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Register a new userLowest Q^2 Measurement of the gamma*p -> Delta Reaction: Probing the Pionic Contribution
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To determine nonspherical angular momentum amplitudes in hadrons at long ranges (low Q^2), data were taken for the p(vec{e},ep)pi^0 reaction in the Delta region at Q^2=0.060 (GeV/c)^2 utilizing the magnetic spectrometers of the A1 Collaboration at MAMI. The results for the dominant transition magnetic dipole amplitude and the quadrupole to dipole ratios at W=1232 MeV are: M_{1+}^{3/2} = (40.33 +/- 0.63_{stat+syst} +/- 0.61_{model}) (10^{-3}/m_{pi^+}),Re(E_{1+}^{3/2}/M_{1+}^{3/2}) = (-2.28 +/- 0.29_{stat+syst} +/- 0.20_{model})%, and Re(S_{1+}^{3/2}/M_{1+}^{3/2}) = (-4.81 +/- 0.27_{stat+syst} +/- 0.26_{model})%. These disagree with predictions of constituent quark models but are in reasonable agreement with lattice calculations with non-linear (chiral) pion mass extrapolations, with chiral effective field theory, and with dynamical models with pion cloud effects. These results confirm the dominance, and general Q^2 variation, of the pionic contribution at large distances.
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The determination of non-spherical angular momentum amplitudes in nucleons at long ranges (low Q^{2}), was accomplished through the $p(vec{e},ep)pi^0$ reaction in the Delta region at $Q^2=0.060$, 0.127, and 0.200 (GeV/c)^2 at the Mainz Microtron (MAMI) with an accuracy of 3%. The results for the dominant transition magnetic dipole amplitude and the quadrupole to dipole ratios have been obtained with an estimated model uncertainty which is approximately the same as the experimental uncertainty. Lattice and effective field theory predictions agree with our data within the relatively large estimated theoretical uncertainties. Phenomenological models are in good agreement with experiment when the resonant amplitudes are adjusted to the data. To check reaction model calculations additional data were taken for center of mass energies below resonance and for the $sigma_{TL}$ structure function. These results confirm the dominance, and general Q^2 variation, of the pionic contribution at large distances.
The circular photon asymmetry for pi0 eta photoproduction on the proton was measured for the first time at the tagged photon facility of the MAMI C accelerator using the Crystal Ball/TAPS photon spectrometer. The experimental results are interpreted within a phenomenological isobar model that confirms the dominant role of the Delta(1700)D33 resonance. The measured asymmetry allows us to identify small contributions from positive-parity resonances via interference terms with the dominant D33 amplitude.
The low $Q^2$ slopes of the the transition form factors provide a unique method to measure the sizes of the neutral pseudo-scalar mesons, since they do not have electromagnetic form factors. From the slope one obtains the axial transition RMS radius $ R_{PS,A} = sqrt{<r^2>}$ for each PS meson. The present status of theory and experiment for these quantities are presented. A comparison of the $ R_{PS,A}$ is presented along with the electromagnetic and scalar radii of the $pi^{pm}$ mesons and the proton. We observe the striking similarity of the values of axial transition radii of all of the pseudoscalar mesons to each other and to the charge radius of the $pi^{pm}$. In the $Q^2$ = 0 limit the transition form factor is a measure of the pseudo-scalar meson radiative width (lifetime) and is a possible fourth (unexploited) method to perform such a measurement. The $pi^{0} rightarrow gamma gamma$ decay rate is a test of QCD at the confinement scale. There is a firm QCD prediction with a theoretical uncertainty of $simeq $ 1 % which calls for an experimental test at the same level of accuracy. There are three methods that have been utilized to perform this measurement and the present status of the experimental tests are outlined. The current accuracy is significantly less than the theoretical uncertainty. The efforts to improve this are briefly summarized.
The first data on target and beam-target asymmetries for the $gamma ptopi^0eta p$ reaction at photon energies from 1050 up to 1450 MeV are presented. The measurements were performed using the Crystal Ball and TAPS detector setup at the Glasgow tagged photon facility of the Mainz Microtron MAMI. The general assumption that the reaction is dominated by the $Delta 3/2^-$ amplitude is confirmed. The data are in particular sensitive to small contributions from other partial waves.
We report new p$(vec{e},e^prime p)pi^circ$ measurements in the $Delta^{+}(1232)$ resonance at the low momentum transfer region utilizing the magnetic spectrometers of the A1 Collaboration at MAMI. The mesonic cloud dynamics are predicted to be dominant and appreciably changing in this region while the momentum transfer is sufficiently low to be able to test chiral effective calculations. The results disagree with predictions of constituent quark models and are in reasonable agreement with dynamical calculations with pion cloud effects, chiral effective field theory and lattice calculations. The reported measurements suggest that improvement is required to the theoretical calculations and provide valuable input that will allow their refinements.
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