Strange Form Factors Of Nucleon Research Articles

Sea quark contributions to the electromagnetic form factors of Σ hyperons

We study the sea quark contributions to the electromagnetic form factors of $\Sigma$ baryons with nonlocal chiral effective theory. Both octet and decuplet intermediate states are included in the one loop calculation. $G_{\Sigma^{-}}^{u}$ and $G_{\Sigma^{+}}^{d}$ could be priority observables for the examination of sea quark contributions to baryon structure because these quantities are much larger than the strange form factors of nucleon. It will be less difficult for lattice simulation to determine the sign of these pure sea quark contributions unambiguously. In $\Sigma^0$, the light sea quark form factors $G_{\Sigma^{0}}^{u}$ and $G_{\Sigma^{0}}^{d}$ are identical. Since the light sea quark form factors in proton are different, it will be more meaningful to compare lattice result of the light sea quark form factors in $\Sigma^0$ with that obtained from effective field theory.

Strange form factors of the nucleon with a nonlocal chiral effective Lagrangian

The strange form factors of nucleon are studied with the nonlocal chiral effective Lagrangian. One loop contributions from both octet and decuplet intermediate states are included. The relativistic regulator is obtained by the nonlocal Lagrangian where the gauge link is introduced to guarantee the local gauge symmetry. With the kaon loop, the calculated charge form factor is positive, while the magnetic form factor is negative. The strange magnetic moment is $-0.041^{+0.012}_{-0.014}$ with $\Lambda=0.9 \pm 0.1$ determined from the nucleon electromagnetic form factors. Our results are comparable with the recent lattice simulation.

How strange is pion electroproduction?

We consider pion production in parity-violating electron scattering (PVES) in the presence of nucleon strangeness in the framework of partial wave analysis with unitarity. Using the experimental bounds on the strange form factors obtained in elastic PVES, we study the sensitivity of the parity-violating asymmetry to strange nucleon form factors. For forward kinematics and electron energies above 1 GeV, we observe that this sensitivity may reach about 20% in the threshold region. With parity-violating asymmetries being as large as tens p.p.m., this study suggests that threshold pion production in PVES can be used as a promising way to better constrain strangeness contributions. Using this model for the neutral current pion production, we update the estimate for the dispersive γZ-box correction to the weak charge of the proton. In the kinematics of the Qweak experiment, our new prediction reads Re□γZV(E=1.165 GeV)=(5.58±1.41)×10−3, an improvement over the previous uncertainty estimate of ±2.0×10−3. Our new prediction in the kinematics of the upcoming MESA/P2 experiment reads Re□γZV(E=0.155 GeV)=(1.1±0.2)×10−3.

Determination of the strange nucleon form factors.

The strange contribution to the electric and magnetic form factors of the nucleon is determined at a range of discrete values of Q^{2} up to 1.4 GeV^{2}. This is done by combining a recent analysis of lattice QCD results for the electromagnetic form factors of the octet baryons with experimental determinations of those quantities. The most precise result is a small negative value for the strange magnetic moment: G_{M}^{s}(Q^{2}=0)=-0.07±0.03μ_{N}. At larger values of Q^{2} both the electric and magnetic form factors are consistent with zero to within 2 standard deviations.

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.

Nucleon strangeness form factors fromNf=2+1clover fermion lattice QCD

We present the ${N}_{f}=2+1$ clover fermion lattice QCD calculation of the nucleon strangeness form factors. We evaluate disconnected insertions using the Z(4) stochastic method, along with unbiased subtractions from the hopping parameter expansion. We find that increasing the number of nucleon sources for each configuration improves the signal significantly. We obtain ${G}_{M}^{s}(0)=\ensuremath{-}0.017(25)(07)$, where the first error is statistical, and the second is the uncertainties in ${Q}^{2}$ and chiral extrapolations. This is consistent with experimental values, and has an order of magnitude smaller error.

Strange quark content in the nucleon

Though none of the experimental evidences for the strange quark contributions to nucleon properties is explained convincingly by an alternative, the recent experiments, even HAPPEX Collab. and A4 Collab., on a measurement of the parity-violating asymmetries show no strangeness in the proton. Despite this conclusion we demonstrate here no accidental compatibility of our theoretical predictions for nucleon strange form factors with some nonzero parity violation experimental results which strengthens our belief in the strangeness in the nucleon.

Isospin breaking in the vector current of the nucleon

Extraction of the nucleon's strange form factors from experimental data requires a quantitative understanding of the unavoidable contamination from isospin violation. A number of authors have addressed this issue during the past decade, and their work is reviewed here. The predictions from early models are largely consistent with recent results that rely as much as possible on input from QCD symmetries and related experimental data. The resulting bounds on isospin violation are sufficiently precise to be of value to on-going experimental and theoretical studies of the nucleon's strange form factors.

