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 • < θe < 40 • . The measured asymmetry is ALR( ep) = (-1.36 ± 0.29stat ± 0.13syst) × 10 −6 . The expectation from the Standard Model assuming no strangeness contribution to the vector current is A0 = (-2.06± 0.14) × 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 s E + 0.106 G s M = 0.071 ± 0.036 at Q 2 = 0.108 (GeV/c) 2 . As in our previous measurement at higher momentum transfer for G s E + 0.230 G s M , we again find the value for G s E + 0.106 G s M to be positive, this time at an improved significance level of 2 σ.

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We report on a measurement of the asymmetry in the scattering of transversely polarized electrons off unpolarized protons, A ⊥ , at two Q 2 values of 0.106 (GeV/c) 2 and 0.230 (GeV/c) 2 and a scattering angle of 30−6 . The first errors denotes the statistical error and the second the systematic uncertainties. A ⊥ arises from the imaginary part of the two-photon exchange amplitude and is zero in the one-photon exchange approximation. From comparison with theoretical estimates of A ⊥ we conclude that πN-intermediate states give a substantial contribution to the imaginary part of the two-photon amplitude. The contribution from the ground state proton to the imaginary part of the two-photon exchange can be neglected. There is no obvious reason why this should be different for the real part of the two-photon amplitude, which enters into the radiative corrections for the Rosenbluth separation measurements of the electric form factor of the proton.

We report on a measurement of the parity-violating asymmetry in the scattering of longitudinally polarized electrons on unpolarized protons at a Q2 of 0.230 (GeV/c)(2) and a scattering angle of theta (e) = 30 degrees - 40 degrees. Using a large acceptance fast PbF2 calorimeter with a solid angle of delta omega = 0.62 sr, the A4 experiment is the first parity violation experiment to count individual scattering events. The measured asymmetry is A(phys)=(-5.44+/-0.54(stat)+/-0.26(sys))x10(-6). The standard model expectation assuming no strangeness contributions to the vector form factors is A(0) = (-6.30+/-0.43) x 10(-6). The difference is a direct measurement of the strangeness contribution to the vector form factors of the proton. The extracted value is G(s)(E) + 0.225G(s)(M) = 0.039+/-0.034 or F(s)(1) + 0.130F(s)(2) = 0.032+/-0.028.

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A new measurement of the parity violating asymmetry in elastic electron scattering on hydrogen at backward angles and at a four momentum transfer of Q 2 ¼ 0:22 ðGeV=cÞ 2 is reported here. The measured asymmetry is A LR ¼ ðÀ17:23 AE 0:82 stat AE 0:89 syst Þ Â 10 À6 . The standard model prediction assuming no strangeness is A 0 ¼ ðÀ15:87 AE 1:22Þ Â 10 À6 . In combination with previous results from measurements at forward angles, it is possible to disentangle for the first time the strange form factors at this momentum transfer, G s E ¼ 0:050 AE 0:038 AE 0:019 and G s M ¼ À0:14 AE 0:11 AE 0:11. DOI: 10.1103/PhysRevLett.102.151803 PACS numbers: 13.40.Gp, 11.30.Er, 12.15.Ày, 14.20.Dh Sea quarks are an important ingredient to describe nucleon properties in terms of fundamental QCD degrees of freedom. Strange quark-antiquark pairs might play a relevant role and affect, e.g., the electromagnetic properties of the nucleon. The contribution of strange quarks to the charge radius and magnetic moment in the nucleon ground state is of specific interest since this is a pure sea quark effect. The strange quark contribution to the electromagnetic form factors of the nucleon can be expressed in terms of the strange electric and magnetic form factors G s E and G s M . There are various theoretical approaches for estimating the strange form factors [1,2], such as quark soliton models [3][4][5], chiral quark models [6], quenched lattice calculations [7], or two-component models [8]. Parity violating electron scattering provides a direct experimental approach [9][10][11].A measurement of parity violation necessarily involves a weak interaction probe of the nucleon. This provides additional information allowing a measurement of G s E and G s M . Within the standard model of electroweak interaction, it is known that electromagnetic and weak currents are related. Assuming isospin symmetry, the weak vector form factors G p E;M of the proton, describing the vector coupling to the Z 0 boson, can be expressed in terms of the electromagnetic nucleon form factors G p;n E;M and the strange form factors G s E;M . The interference between tree level electromagnetic and weak amplitudes leads to a parity violating asymmetry in the elastic scattering cross section of left-and righthanded electrons (LR) L , R :This asymmetry can be written as a sum of three terms, A LR ¼ A V þ A S þ A A . A V represents the vector coupling on the proton vertex without strangeness contribution, A S contains the strange quark vector contribution, and A A represents the axial coupling to the proton vertex [11]: PRL 102, 151803 (2009)

The cross section for exclusive zr + electroproduction on the proton has been measured near threshold for the first time at two different values of the virtual photon polarization (e-0.2 and e~0.7). Using the low energy theorem for this reaction we deduce the axial and pseudoscalar weak form factors GA and G P at |f | =0.073, 0.139, and 0.179 (GeV/c) 2 . The slope of G A agrees with the value obtained in neutrino experiments. Gp satisfies the pion pole dominance hypothesis, which is thus verified for the first time in this range of transfer.PACS numbers: 13.60. Le, 11.40.Ha, 14.20.Dh The electroweak form factors provide a significant test of our understanding of the nucleon structure. Continuous efforts are devoted to their experimental determination but, as compared to the electromagnetic form factors of the nucleon (Ff'^Ff'"), the axial {GA) and pseudoscalar (Gp) weak form factors are still poorly known. In particular, very little is known about Gp which is very sensitive to the pion cloud of the nucleon. This Letter reports the first determination of both GA and Gp at / =-0.073, -0.139, and -0.179 (GeV/c) 2 , using near threshold n + electroproduction on the proton with detection of the pion and electron in coincidence.To fix our conventions we write the matrix element of the axial current between nucleon states of momenta p\ and/?2 U == (pi -pi)(1) From p decay and muon capture one can determine the weak form factors in the range |/1 -0-0.01 (GeV/c) 2 , but at larger / the only direct information comes from neutrino scattering on nuclei. From these experiments, only the mass parameter MA of the dipole parametrization of GA can be obtained, assuming that the vector and magnetic weak form factors can be taken from electron scattering using the isotriplet hypothesis and that Gp is given by pion pole dominance [1], which we write in the form Gp(t) = -2MG A (t)/U-mZ). The well established approximate chiral symmetry of strong interactions allows us to write a low energy theorem [1,2] which relates the low energy pion electroproduction amplitude to GA, Gp, F p,n , and Ff ,n , up to corrections which vanish in the chiral limit (pion mass going to zero). For our purpose, F p>n and Ff'" are known with sufficient accuracy. Therefore, a measurement at the same t for two values of the virtual photon polarization e allows a simultaneous determination of GA and Gp. Previous experiments (see Ref.[3] for a list of references) performed the measurement only at a single value of e except in Ref. [4] where the neutral pions were not separated from the charged ones. Therefore the determination of GA relied either on the pion pole dominance hypothesis for Gp or on a model for the estimation of the neutral pion contribution. Our experiment is the first exclusive experiment which uses two values of e (e~0.2 and £~~0.7) different enough to allow an independent determination of GA and Gp. Up to now, the latter is known only at the muon point [5]: G P =0.082 ±0.018 MeV _1 c 2 at t 0.0112 (GeV/c) 2 . In the following, p\, pi, q, and...