Strange Magnetic Form Factor Research Articles

Measurement of the parity violating asymmetry in the quasielastic electron-deuteron scattering and improved determination of the magnetic strange form factor and the isovector anapole radiative correction

A new measurement of the parity-violating asymmetry in the electron-deuteron quasielastic scattering for backward angles at $⟨{Q}^{2}⟩=0.224\text{ }\text{ }{(\mathrm{GeV}/c)}^{2}$, obtained in the A4 experiment at the Mainz Microtron accelerator (MAMI) facility, is presented. The measured asymmetry is ${A}_{PV}^{d}=(\ensuremath{-}20.11\ifmmode\pm\else\textpm\fi{}0.8{7}_{\mathrm{stat}}\ifmmode\pm\else\textpm\fi{}\phantom{\rule{0ex}{0ex}}1.0{3}_{\mathrm{sys}})\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}$. A combination of these data with the proton measurements of the parity-violating asymmetry in the A4 experiment yields a value for the effective isovector axial-vector form factor of ${G}_{A}^{e,(T=1)}=\ensuremath{-}0.19\ifmmode\pm\else\textpm\fi{}0.43$ and ${R}_{A}^{(T=1),\mathrm{anap}}=\ensuremath{-}0.41\ifmmode\pm\else\textpm\fi{}0.35$ for the anapole radiative correction. When combined with a reanalysis of measurements obtained in the G0 experiment at the Thomas Jefferson National Accelerator Facility, the uncertainties are further reduced to ${G}_{M}^{s}=0.17\ifmmode\pm\else\textpm\fi{}0.11$ for the magnetic strange form factors, and ${R}_{A}^{(T=1),\mathrm{anap}}=\ensuremath{-}0.54\ifmmode\pm\else\textpm\fi{}0.26$.

Lattice QCD Evidence that theΛ(1405)Resonance is an Antikaon-Nucleon Molecule

For almost 50 years the structure of the Λ(1405) resonance has been a mystery. Even though it contains a heavy strange quark and has odd parity, its mass is lower than any other excited spin-1/2 baryon. Dalitz and co-workers speculated that it might be a molecular state of an antikaon bound to a nucleon. However, a standard quark-model structure is also admissible. Although the intervening years have seen considerable effort, there has been no convincing resolution. Here we present a new lattice QCD simulation showing that the strange magnetic form factor of the Λ(1405) vanishes, signaling the formation of an antikaon-nucleon molecule. Together with a Hamiltonian effective-field-theory model analysis of the lattice QCD energy levels, this strongly suggests that the structure is dominated by a bound antikaon-nucleon component. This result clarifies that not all states occurring in nature can be described within a simple quark model framework and points to the existence of exotic molecular meson-nucleon bound states.

Strange magnetic form factor of the nucleon in a chiral effective model at next to leading order

The strange magnetic form factor of the nucleon is studied in a chiral effective model motivated by the heavy baryon formalism at next-to-leading order. The one-loop contributions from kaon and intermediate octet and decuplet hyperons are included, using finite-range regularization to deal with the ultraviolet divergences. Drawing on an established connection between quenched and full QCD, this model makes it possible to predict the strange magnetic form factor under the hypothesis that for a dipole regulator mass $\mathrm{\ensuremath{\Lambda}}$ around 0.8 GeV, strangeness in the core is negligible. The strange magnetic form factor is found to be small and negative over a range of momentum transfer, while the strange magnetic moment is consistent with the best lattice QCD determinations.

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

Measurements of the neutral weak-vector form factors of the nucleon provide unique access to the strange quark content of the nucleon. These form factors can be studied by using parity-violating electron scattering. A comprehensive program of experiments has been performed at three accelerator laboratories to determine the role of strange quarks in the electromagnetic form factors of the nucleon. This article reviews the remarkable technical progress associated with this program, describes the various methods used in the different experiments, and summarizes the physics results along with recent theoretical calculations.

Strange magnetic moments of octet baryons under SU(3) breaking

Magnetic moments of octet baryons are parameterized to all orders of the flavor SU(3) breaking with the irreducible tensor technique in order to extract the contribution of each flavor quark to the magnetic moments of the octet baryons. The not-yet measured magnetic moment of Σ0 is predicted to be 0.649 μN. Our parameterized forms for the magnetic moments are explicitly flavor-dependent, and hence each flavor component of the magnetic moments can be evaluated directly via the flavor projection operator. It is found that the strange magnetic moment of the nucleon is suppressed due to the small isoscalar anomalous magnetic moment of the nucleon. In particular, the strange magnetic form factor of the nucleon turns out to be positive, G(s)N(0) = 0.428 μN, which is consistent with recent data.

