Compton scattering by the proton

Abstract

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 • < ∼ θ lab γ < ∼ 150 • , 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 ± 0.4 stat+syst ± 0.2 model )%. From all data below 455 MeV the proton's backward spin polarizability was determined to be γ π = (−37.9 ± 0.6 stat+syst ± 3.5 model ) × 10 −4 fm 4 .

Mass generation via the Higgs boson and the quark condensate of the QCD vacuum

“…(38) together with the mass m σ = 666 MeV leads to an excellent agreement of the predicted with the experimental polarizabilities of the nucleon as shown in Table 1. Furthermore, there is a Compton-scattering experiment on the proton where the σ meson as part of the constituent-quark structure is directly visible in the differential cross section for Compton scattering in the energy range from 400-700 MeV and at large scattering angles [15][16][17]. This latter experiment leads to a σ meson mass of m σ = 600±70 MeV [14] in good agreement with the standard value m σ = 666 MeV.…”

Research opportunities at the upgraded HIγS facility

“…The LEGS result is in disagreement with the TAPS result γ π = (−36.1 ± 2.2) × 10 −4 fm 4 [174]. The most recent Mainz experiment [203] using the LARA detector extracted a backward polarizability value of (−37.9 ± 0.6 stat + syst ± 3.5 model ) ×10 −4 fm 4 , and is in agreement with the earlier result from Mainz [174]. The new data confirm the previous observation that there is a systematic discrepancy between Mainz results and LEGS' results.…”

Chiral perturbation theory and baryon properties

“…A first experiment by the LEGS group [288] has challenged the theoretical predictions with a value much smaller than the absolute value. However there is now contradicting evidence from recent MAMI data [289,290], their new results being well in the range from both dispersion theory [291] and CHPT [292,293]. Extension of these polarizabilities to non-zero energies have been defined in [294].…”