Evidence for Strange Quark Contributions to the Nucleon's Form Factors at $Q^2$ = 0.108 (GeV/c)${}^2$

Abstract

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${}^{\circ }<\hat{I}{¸}_e<40^{\circ }$. 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 $\ddot{I}ƒ$.

4 pages, 3 figures