Exploring strange nucleon form factors on the lattice

We discuss techniques for evaluating sea quark contributions to hadronic form factors on the lattice and apply these to an exploratory calculation of the strange electromagnetic, axial, and scalar form factors of the nucleon. We employ the Wilson gauge and fermion actions on an anisotropic ${24}^{3}\ifmmode\times\else\texttimes\fi{}64$ lattice, probing a range of momentum transfer with ${Q}^{2}<1\text{ }\text{ }{\mathrm{GeV}}^{2}$. The strange electric and magnetic form factors, ${G}_{E}^{s}({Q}^{2})$ and ${G}_{M}^{s}({Q}^{2})$, are found to be small and consistent with zero within the statistics of our calculation. The lattice data favor a small negative value for the strange axial form factor ${G}_{A}^{s}({Q}^{2})$ and exhibit a strong signal for the bare strange scalar matrix element $⟨N|\overline{s}s|N{⟩}_{0}$. We discuss the unique systematic uncertainties affecting the latter quantity relative to the continuum, as well as prospects for improving future determinations with Wilson-like fermions.

Results from the Mainz A4 experiment

The A4 collaboration at the MAMI accelerator at Mainz investigates the contribution of strange quarks to the form factors of the nucleon by measuring the parity violating asymmetry APV in the cross section of elastic scattering of longitudinally polarized electrons off hydrogen and deuterium. Recently, measurements at backward angles at a four momentum transfer of Q2 = 0.22 GeV2 were completed. Together with previous results at forward angles at the same momentum transfer, the strange electric and magnetic form factors \(G_E^s\) and \(G_M^s\) can be disentangled.

Strange magnetic form factor of the proton atQ2=0.23 GeV2

We determine the $u$ and $d$ quark contributions to the proton magnetic form factor at finite momentum transfer by applying chiral corrections to quenched lattice data. Heavy baryon chiral perturbation theory is applied at next-to-leading order in the quenched and full QCD cases for the valence sector using finite range regularization. Under the assumption of charge symmetry these values can be combined with the experimental values of the proton and neutron magnetic form factors to deduce a relatively accurate value for the strange magnetic form factor at ${Q}^{2}=0.23 {\mathrm{GeV}}^{2}$, namely, ${G}_{M}^{s}=\ensuremath{-}0.034\ifmmode\pm\else\textpm\fi{}0.021 {\ensuremath{\mu}}_{N}$.

Isospin-symmetry breaking effects on the strange electric and magnetic form factors of the nucleon

We examine the electric and magnetic strange form factors of the nucleon in the pseudoscalar-vector SU(3) Skyrme model, with special emphasis on the effects of isospin symmetry breaking (ISB). It is found that ISB has a nontrivial effect on the strange vector form factors of the nucleon and its contribution to the nucleon strangeness is significantly larger than one might naively expect. Our calculations and discussions may be of some significance for the experimental extraction of the authentic strangeness.

Global analysis of nucleon strange form factors at lowQ2

We perform a global analysis of all recent experimental data from elastic parity-violating electron scattering at low Q^2. The values of the electric and magnetic strange form factors of the nucleon are determined at Q^2=0.1 GeV/c^2 to be G E s =-0.008±0.016 and G M s =0.29±0.21.

The parity violation A4 experiment at forward and backward angles

The A4 Collaboration at the MAMI accelerator in Mainz measures the parity-violating asymmetry in the cross-section of elastic scattering of longitudinally polarized electrons off unpolarized protons from which the contribution of strange sea quarks to the vector form factors of the proton can be derived. Measurements at forward angles 30° ≤ ϑ ≤ 40° and two different momentum transfers Q2 of 0.23 (GeV/c)2 and 0.11 (GeV/c)2 have been performed in the past. Measurements at backward angles 140° ≤ ϑ <- 150° are underway for a separate determination of the strange electric and the strange magnetic form factor at Q 2 = 0.23 (GeV/c)2.

Strange nucleon form factors: Solitonic approach to GM s , GE s , $ \tilde{{G}}_{{A}}^{{p}}$ and $ \tilde{{G}}_{{A}}^{{n}}$ and comparison with world data

We summarize the results of the chiral quark-soliton model (\( X \) QSM) concerning basically all form factors necessary to interpret the present data of the parity-violating electron scattering experiments SAMPLE, HAPPEX, A4 and G0. The results particularly focus on the recently measured asymmetries and the detailed data for various combinations of G M s , G E s , \( \tilde G_A^p \) and \( \tilde G_A^n \) at Q 2 = 0.1 GeV2. The calculations yield positive strange magnetic and electric form factors and a negative axial vector one, all being rather small. The results are very close to the combined experimental world data from parity-violating electron scattering and elastic \( \nu p \) and \( \mathop v\limits^ - p \) scattering.

Meson cloud and nucleon strangeness: An update

We use a version of the meson cloud model, including kaon, κ and K* contributions, to estimate the electric and magnetic strange form factors of the nucleon. We compare our results with the recent measurements of the strange-quark contribution to parity-violating asymmetries in electron-proton scattering experiments.

