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

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 $Q^2=0.108(\mathrm{GeV}/c{)}^2$ and at a forward electron scattering angle of $30°<{\theta }_e<40°$. The measured asymmetry is $A_{LR}(\overrightarrow{e}p)=[-1.36\pm 0.29(\mathrm{stat})\pm 0.13(\mathrm{syst})]\times {10}^{-6}$. The expectation from the standard model assuming no strangeness contribution to the vector current is $A_0=(-2.06\pm 0.14)\times {10}^{-6}$. 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 $G_E^s+0.106G_M^s=0.071\pm 0.036$ at $Q^2=0.108(\mathrm{GeV}/c{)}^2$. We again find the value for $G_E^s+0.106G_M^s$ to be positive, this time at an improved significance level of two $\sigma$.