst)+-3.0(model))x10^{-4}fm^4 has been determined from data of the first resonance region below 455 MeV. This value is in a good agreement with predictions of dispersion relations and chiral pertubation theory. From a subset of data between 280 and 360 MeV the resonance pion-photoproduction amplitudes were evaluated leading to a E2/M1 multipole ratio of the p-to-Delta radiative transition of EMR(340 MeV)=(-1.7+-0.4(stat+syst)+-0.2(model))%. It was found that this number is dependent on the parameterization of photo-meson amplitudes. With the MAID2K parameterization an E2/M1 multipole ratio of EMR(340 MeV)=(-2.0+-0.4(stat+syst)+-0.2(model))% is obtained.

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arXiv2001

Compton Scattering by the Proton

Compton scattering by the proton has been measured over a wide range covering photon energies 250 MeV < E_γ< 800 MeV and photon scattering angles 30^0 < θ^{lab}_γ< 150^0, using the tagged-photon facility at MAMI (Mainz) and the large-acceptance arrangement LARA. The data are in good agreement with the dispersion theory based on the SAID-SM99K parameterization of photo-meson amplitudes. From the subset of data between 280 and 360 MeV the resonance pion-photoproduction amplitudes were evaluated leading to the multipole E2/M1 ratio EMR(340 MeV) =(-1.6 \pm 0.4(stat+syst) \pm 0.2(model)%. From all data below 455 MeV the proton's backward spin polarizability was determined to be γ_π=(-37.9 \pm 0.6(stat+syst) \pm 3.5(model))x10^{-4}fm^4.

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arXiv2001

Relativistic corrections to the electromagnetic polarizabilities of compound systems

The low-energy amplitude of Compton scattering on the bound state of two charged particles of arbitrary masses, charges and spins is calculated. A case in which the bound state exists due to electromagnetic interaction (QED) is considered. The term, proportional to $ω^2$, is obtained taking into account the first relativistic correction. It is shown that the complete result for this correction differs essentially from the commonly used term $Δα$, proportional to the r.m.s. charge radius of the system. We propose that the same situation can take place in the more complicated case of hadrons.

hep-ph↗

arXiv2000

The inclusive reaction d(γ,π)NN in the first resonance region

Inclusive single pion photoproduction on the deuteron is studied in the first resonance region. The calculation is based on the use of the diagrammatic approach. Pole diagrams and one-loop diagrams with NN rescattering in the final state are taken into account. The elementary operator for pion photoproduction from the nucleon is taken in on-shell form and calculated using the SAID and MAID multipole analyses. Our predictions for total and differential cross section show good agreement with the available experimental data. Invoking some information on the reactions $gamma d\toπ^0 d and $γd\to np$ we predict the total photoabsorption cross section for deuterium. We find that our results overestimate the experimental data in the center of the Δ-peak (\sim 320 MeV) by about 10%.

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arXiv2000

The reaction $^2H(γ,π^0)np$ in the threshold region

Neutral pion photoproduction on the deuteron in the inelastic channel is studied in the threshold region from 142 to 160 MeV. The calculation is based on the use of the diagrammatic approach. Contributions from the pole diagrams, one-loop diagrams both with $n$-$p$ and $π$-$N$ rescattering as well as two-loop diagrams with simultaneous inclusion of these rescattering mechanisms are taken into account. We have found a reasonable agreement with a very simple calculation by J.C. Bergstrom et al. [Phys. Rev. C 57 (1998) 3203]. It is shown that the `effective' electric dipole deuteron amplitude $E_d$ is negative in agreement with a prediction of ChPT. We conclude that a deviation of about 20% from a ChPT prediction for $E_d$ discovered in an experiment on coherent $π^0$ photoproduction off the deuteron cannot be attributed to inadequate estimates of inelastic channel contributions. We suggest that $π$-$N$ rescattering is responsible for the deviation of the free-nucleon $p_1$ threshold amplitude from the value measured in that experiment

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arXiv1999

Coherent π^\circ photoproduction from ^4He

Differential cross sections and beam asymmetries for coherent π^\circ photoproduction from ^4He in the Δenergy-range have been measured with high statistical and systematic precisions using both decay photons for identifying the process.The experiment was performed at the MAinz MIcrotron using the TAPS photon spectrometer and the Glasgow/Mainz tagged photon facility. The differential cross sections are in excellent agreement with predictions based on the DWIA if an appropriate parametrization of the Δ-nuclear interaction is applied. The beam asymmetries are interpreted in terms of degrees of linear polarization of collimated coherent bremsstrahlung. The expected increase of the degree of linear polarization with decreasing collimation angle is confirmed. Agreement with calculations is obtained on a few-percent level of precision in the maxima of the coherent peaks.

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arXiv1999

Compton Scattering by Nuclei

The concept of Compton scattering by even-even nuclei from giant-resonance to nucleon-resonance energies and the status of experimental and theoretical researches in this field are outlined. Nuclear Compton scattering in the giant-resonance energy-region provides information on the dynamical properties of the in-medium mass of the nucleon. The electromagnetic polarizabilities of the nucleon in the nuclear medium can be extracted from nuclear Compton scattering data obtained in the quasi-deuteron energy-region. Recent results are presented for two-body effects due to the mesonic seagull amplitude and due to the excitation of nucleon internal degrees of freedom accompanied by meson exchanges. Due to these studies the in-medium electromagnetic polarizabilities are by now well understood, whereas the understanding of nuclear Compton scattering in the Delta-resonance range is only at the beginning. Phenomenological methods how to include retardation effects in the scattering amplitude are discussed and compared with model predictions.

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arXiv1999

Electromagnetic Polarizabilities of Nucleons bound in $^{40}$Ca, $^{16}$O and $^4$He

Differential cross sections for elastic scattering of photons have been measured for $^{40}$Ca at energies of 58 and 74 MeV and for $^{16}$O and $^4$He at 61 MeV, in the angular range from 45$^o$ to 150$^o$. Evidence is obtained that there are no significant in-medium modifications of the electromagnetic polarizabilities except for those originating from meson exchange currents.

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arXiv1996

Contribution of real and virtual Pions to nuclear Photoabsorption at Intermediate Energies

The universal curve $σ/A$ of nuclear photoabsorption is investigated within a Fermi gas model of nuclear matter. An energy range from pion threshold up to 400 MeV is considered. The interactions between nucleon, pion, $Δ$-isobar and photon are considered in the non-relativistic approximation with corrections of the order $1/M$ taken into account with respect to proton mass. Analytical expressions are obtained, in which the influence of nuclear correlations, two-nucleon contributions and relativistic corrections is studied explicitely. An extension of the model calculation to nucleon knock-out reactions is discussed.

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