Evidence for Strange-Quark Contributions to the Nucleon's Form Factors at Q~2=0.108 (Ge V/c)~2

摘要

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° < θ_e < 40°. The measured asymmetry is A_(LR)(ep) = [-1.36 ± 0.29(stat) ± 0.13(syst)] X 10~(-6). The expectation from the standard model assuming no strangeness contribution to the vector current is A_0 = (—2.06 ± 0.14) X 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 ± 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 σ.