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The violation of mirror symmetry in the weak force provides a powerful tool to study the internal structure of the proton. Experimental results have been obtained that address the role of strange quarks in generating nuclear magnetism. The measurement reported here provides an unambiguous constraint on strange quark contributions to the proton's magnetic moment through the electron-proton weak interaction. We also report evidence for the existence of a parity-violating electromagnetic effect known as the anapole moment of the proton. The proton's anapole moment is not yet well understood theoretically, but it could have important implications for precision weak interaction studies in atomic systems such as cesium.

This paper reports the first measurement of the tensor polarization t 2 o ine-d elastic scattering. The polarization of the recoil deuterons was measured for two values of momentum transfer, # = 1.74 and 2.03 fm" 1 , with a high-efficiency polarimeter. The results are in good agreement with reasonable models for the deuteron.PACS numbers: 25.30.Bf, 24.70,+ s, 25.10.+ S A complete experimental determination 1 ' 2 of the electromagnetic current of the deuteron requires the measurement of at least one polarization observable, in addition to the differential cross section. Here we report the first measurement of t 20 in elastic e-d scattering. Measurements of t 20 have previously not been feasible because of the absence of high-efficiency deuteron tensor polarimeters or tensor-polarized targets, and the lack of high-intensity electron beams and large-acceptance magnetic spectrometers. The results are found to be in good agreement with the predictions of t 20 for "reasonable" models of the deuteron, but in disagreement with those of separable-potential models.Electron elastic scattering from the deuteron can be described by three form factors: charge (F c ), quadrupole (FQ), and magnetic (F M ). Thus far, only F M has been isolated 3 in measurements of the cross section. The form factors F c and F Q have not been isolated previously from measurements of the structure function. 4 A determination of these form factors separately would discriminate further among different deuteron wave functions. The sensitivity of t 20 to the deuteron wave function arises from the fact that the leading term in t 2Q is proportional to the ratio of F Q to F c . in this ratio, the poorly known isoscalar electric nucleon form factor drops out. The expression for t 2Q is given 2 by t 20 2 , and/(6>) = i + (l+77)tan 2 (6i/2). Here, q is the four-momentum transfer, M d is the rest mass of the deuteron, and 6 is the angle of the scattered electron. The terms involving powers of X in the numerator are dominant in the momentum transfer region of 1-5 fm" 1 . Thus, t 20 is sensitive to the ratio of F Q to F c . The quantity F Q is sensitive to the tensor part of the N-N interaction, while F c is dominated by the S-wave part of the deuteron wave function at these values of low momentum transfer. Additionally, recent work 1 ' 2 has shown that t 2Q is also sensitive to the isoscalar meson exchange current (MEC) and relativistic corrections, about which there is much controversy. 5 The experiment was performed at the South Experimental Hall of the Massachusetts Institute of Technology-Bates Linear Accelerator Center. A schematic diagram of the experimental arrangement is given in Fig. 1. The electrons from the linac were focused on a windowless D 2 0 target which consisted of a 0.38-or 0.64-mm-thick laminar flow of heavy water. The incident electron energies were 371 ±2 MeVfor ^r=2.03fm" 1 , and 310± 1.8 MeVfor #=1.74 fm" 1 . During the experiment the average current and duty factor of the electron beam varied from 15 to 50 /iA and 0.3% to 0.4%, respec...

We report the first measurement of the parity-violating asymmetry in elastic electron scattering from the proton. The asymmetry depends on the neutral weak magnetic form factor of the proton which contains new information on the contribution of strange quark-antiquark pairs to the magnetic moment of the proton. We obtain the value G Z M 0.34 6 0.09 6 0.04 6 0.05 n.m. at Q 2 0.1 ͑GeV͞c͒ 2 . [S0031-9007(97)03181-5] PACS numbers: 13.60. Fz, 11.30.Er, 13.40.Gp, 14.20.Dh The measurement of strange quark-antiquark (ss) effects in the nucleon offers a unique window to study the effects of the qq "sea" at low momentum transfers. This information is an important clue to the dynamical effects of QCD that are responsible for form factors in the nonperturbative regime, and may lead to new insight into the origins of these effects.It has been shown [1] that the neutral weak current can be used to determine the ss contributions to nucleon form factors. The magnetic moment is one important nucleon property that can be studied in this fashion. The neutral weak magnetic form factor of the proton can be measured in parity-violating electron scattering, [2], thus providing information on the ss content of the nucleon's magnetic moment. In this Letter, we report the first such measurement and obtain the first direct experimental data relevant to determination of the strange magnetic moment of the proton.To lowest order (tree-level), the neutral weak magnetic form factor of the proton G Z M can be related to nucleon electromagnetic form factors and a contribution from strange quarks: As mentioned above, the quantity G Z M for the proton can be measured via elastic parity-violating electron scattering at backward angles [2]. The difference in cross sections for right and left handed incident electrons arises from interference of the electromagnetic and neutral weak amplitudes, and so contains products of electromagnetic and neutral weak form factors. The expression for elastic scattering from the proton is given by

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