Evidence for Strange-Quark Contributions to the Nucleon's Form Factors at Q$^2$=0.108 (GeV/c)$^2$
Résumé
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 (GeV/c)$^2$ and at a forward electron scattering angle of 30$\degres<\theta_e<40\degres$. The measured asymmetry is A$_{LR}(e \to p)=[-1.36\pm 0.29$(stat)$\pm$ 0.13(syst)]$\times 10^{-6}$. The expectation from the standard model assuming no strangeness contribution to the vector current is A$_0=(-2.06\pm 0.14)\times10^{-6}$. 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 (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 $\sigma$.