Measurement of strange-quark contributions to the nucleon's form factors at Q(2) = 0.230 (GeV/c)(2).
/ Authors
F. Maas, P. Achenbach, K. Aulenbacher, S. Baunack, L. Capozza, J. Diefenbach, K. Grimm, Y. Imai, T. Hammel, D. Harrach
and 23 more authors
E. Kabuß, R. Kothe, J. Lee, A. Lorente, A. Ginja, L. Nungesser, E. Schilling, G. Stephan, C. Weinrich, I. Altarev, J. Arvieux, B. Collin, R. Frascaria, M. Guidal, R. Kunne, D. Marchand, M. Morlet, S. Ong, J. Wiele, S. Kowalski, B. Plaster, R. Suleiman, S. Taylor
/ Abstract
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.
Journal: Physical review letters