Test of Low-Energy Theorems forH1(γ→,

Abstract

The photon asymmetry in the reaction 1H(--> gamma,pi(0))(1)H 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 values obtained at threshold are E(0+) = [-1.33+/-0.08(stat)+/-0.03(syst)] x 10(-3)/m(pi(+)), P(1) = [9.47 +/- 0.08(stat) +/- 0.29(syst)] x 10(-3)q/m(2)(pi(+)), P(2) = [-9.46 +/- 0.1(stat) +/- 0.29(syst)] x 10(-3)q/m(2)(pi(+)), and P(3) = [11.48 +/- 0.06(stat) +/- 0.35(syst)] x 10(-3)q/m(2)(pi(+)).

Analysis of pion photoproduction data

“…While we have obtained a reasonable fit to the available π 0 p differential and total cross sections (we fit rather well the new threshold TRIUMF-SAL π + n cross sections [22], and Mainz π 0 p Σ at 160 MeV [19]), the multipole amplitudes have no cusp built into the π + n threshold region. Both energy-dependent and single-energy solutions (SES) were obtained from fits to the combined π 0 p, π + n, π − p, and π 0 n databases to 2 GeV.…”

“…The plotted curves differ mainly in the P-wave multipoles. Our SES at 162 MeV, which covers 158 − 165 MeV (see Table IV,) fits the new Mainz data [19] rather well, while the energy-dependent MAID2000 and SM02 solutions are less successful.…”

“…Since 1995, 87% (21%) of all new π 0 p (π + n) data have been produced at Mainz using the MAMI facility [9][10][11][12][13][14][15][16][17][18][19]. These measurements of total and differential cross sections, Σ beam asymmetry, and the GDH-related quantity (σ 1/2 − σ 3/2 ) have increased the database by a factor of about two over the energy range from the threshold to 800 MeV.…”

Measurement of the beam asymmetryΣin the forward direction forγ⃗p→pπ0

“…The total π 0 cross section exhibits three clear peaks and a broad enhancement around W ≈ 1900 MeV/c 2 , which represent the four known resonance regions below 2 GeV/c 2 . The first resonance region below 1500 MeV/c 2 is dominated by the wellknown (1232)P 33 resonance with very small contributions of the N (1440)P 11 Roper resonance. The N (1520)D 13 and the two S 11 resonances combined, N (1535)S 11 and N (1650)S 11 , contribute with about equal strength to the second resonance region around 1550 MeV/c 2 .…”

“…The first resonance region below 1500 MeV/c 2 is dominated by the wellknown (1232)P 33 resonance with very small contributions of the N (1440)P 11 Roper resonance. The N (1520)D 13 and the two S 11 resonances combined, N (1535)S 11 and N (1650)S 11 , contribute with about equal strength to the second resonance region around 1550 MeV/c 2 . The third bump in the pπ 0 total cross section is mainly caused by three major resonance contributions: (1700)D 33 , N (1680)F 15 , and N (1650)S 11 (e.g., Refs.…”