Maas F E, Achenbach P, Aulenbacher K, Baunack S, Capozza L, Diefenbach J, Grimm K, Imai Y, Hammel T, von Harrach D, Kabuss E M, Kothe R, Lee J H, Lorente A, Lopes Ginja A, Nungesser L, Schilling E, Stephan G, Weinrich C, Altarev I, Arvieux J, Collin B, Frascaria R, Guidal M, Kunne R, Marchand D, Morlet M, Ong S, van de Wiele J, Kowalski S, Plaster B, Suleiman R, Taylor S
Institut für Kernphysik, Johannes Gutenberg Universität Mainz, J. J. Becherweg 45, D-55099 Mainz, Germany.
Phys Rev Lett. 2004 Jul 9;93(2):022002. doi: 10.1103/PhysRevLett.93.022002. Epub 2004 Jul 7.
We report on a measurement of the parity-violating asymmetry in the scattering of longitudinally polarized electrons on unpolarized protons at a Q2 of 0.230 (GeV/c)(2) and a scattering angle of theta (e) = 30 degrees - 40 degrees. Using a large acceptance fast PbF2 calorimeter with a solid angle of delta omega = 0.62 sr, the A4 experiment is the first parity violation experiment to count individual scattering events. The measured asymmetry is A(phys)=(-5.44+/-0.54(stat)+/-0.26(sys))x10(-6). The standard model expectation assuming no strangeness contributions to the vector form factors is A(0) = (-6.30+/-0.43) x 10(-6). The difference is a direct measurement of the strangeness contribution to the vector form factors of the proton. The extracted value is G(s)(E) + 0.225G(s)(M) = 0.039+/-0.034 or F(s)(1) + 0.130F(s)(2) = 0.032+/-0.028.
