Evidence for Strange Quark Contributions to the Nucleon's Form Factors at \(Q^2\) = 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 \(Q^2\) = \Qsquare (GeV/c)\(^2\) and at a forward electron scattering angle of 30\(^\circ < \theta_e < 40^\circ\). The measured asymmetry is \(A_{LR}(\vec{e}p)\) = (\Aphys \(\pm\) \Deltastat\(_{stat}\) \(\pm\) \Deltasyst\(_{syst}\)) \(\times\) 10\(^{-6}\). The expectation from the Standard Model assuming no strangeness contribution to the vector current is A\(_0\) = (\Azero \(\pm\) \DeltaAzero) \(\times\) 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 \(G_E^s\) + \FakGMs \(G_M^s\) = \GEsGMs \(\pm \) \DeltaGEsGMs at \(Q^2\) = \Qsquare (GeV/c)\(^2\). As in our previous measurement at higher momentum transfer for \(G_E^s\) + 0.230 \(G_M^s\), we again find the value for \(G_E^s\) + \FakGMs \(G_M^s\) to be positive, this time at an improved significance level of 2 \(\sigma\).