Differential cross sections of the reactions $(\gamma,\pi^\circ\pi^\circ)$ and $(\gamma,\pi^\circ\pi^++\pi^\circ\pi^-)$ have been measured for several nuclei ($^1$H,$^{12}$C, and $^{\rm nat}$Pb) at an incident-photon energy of $E_{\gamma}$=400-460 MeV at the tagged-photon facility at MAMI-B using the TAPS spectrometer. A significant nuclear-mass dependence of the $\pi\pi$ invariant-mass distribution is found in the $\pi^\circ\pi^\circ$ channel. This dependence is not observed in the $\pi^\circ\pi^{+/-}$ channel and is consistent with an in-medium modification of the $\pi\pi$ interaction in the $I$=$J$=0 channel. The data are compared to $\pi$-induced measurements and to calculations within a chiral-unitary approach.
Differential cross section for PI0PI0 production with a proton target. Errors are statistical only. Note that the data given in this table are sightly different (newer) than the data points presented in the paper.
Differential cross section for PI0PI0 production with a Carbon target. Errors are statistical only.
Differential cross section for PI0PI0 production with a Lead target. Errors are statistical only.
The C12(γ,p)11B differential cross section has been measured over proton angles ranging from 58° to 128°, using tagged photons of energy 80–157 MeV, for low-lying regions of residual excitation energy in B11. The data have been compared with four different types of calculation. It is shown that scaling of the cross section with momentum mismatch occurs for both the ground-state and excited-state data.
EX IN 0 MEV REGION (FROM 0 TO 1.5 MEV). ANGULAR BINS OF 5 DEG WIDTH.
EX = 7 (FROM 6.5 TO 8.0 MEV). ANGULAR BINS OF 5 DEG WIDTH.
EX = 13 (FROM 12.0 TO 13.5 MEV). ANGULAR BINS OF 5 DEG WIDTH.
The reaction $\gamma p \to \pi^0 \pi^0 p$ 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$\pi^0$ photoproduction. The consequence is the dominance of pion loops in the 2$\pi^0$ 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.
Total cross section measurement. Statistical errors only.
The cross section for the reaction 2H(γ, p)n has been measured at laboratory photon energies Eγ = 133−158 MeV and c.m. angles between 30° and 150°. The reaction was induced by a tagged bremsstrahlung photon beam incident on a liquid deuterium target. The uncertainty in the absolute cross sections is ⩽ 5%. There is now reasonable agreement between recent measurements in this energy region and the overall data set now defines the cross section sufficiently well to provide a test of current models of the reaction.
No description provided.
No description provided.
A direct measurement of the helicity dependence of the total photoabsorption cross section on the proton was carried out at MAMI (Mainz) in the energy range 200 < E_gamma < 800 MeV. The experiment used a 4$\pi$ detection system, a circularly polarized tagged photon beam and a frozen spin target. The contributions to the Gerasimov-Drell-Hearn sum rule and to the forward spin polarizability $\gamma_0$ determined from the data are 226 \pm 5 (stat)\pm 12(sys) \mu b and -187 \pm 8 (stat)\pm 10(sys)10^{-6} fm^4, respectively, for 200 < E_\gamma < 800 MeV.
Two absorption cross sections, SIG(C=3/2) and SIG(C=1/2), are determined bytwo relative spin configurations, namely parallel and antiparallel. N=RE.
The results of tagged photon measurements of the 16O(γ, pn) and 16O(γ, pp) reactions, carried out with photons of energies of 80–131 MeV, are presented. Missing energy spectra for both reactions, with an energy resolution of 7 MeV have been obtained. The 16O(γ, pn) missing energy spectrum is very similar to that recently measured for the 12C(γ, pN) reaction. In both cases the recoil momentum distributions are quantitavely described by a quasideuteron mechanism. Using normalisation factors based on this mechanism the average cross section for the 16O(γ, pn) reaction, for nucleons ejected from the 1p shell, is 510 ± 95 μb. The corresponding cross section for the 16O(γ, pp) reaction is 10.0 ± 3.0 μb.
PROPOSED THAT THE REACTION IS DUE TO A QUASI DEUTERON MECHANISM AND P N PAIRS E]ECT FROM THE 1P SHELL.
PROPOSED THAT THE REACTION IS DUE TO A QUASI DEUTERON MECHANISM AND P N PAIRS E]ECT FROM THE 1P SHELL.
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 G\"ottingen SENECA recoil detector. The data cover photon energies ranging from 200 MeV to 400 MeV at $\theta^{LAB}_\gamma=136.2^\circ$. 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 $p(\gamma,\pi^+ n)$. 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 $\alpha-\beta= 9.8\pm 3.6(stat){}^{2.1}_1.1(syst)\pm 2.2(model)$ in units $10^{-4}fm^3$. In combination with the polarizability sum $\alpha +\beta=15.2\pm 0.5$ deduced from photoabsorption data, the neutron electric and magnetic polarizabilities, $\alpha_n=12.5\pm 1.8(stat){}^{+1.1}_{-0.6}\pm 1.1(model)$ and $\beta_n=2.7\mp 1.8(stat){}^{+0.6}_{-1.1}(syst)\mp 1.1(model)$ are obtained. The backward spin polarizability of the neutron was determined to be $\gamma^{(n)}_\pi=(58.6\pm 4.0)\times 10^{-4}fm^4$.
Energy dependence of the free-proton differential cross section.
Energy dependence of the quasi-free proton differential cross section.
Energy dependence of the triple differential cross section w.r.t. the scattered proton.
The photon asymmetry in the reaction p(\vec{\gamma},\pi^{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.
Polarized photon beam.
Total and differential cross sections for photoproduction of η mesons from 12 C, 40 Ca, 93 Nb, and nat Pb have been obtained up to 790 MeV incident photon energy at the Mainz Microtron (MAMI) with the TAPS spectrometer. The absorption cross section σ ηN abs = (30 ± 2.5 ± 6)mb of η mesons in nuclear matter and the absorption length λ η = (2.0 ± 0.2 ± 0.4) fm are extracted. No significant depletion of the S 11 (1535) strength in the η photoproduction on nuclei is observed.
THE TOTAL SIG WAS PARAMETRIZED BY A**POWER.
The 4 He( γ ,p) 3 H reaction cross section has been measured with tagged photons of energy 81–158 MeV. Protons were detected over the angular range 55°–125°. The scaling of the data as a function of momentum mismatch has been examined for two reaction models, the modified quasideuteron mechanism and quasifree knockout. Both models show clear scaling behaviour but comparison of the predicted and measured scaling function shapes favours the modified quasideuteron mechanism at momenta up to ≈500 MeV/ c .
No description provided.
No description provided.
No description provided.
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