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
We report a measurement of the asymmetry in spin-dependent quasielastic scattering of longitudinally polarized electrons from a polarized 3~e target. The neutron magnetic form factor G' ;, has been extracted from the measured asymmetry based on recent PWIA calculations using spin-dependent spectral functions. Our determination of G& at ~~= 0 . 1 9 (G~VIC)' agrees with the dipole parametrization. This experiment represents the first measurement of the neutron magnetic form factor using spin-dependent electron scattering. PACS nuniber(s): 25.30. Fj, 13.40.Gp, 14.20.Dh, 24.70.fs Electromagnetic form factors are of fundamental importance for an understanding of the underlying structure of nucleons. Knowledge of the distribution of charge and magnetization within the nucleons provides a sensitive test of models based on QCD, as well as a basis for calculations of processes involving the electromagnetic interaction with complex nuclei. Due to the lack of a free neutron target, the neutron electromagnetic form factors are known with less precision than the proton electric and magnetic form factors. They have been deduced in the past from elastic or quasielastic electron-deuteron scattering. This procedure involves considerable model dependence. The development of polarized targets and beams has allowed more complete studies of electromagnetic structure than has been possible with unpolarized reactions. In quasielastic scattering, the spin degrees of freedom introduce new response functions into the inclusive cross section, thus providing additional information on nuclear structure [I].3~e is an interesting nucleus for polarization studies because its ground state wave function is predominantly a spa- tially symmetric S state in which the spin of the nucleus is carried mainly by the unpaired neutron. Therefore, inelastic scattering of polarized electrons from polarized '~e in the vicinity of the quasielastic peak should be useful for studying the neutron electromagnetic form factors. This idea was first investigated by Blankleider and Woloshyn in closure approximation [2]. Friar et al. [3] have studied the model dependence in the spin structure of the 3~e wave function and its effect on the quasielastic asymmetry. Recently the plane wave impulse approximation (PWIA) calculations performed independently by two groups [4,5] using a spin-dependent spectral function show that the spin-dependent asymmetry is very sensitive to the neutron electric or magnetic form factors at certain kinematics near the top of the quasielastic peak. Two previous experiments [6,7] measured the spindependent asymmetry in quasielastic scattering of polarized electrons from oolarized h e . and demonstrated that this new experimental technique is feasible for studying the neutron electromagnetic structure. As a result, new experimental programs utilizing polarized electrons and polarized %e targets to study the neutron electromagnetic structure and the nucleon spin structure are under way at several electron accelerator laboratories (SLAC, MIT-B...
Parity-violating electron scattering measurements on hydrogen and deuterium, such as those underway at the Bates and CEBAF laboratories, require luminosities exceeding 10 38 cm −2 s −1 , resulting in large beam power deposition into cryogenic liquid. Such targets must be able to absorb 500 watts or more with minimal change in target density. A 40 cm long liquid hydrogen target, designed to absorb 500 watts of beam power without boiling, has been developed for the SAMPLE experiment at Bates. In recent tests with 40 µA of incident beam, no evidence was seen for density fluctuations in the target, at a sensitivity level of better than 1%. A summary of the target design and operational experience will be presented.
The transverse-longitudinal asymmetry A&« in 'He(e, e') quasielastic scattering at momentum transfer Q' = 0.14(GeV/c)2 has been measured to be 1.52~0.55(stat)~0.15(syst)%. The plane wave impulse approximation (PWIA) prediction for this measurement ranges from 2.1% to 2.9%, where the variation is due to uncertainty in the initial state wave function, nucleon form factors, and off-shell prescription. The data may suggest a suppression with respect to the PWIA, which has also been observed for the unpolarized longitudinal response function.
We report a measurement of the asymmetry in spin-dependent quasielastic scattering of longitudinally polarized electrons from a polarized 3~e target. The neutron magnetic form factor G' ;, has been extracted from the measured asymmetry based on recent PWIA calculations using spin-dependent spectral functions. Our determination of G& at ~~= 0 . 1 9 (G~VIC)' agrees with the dipole parametrization. This experiment represents the first measurement of the neutron magnetic form factor using spin-dependent electron scattering. PACS nuniber(s): 25.30. Fj, 13.40.Gp, 14.20.Dh, 24.70.fs Electromagnetic form factors are of fundamental importance for an understanding of the underlying structure of nucleons. Knowledge of the distribution of charge and magnetization within the nucleons provides a sensitive test of models based on QCD, as well as a basis for calculations of processes involving the electromagnetic interaction with complex nuclei. Due to the lack of a free neutron target, the neutron electromagnetic form factors are known with less precision than the proton electric and magnetic form factors. They have been deduced in the past from elastic or quasielastic electron-deuteron scattering. This procedure involves considerable model dependence. The development of polarized targets and beams has allowed more complete studies of electromagnetic structure than has been possible with unpolarized reactions. In quasielastic scattering, the spin degrees of freedom introduce new response functions into the inclusive cross section, thus providing additional information on nuclear structure [I].3~e is an interesting nucleus for polarization studies because its ground state wave function is predominantly a spa- tially symmetric S state in which the spin of the nucleus is carried mainly by the unpaired neutron. Therefore, inelastic scattering of polarized electrons from polarized '~e in the vicinity of the quasielastic peak should be useful for studying the neutron electromagnetic form factors. This idea was first investigated by Blankleider and Woloshyn in closure approximation [2]. Friar et al. [3] have studied the model dependence in the spin structure of the 3~e wave function and its effect on the quasielastic asymmetry. Recently the plane wave impulse approximation (PWIA) calculations performed independently by two groups [4,5] using a spin-dependent spectral function show that the spin-dependent asymmetry is very sensitive to the neutron electric or magnetic form factors at certain kinematics near the top of the quasielastic peak. Two previous experiments [6,7] measured the spindependent asymmetry in quasielastic scattering of polarized electrons from oolarized h e . and demonstrated that this new experimental technique is feasible for studying the neutron electromagnetic structure. As a result, new experimental programs utilizing polarized electrons and polarized %e targets to study the neutron electromagnetic structure and the nucleon spin structure are under way at several electron accelerator laboratories (SLAC, MIT-B...
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