Measurement of Strange Quark Contributions to the Nucleon's Form Factors at Q^2=0.230 (GeV/c)^2

A4 Collaboration; F. E. Maas; P. Achenbach; K. Aulenbacher; S. Baunack; L. Capozza; J. Diefenbach; K. Grimm; Y. Imai; T. Hammel; D. von Harrach; E. -M. Kabuss; R. Kothe; J. H. Lee; A. Lorente; A. Lopes 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. van de Wiele; S. Kowalski; B. Plaster; R. Suleiman; S. Taylor

doi:10.1103/physrevlett.93.022002

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Measurement of Strange Quark Contributions to the Nucleon's Form Factors at Q^2=0.230 (GeV/c)^2

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Abstract

We report on a measurement of the parity-violating asymmetry in the scattering of longitudinally polarized electrons on unpolarized protons at a $Q^2$ of 0.230 (GeV/c)^2 and a scattering angle of θ_e = 30^o - 40^o. Using a large acceptance fast PbF_2 calorimeter with a solid angle of ΔΩ= 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} +- 0.27_{\rm sys}) 10^{-6}. The Standard Model expectation assuming no strangeness contributions to the vector form factors is $A_0=(-6.30 +- 0.43) 10^{-6}$. 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 + 0.225 G^s_M = 0.039 +- 0.034 or F^s_1 + 0.130 F^s_2 = 0.032 +- 0.028.

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A4 Collaboration, F. E. Maas, P. Achenbach, K. Aulenbacher, S. Baunack, L. Capozza, J. Diefenbach, K. Grimm, Y. Imai, T. Hammel, D. von Harrach, E. -M. Kabuss, R. Kothe, J. H. Lee, A. Lorente, A. Lopes 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. van de Wiele, S. Kowalski, B. Plaster, R. Suleiman, S. Taylor. 2004-01-16. Measurement of Strange Quark Contributions to the Nucleon's Form Factors at Q^2=0.230 (GeV/c)^2. https://doi.org/10.1103/physrevlett.93.022002

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