Precision Measurements of the Nucleon Strange Form Factors atQ2∼0.1 GeV2

We report new measurements of the parity-violating asymmetry APV in elastic scattering of 3 GeV electrons off hydrogen and He4 targets with ⟨θlab⟩≈6.0°. The He4 result is APV=(+6.40±0.23(stat)±0.12(syst))×10−6. The hydrogen result is APV=(−1.58±0.12(stat)±0.04(syst))×10−6. These results significantly improve constraints on the electric and magnetic strange form factors GEs and GMs. We extract GEs=0.002±0.014±0.007 at ⟨Q2⟩=0.077 GeV2, and GEs+0.09GMs=0.007±0.011±0.006 at ⟨Q2⟩=0.109 GeV2, providing new limits on the role of strange quarks in the nucleon charge and magnetization distributions.Received 6 September 2006DOI:https://doi.org/10.1103/PhysRevLett.98.032301©2007 American Physical Society

Parity-violating asymmetries in elastice→pscattering in the chiral quark-soliton model: Comparison with the A4, G0, HAPPEX and SAMPLE experiments

We investigate parity-violating electroweak asymmetries in the elastic scattering of polarized electrons off protons within the framework of the chiral quark-soliton model ($\ensuremath{\chi}\mathrm{QSM}$). We use as input the former results of the electromagnetic and strange form factors and newly calculated SU(3) axial-vector form factors, all evaluated with the same set of four parameters adjusted several years ago to general mesonic and baryonic properties. Based on this scheme, which yields positive electric and magnetic strange form factors with ${\ensuremath{\mu}}_{s}=(0.08--0.13){\ensuremath{\mu}}_{N}$, we determine the parity-violating asymmetries of elastic polarized electron-proton scattering. The results are in a good agreement with the data of the A4, HAPPEX, and SAMPLE experiments and reproduce the full ${Q}^{2}$ range of the G0 data. We also predict the parity-violating asymmetries for the backward G0 experiment.

Extracting Nucleon Strange and Anapole Form Factors from World Data

The complete world set of parity-violating electron scattering data up to Q2 approximately 0.3 GeV2 is analyzed. We extract the current experimental determination of the strange electric and magnetic form factors of the proton, as well as the weak axial form factors of the proton and neutron, at Q2=0.1 GeV2. Within experimental uncertainties, we find that the strange form factors are consistent with zero, as are the anapole contributions to the axial form factors. Nevertheless, the correlation between the strange and anapole contributions suggest that there is only a small probability that these form factors all vanish simultaneously.

Can the meson cloud explain the nucleon strangeness?

We use the meson cloud model, including the kaon and the $K^*$ contributions, to estimate the electric and magnetic strange form factors of the nucleon. We compare our results with the recent measurements of the strange quark contribution to parity-violating asymmetries in the forward G0 electron-proton scattering experiment. We conclude that it is not possible to explain the data using this model.

The Strange Form Factors of the Proton and the G 0 Experiment

The measurement of parity violating asymmetries in elastic electron-nucleon scattering can be used to determine the contribution of strange quark-antiquark pairs to the form factors. It is the aim of the G 0 experiment at Jefferson Lab to perform the complete separation of the electric and magnetic strange form factors over a broad range of Q 2 . The G 0 forward-angle measurements were completed in 2004 and preliminary results will be presented. The first G 0 backward-angle measurements will take place in late 2005. Preparations for those measurements are under way and will be discussed.

The next generation HAPPEX experiments

A new generation of parity violating electron scattering experiments began running in Summer 2004 at the Thomas Jefferson National Accelerator Facility. HAPPEX-H will measure the parity violating asymmetry in polarized electron scattering from protons at Q 2 = 0.11 (GeV/c)2. This asymmetry is sensitive to a linear combination of the strange electric and magnetic form factors of the proton. HAPPEX- He will measure the parity violating asymmetry in polarized electron scattering from a 4He target at the same value of Q 2, using the same spectrometers and similar detectors, accessing the strange electric form factor cleanly. PREX will use parity violating electron scattering to determine the neutron radius of the 206Pb nucleus.

Strange and singlet form factors of the nucleon: Predictions for G0, A4, and HAPPEX II experiments

We investigate the strange and flavor-singlet electric and magnetic form factors of the nucleon within the framework of the SU(3) chiral quark-soliton model. Isospin symmetry is assumed and the symmetry-conserving SU(3) quantization is employed, rotational and strange-quark mass corrections being included. For the experiments G0, A4, and HAPPEX II we predict the quantities G0E + βG0M and GsE + βGsM. The dependence of the results on the parameters of the model and the treatment of the Yukawa asymptotic behavior of the soliton are investigated.