arXiv subjects

J. C. McGeorge

Publications and source records attributed to J. C. McGeorge.

62 records · Page 4Linked to original sources

arXiv2003

In-medium properties of the D13(1520) nucleon resonance

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.

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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

Three-nucleon mechanisms in photoreactions

The $^{12}$C$(γ,ppn)$ reaction has been measured for E$_γ$=150-800 MeV in the first study of this reaction in a target heavier than $^3$He. The experimental data are compared to a microscopic many body calculation. The model, which predicts that the largest contribution to the reaction arises from final state interactions following an initial pion production process, overestimates the measured cross sections and there are strong indications that the overestimate arises in this two-step process. The selection of suitable kinematic conditions strongly suppresses this two-step contribution leaving cross sections in which up to half the yield is predicted to arise from the absorption of the photon on three interacting nucleons and which agree with the model. The results indicate $(γ,3N)$ measurements on nuclei may be a valuable tool for obtaining information on the nuclear three-body interaction.

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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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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

Test of low-energy theorems for p(\vec{gamma},π^{0})p in the threshold region

The photon asymmetry in the reaction p(\vecγ,π^{0})p close to threshold has been measured for the first time with the photon spectrometer TAPS using linearly polarized photons from the tagged-photon facility at the Mainz Microtron MAMI. The total and differential cross sections were also measured simultaneously with the photon asymmetry. This allowed determination of the S-wave and all three P-wave amplitudes. The low-energy theorems based on the parameter-free third-order calculations of heavy-baryon chiral perturbation theory for P1 and P2 agree with the experimental values.

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