6533b7d9fe1ef96bd126d567

RESEARCH PRODUCT

Evidence for Strange-Quark Contributions to the Nucleon’s Form Factors atQ2=0.108   (GeV/c)2

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We report on a measurement of the parity violating asymmetry in the elastic scattering of polarized electrons off unpolarized protons with the A4 apparatus at MAMI in Mainz at a four momentum transfer value of ${Q}^{2}=0.108\text{ }(\mathrm{GeV}/c{)}^{2}$ and at a forward electron scattering angle of $30\ifmmode^\circ\else\textdegree\fi{}l{\ensuremath{\theta}}_{e}l40\ifmmode^\circ\else\textdegree\fi{}$. The measured asymmetry is ${A}_{LR}(\stackrel{\ensuremath{\rightarrow}}{e}p)=[\ensuremath{-}1.36\ifmmode\pm\else\textpm\fi{}0.29(\mathrm{stat})\ifmmode\pm\else\textpm\fi{}0.13(\mathrm{syst})]\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}$. The expectation from the standard model assuming no strangeness contribution to the vector current is ${A}_{0}=(\ensuremath{-}2.06\ifmmode\pm\else\textpm\fi{}0.14)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}$. We have improved the statistical accuracy by a factor of 3 as compared to our previous measurements at a higher ${Q}^{2}$. We have extracted the strangeness contribution to the electromagnetic form factors from our data to be ${G}_{E}^{s}+0.106{G}_{M}^{s}=0.071\ifmmode\pm\else\textpm\fi{}0.036$ at ${Q}^{2}=0.108\text{ }(\mathrm{GeV}/c{)}^{2}$. We again find the value for ${G}_{E}^{s}+0.106{G}_{M}^{s}$ to be positive, this time at an improved significance level of two $\ensuremath{\sigma}$.