Parity-Violating Electron Scattering and the Electric and Magnetic Strange Form Factors of the Nucleon

Armstrong, David; McKeown, R. D.

doi:10.1146/annurev-nucl-102010-130419

Cited by 73 publications

(70 citation statements)

References 58 publications

(63 reference statements)

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“…[10] and references therein as an indication of the possibility of sizable nonzero strange matrix elements at large Q 2 or the importance of diquark degrees of freedom. While existing measurements of parityviolating elastic scattering yield very small contributions from the strange quarks up to Q 2 ≈ 1 (GeV/c) 2 [88][89][90][91], they still leave open the possibility for significant contributions from G s E and G s M which cancel in the parityviolating observables [91,92], although there are also results from lattice QCD that the strange-quark contribution is small for the charge and magnetic form factors [93,94]. In the diquark model, the singly occurring down-quark in the proton is more likely to be associated with an axial-vector diquark than a scalar diquark, and the contribution of the axial-vector diquark yields a more rapid falloff of the form factors.…”

Section: Flavor-dependent Contributions To σRmentioning

confidence: 99%

Flavor decomposition of the nucleon electromagnetic form factors at lowQ2

2015

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Background The spatial distribution of charge and magnetization within the proton is encoded in the elastic form factors.These have been precisely measured in elastic electron scattering, and the combination of proton and neutron form factors allows for the separation of the up-and down-quark contributions.Purpose In this work, we extract the proton and neutron form factors from world's data with an emphasis on precise new data covering the low-momentum region, which is sensitive to the large-scale structure of the nucleon. From these, we separate the up-and down-quark contributions to the proton form factors.Method We combine cross section and polarization measurements of elastic electron-proton scattering to separate the proton form factors and two-photon exchange (TPE) contributions. We combine the proton form factors with parameterization of the neutron form factor data and uncertainties to separate the up-and down-quark contributions to the proton's charge and magnetic form factors.Results The extracted TPE corrections are compared to previous phenomenological extractions, TPE calculations, and direct measurements from the comparison of electron and positron scattering. The flavor-separated form factors are extracted and compared to models of the nucleon structure.Conclusions With the inclusion of the precise new data, the extracted TPE contributions show a clear change of sign at low Q 2 , necessary to explain the high-Q 2 form factor discrepancy while being consistent with the known Q 2 → 0 limit. We find that the new Mainz data yield a significantly different result for the proton magnetic form factor and its flavorseparated contributions. We also observe that the RMS radius of both the up-and down-quark distributions are smaller than the RMS charge radius of the proton.

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Section: Flavor-dependent Contributions To σRmentioning

confidence: 99%

Flavor decomposition of the nucleon electromagnetic form factors at lowQ2

2015

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“…Recognition that the spatial distributions of strange quarks and antiquarks could be different further motivated searches for strange contributions to the nucleon's electroweak form factors [3][4][5][6][7][8]. Dedicated programs of strange form factor measurements through parityviolating electron scattering at Jefferson Lab and other facilities [9][10][11] subsequently yielded very precise determinations of both the strange electric and magnetic form factors of the nucleon [12], enabling rigorous comparisons with lattice QCD and chiral effective theory [13,14], as well as fundamental tests of the Standard Model [15].…”

Section: Introductionmentioning

confidence: 99%

Strange-quark asymmetry in the proton in chiral effective theory

Wang

Melnitchouk

et al. 2016

Phys. Rev. D

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We perform a comprehensive analysis of the strange-antistrange parton distribution function (PDF) asymmetry in the proton in the framework of chiral effective theory, including the full set of lowest order kaon loop diagrams with off-shell and contact interactions, in addition to the usual on-shell contributions previously discussed in the literature. We identify the presence of δ-function contributions to thes PDF at x = 0, with a corresponding valence-like component of the s-quark PDF at larger x, which allows greater flexibility for the shape of s −s. Expanding the moments of the PDFs in terms of the pseudoscalar kaon mass, we compute the leading nonanalytic behavior of the number and momentum integrals of the s ands distributions, consistent with the chiral symmetry of QCD. We discuss the implications of our results for the understanding of the NuTeV anomaly and for the phenomenology of strange quark PDFs in global QCD analysis.

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“…This was done by exploiting previously-measured PVES asymmetries from the SAMPLE, HAPPEX, G0, and PVA4 experiments (see Ref. [10] and references therein), all of which were at significantly higher Q 2 than the present result. These included data for hydrogen, deuterium and 4 He, which were combined with our result in a global fit following the method outlined in [9].…”

Section: Resultsmentioning

confidence: 98%

“…The experiment built on techniques developed at Jefferson Lab over the last two decades for conducting precision parity-violating electron scattering measurements [10]. Of particular importance is the superbly small level of helicity-correlated fluctuations in beam properties available (through close cooperation with the electron-gun group and the accelerator operations group).…”

Section: Methodsmentioning

confidence: 99%

First result from Q_weak

Armstrong

2014

EPJ Web of Conferences

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Abstract.Initial results are presented from the recently-completed Q weak experiment at Jefferson Lab. The goal is a precise measurement of the proton's weak charge Q p w , to yield a test of the standard model and to search for evidence of new physics. The weak charge is extracted from the parity-violating asymmetry in elastic ep scattering at low momentum transfer, Q 2 = 0.025 GeV 2 . A 180 A longitudinally-polarized 1.16 GeV electron beam was scattered from a 35 cm long liquid hydrogen at small angles, 6 • < < 12 • . Scattered electrons were analyzed in a toroidal magnetic field and detected using an array of eight Cerenkov detectors arranged symmetrically about the beam axis. The initial result, from 4% of the complete data set, is Q p W = 0.064 ± 0.012, in excellent agreement with the standard model expectation. Full analysis of the data is expected to yield a value for the weak charge to about 5% precision.

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Parity-Violating Electron Scattering and the Electric and Magnetic Strange Form Factors of the Nucleon

Cited by 73 publications

References 58 publications

Flavor decomposition of the nucleon electromagnetic form factors at lowQ2

Flavor decomposition of the nucleon electromagnetic form factors at lowQ2

Strange-quark asymmetry in the proton in chiral effective theory

First result from Q_weak

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Parity-Violating Electron Scattering and the Electric and Magnetic Strange Form Factors of the Nucleon

Cited by 73 publications

References 58 publications

Flavor decomposition of the nucleon electromagnetic form factors at lowQ2

Flavor decomposition of the nucleon electromagnetic form factors at lowQ2

Strange-quark asymmetry in the proton in chiral effective theory

First result from Qweak

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Product

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First result from Q_weak