The double-polarization observable E and helicity-dependent cross sections σ1/2, σ3/2 have been measured for the photoproduction of π0 pairs off quasifree protons and neutrons at the Mainz MAMI ...accelerator with the Crystal Ball/TAPS setup. A circularly polarized photon beam was produced by bremsstrahlung from longitudinally polarized electrons and impinged on a longitudinally polarized deuterated butanol target. The reaction products were detected with an almost 4 π covering calorimeter. The results reveal for the first time the helicity- and isospin-dependent structure of the γN → Nπ0 π0 reaction. They are compared to predictions from reaction models in view of nucleon resonance contributions and also to a refit of one model that predicted results for the proton and for the neutron target. The comparison of the prediction and the refit demonstrates the large impact of the new data.
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The scalar dipole polarizabilities,
α
E
1
and
β
M
1
, are fundamental properties related to the internal dynamics of the nucleon. The currently accepted values of the proton polarizabilities were ...determined by fitting to unpolarized proton Compton scattering cross section data. The measurement of the beam asymmetry
Σ
3
in a certain kinematical range provides an alternative approach to the extraction of the scalar polarizabilities. At the Mainz Microtron (MAMI) the beam asymmetry was measured for Compton scattering below pion photoproduction threshold for the first time. The results are compared with model calculations and the influence of the experimental data on the extraction of the scalar polarizabilities is determined.
We review the present status of the Standard Model calculation of the anomalous magnetic moment of the muon. This is performed in a perturbative expansion in the fine-structure constant α and is ...broken down into pure QED, electroweak, and hadronic contributions. The pure QED contribution is by far the largest and has been evaluated up to and including O ( α 5 ) with negligible numerical uncertainty. The electroweak contribution is suppressed by ( m μ / <msub is="true' M W ) 2 and only shows up at the level of the seventh significant digit. It has been evaluated up to two loops and is known to better than one percent. Hadronic contributions are the most difficult to calculate and are responsible for almost all of the theoretical uncertainty. The leading hadronic contribution appears at O ( α 2 ) and is due to hadronic vacuum polarization, whereas at O ( α 3 ) the hadronic light-by-light scattering contribution appears. Given the low characteristic scale of this observable, these contributions have to be calculated with nonperturbative methods, in particular, dispersion relations and the lattice approach to QCD. The largest part of this review is dedicated to a detailed account of recent efforts to improve the calculation of these two contributions with either a data-driven, dispersive approach, or a first-principle, lattice-QCD approach. The final result reads a μ SM = 116 591 810 ( 43 ) × 1 0 – 11 and is smaller than the Brookhaven measurement by 3.7 σ . The experimental uncertainty will soon be reduced by up to a factor four by the new experiment currently running at Fermilab, and also by the future J-PARC experiment. This and the prospects to further reduce the theoretical uncertainty in the near future – which are also discussed here – make this quantity one of the most promising places to look for evidence of new physics.
Track finding efficiency in B A B AR Allmendinger, T.; Bhuyan, B.; Brown, D.N. ...
Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment,
03/2013, Volume:
704
Journal Article
The double-polarization observable E and helicity-dependent cross sections σ1/2, σ3/2 have been measured for the photoproduction of π0 pairs off quasifree protons and neutrons at the Mainz MAMI ...accelerator with the Crystal Ball/TAPS setup. A circularly polarized photon beam was produced by bremsstrahlung from longitudinally polarized electrons and impinged on a longitudinally polarized deuterated butanol target. The reaction products were detected with an almost 4π covering calorimeter. The results reveal for the first time the helicity- and isospin-dependent structure of the γN → Nπ0π0 reaction.They are compared to predictions from reaction models in view of nucleon resonance contributions and also to a refit of one model that predicted results for the proton and for the neutron target. As a result, the comparison of the prediction and the refit demonstrates the large impact of the new data.
Full text
Available for:
CMK, CTK, FMFMET, NUK, UL
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