Implications of the JLab proton polarization data for the behavior of strange nucleon form factors

Special eight-resonance unitary and analytic model of nucleon electromagnetic structure is used to analyze, first the classical proton form factor data obtained by the Rosenbluth technique and then also the contradicting JLab proton polarization data on the ratio $\mu_p G_{Ep}(Q^2)/G_{Mp}(Q^2)$ with the aim to investigate a manifestation of the latter on the strange nucleon form factors behaviour.

The G0 experiment at Jefferson Laboratory: The nucleon strangeness form factors

The G 0 experiment is dedicated to the determination of the strange quark's contribution to the electric and magnetic nucleon form factors, provided by parity-violating asymmetries of cross-sections measured with longitudinaly polarized electrons in elastic electron-proton scattering and quasi-elastic electron-deuteron scattering. Forward angle measurements, which have been performed in Hall C of Jefferson Laboratory, have provided a linear combination of electric and magnetic vector form factors for momentum transfers in the range 0.1 to 1 ( GeV/c ) 2 . Backward angle measurements, which will be performed starting in 2006, will allow the complete separation of the form factors at Q 2 of 0.23 and 0.63 ( GeV/c ) 2 .

Constraints on the nucleon strange form factors at [formula omitted

We report the most precise measurement to date of a parity-violating asymmetry in elastic electron–proton scattering. The measurement was carried out with a beam energy of 3.03 GeV and a scattering angle 〈θlab〉=6.0○, with the result APV=(−1.14±0.24(stat)±0.06(syst))×10−6. From this we extract, at Q2=0.099 GeV2, the strange form factor combination GEs+0.080GMs=0.030±0.025(stat)±0.006(syst)±0.012(FF) where the first two errors are experimental and the last error is due to the uncertainty in the neutron electromagnetic form factor. This result significantly improves current knowledge of GEs and GMs at Q2∼0.1 GeV2. A consistent picture emerges when several measurements at about the same Q2 value are combined: GEs is consistent with zero while positive values are favored for GMs, though GEs=GMs=0 is compatible with the data at 95% C.L.

Strange nucleon form factors in the perturbative chiral quark model

We apply the perturbative chiral quark model at one loop to calculate the strange form factors of the nucleon. A detailed numerical analysis of the strange magnetic moments and radii of the nucleon, and also the momentum dependence of the form factors is presented.

Observation of scattering and absorption centers in lead fluoride crystals

For the first time, lead fluoride is used as a fast and compact material in electromagnetic calorimetry. Excellent optical and mechanical properties of the pure Cherenkov crystals are necessary for the A4 collaboration to perform a measurement of the nucleon's strange form factors. Visible scattering and absorption centers as well as surface damages have been investigated to characterize the quality of more than one thousand crystals. Besides, transmittance measurements have been performed on all crystals to reveal absorption bands produced by intrinsic or impurity related point-structure defects. As a consequence, 89 crystals had to be replaced by the Chinese manufacturer SICCAS.

Applications of effective field theory methods in nuclear and particle physics

I review recent chiral perturbation theory results for the scalar and vector form factor of the pion in the presence of virtual photons, pion-nucleon scattering inside the Mandelstam triangle, the electromagnetic hyperon and the strange nucleon form factors. Electromagnetic corrections to the energy-momentum tensor of chiral effective field theories are briefly discussed. I also present a precise chiral nucleon-nucleon potential based on a modified Weinberg power counting and discuss the results in the two-nucleon system and their implications.

Baryon form factors: Model-independent results

Baryon form factors can be analyzed in a largely model-independent fashion in terms of two complementary approaches. These are chiral perturbation theory and dispersion relations. I review the status of dispersive calculations of the nucleon electromagnetic form factors in the light of new data. Then, I present the leading one-loop chiral perturbation theory analysis of the hyperon and the strange nucleon form factors. Open problems and challenges are also discussed.

Chiral symmetry and the nucleon’s vector strangeness form factors

The nucleon's strange-quark vector current form factors are studied from the perspective of chiral symmetry. It is argued that chiral perturbation theory cannot yield a prediction for the strangeness radius and magnetic moment. Arrival at definite predictions requires the introduction of additional, model-dependent assumptions which go beyond the framework of chiral perturbation theory. A variety of such model predictions is surveyed, and the credibility of each is evaluated. The most plausible prediction appears in a model where the unknown chiral counterterms are identified with $t$-channel vector meson exchange amplitudes. The corresponding prediction for the mean square Dirac strangeness radius is $\langle r_s^2\rangle = 0.24$ fm$^2$, which would be observable in up-coming semileptonic determinations of the nucleon's strangeness form factors.