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Register a new userNew Precision Limit on the Strange Vector Form Factors of the Proton
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The parity-violating cross-section asymmetry in the elastic scattering of polarized electrons from unpolarized protons has been measured at a four-momentum transfer squared Q2 = 0.624 GeV and beam energy E =3.48 GeV to be A_PV = -23.80 +/- 0.78 (stat) +/- 0.36 (syst) parts per million. This result is consistent with zero contribution of strange quarks to the combination of electric and magnetic form factors G_E^s + 0.517 G_M^s = 0.003 +/- 0.010 (stat) +/- 0.004 (syst) +/- 0.009 (ff), where the third error is due to the limits of precision on the electromagnetic form factors and radiative corrections. With this measurement, the world data on strange contributions to nucleon form factors are seen to be consistent with zero and not more than a few percent of the proton form factors.
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We report the results of a new Rosenbluth measurement of the proton form factors at Q^2 values of 2.64, 3.20 and 4.10 GeV^2. Cross sections were determined by detecting the recoiling proton in contrast to previous measurements in which the scattered electron was detected. At each Q^2, relative cross sections were determined to better than 1%. The measurement focussed on the extraction of G_E/G_M which was determined to 4-8% and found to approximate form factor scaling, i.e. mu_p G_E approx G_M. These results are consistent with and much more precise than previous Rosenbluth extractions. However, they are inconsistent with recent polarization transfer measurements of comparable precision, implying a systematic difference between the two techniques.
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=0.22 (GeV/c)^2 is reported here. The measured asymmetry is A_LR=(-17.23 +- 0.82_stat +-0.89_syst) ppm. The Standard Model prediction assuming no strangeness is A_0=(-15.87 +- 1.22) ppm. In combination with previous results from measurements at forward angles, it it possible to disentangle for the first time the strange electric and magnetic form factors at this momentum transfer, G_E^s(0.22)=0.050 +- 0.038 +- 0.019 and G_M^s(0.22)=-0.14 +- 0.11 +- 0.11.
New precise results of a measurement of the elastic electron-proton scattering cross section performed at the Mainz Microtron MAMI are presented. About 1400 cross sections were measured with negative four-momentum transfers squared up to Q^2=1 (GeV/c)^2 with statistical errors below 0.2%. The electric and magnetic form factors of the proton were extracted by fits of a large variety of form factor models directly to the cross sections. The form factors show some features at the scale of the pion cloud. The charge and magnetic radii are determined to be r_E=0.879(5)(stat.)(4)(syst.)(2)(model)(4)(group) fm and r_M=0.777(13)(stat.)(9)(syst.)(5)(model)(2)(group) fm.
In arXiv:1108.3058v1 [nucl-ex], Arrington criticizes the Coulomb corrections we applied in the analysis of high precision form factor data (see Phys.Rev.Lett.105:242001, 2010, arXiv:1007.5076v3 [nucl-ex]). We show, by comparing different calculations cited in the Comment, that the criticism of the Comment neglects the large uncertainty of more modern TPE corrections. This uncertainty has also been seen in recent polarized measurements. We rerun our analysis using one of these calculations. The results show that the Comment exaggerates the quantitative effect at small Q^2.
The paper describes a precise measurement of electron scattering off the proton at momentum transfers of $0.003 lesssim Q^2 lesssim 1$ GeV$^2$. The average point-to-point error of the cross sections in this experiment is $sim$ 0.37%. These data are used for a coherent new analysis together with all world data of unpolarized and polarized electron scattering from the very smallest to the highest momentum transfers so far measured. The extracted electric and magnetic form factors provide new insight into their exact shape, deviating from the classical dipole form, and of structure on top of this gross shape. The data reaching very low $Q^2$ values are used for a new determination of the electric and magnetic radii. An empirical determination of the Two-Photon-Exchange (TPE) correction is presented. The implications of this correction on the radii and the question of a directly visible signal of the pion cloud are addressed.
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