Strangeness Radius Research Articles

Configuration mixing in the quark model

We discuss the flavor asymmetry and the strangeness content of the proton in an unquenched quark model. It is shown that the inclusion of hadron loops leads automatically to an excess of d̄ over ū, but gives very small contributions to the strange magnetic moment and strangeness radius of the proton, both of which are in agreement with experimental data.

Strangeness of the proton

The contribution of strange quarks to the radius and the magnetic moment of the proton is discussed in the framework of the unquenched quark model in which the effects of the ss̄ pairs (strange sea) are taken into account through a 3P0 quark-antiquark pair creation mechanism. The results for the the strange magnetic moment and the strangeness radius of the proton are found to be compatible with zero, in agreement with the latest experimental results from parity-violating electron scattering experiments and recent lattice calculations.

Ss¯sea pair contribution to electromagnetic observables of the proton in the unquenched quark model

We present an unquenched quark model calculation of the $s\overline{s}$ sea pairs' contribution to the magnetic moment of the proton (the strange magnetic moment) and its charge radius (the strange radius). In our approach the effects of the $s\overline{s}$ pairs are taken explicitly into account through a microscopic, QCD-inspired, quark-antiquark pair creation mechanism. Our results for the two ``strangeness'' observables are compatible with the latest experimental results and recent lattice calculations.

Unquenching the Quark Model

The formalism for a new generation of unquenched quark models for baryons in which the effects of quark–antiquark pairs are taken into account in an explicit form via a microscopic, QCD-inspired, quark–antiquark creation mechanism, is presented. As an application the quark–antiquark contribution to the spin of the proton and the Σ+ is discussed. Finally the strangeness radius of the Σ+ in the closure limit is calculated.

Strangeness in the proton and N*(1535)

The newest progress on the study of the strangeness in the proton and in the lowest negative parity nucleon excited state N ∗ ( 1535 ) is reviewed. Implications on the internal quark structure of the proton, N ∗ ( 1535 ) and other baryons are discussed. The diquark cluster picture for the 5-quark components in baryons gives a natural explanation not only to the empirical indications for a positive strangeness magnetic moment μ s and positive strangeness radius of the proton but also the longstanding mass-reverse problem of N ∗ ( 1535 ) , N ∗ ( 1440 ) and Λ ∗ ( 1405 ) resonances as well as the unusual decay pattern of the N ∗ ( 1535 ) resonance. Evidence for possible existence of N ∗ ( 1535 ) ′ s 1 / 2 − SU ( 3 ) nonet partners in this picture is pointed out, and suggestion is made to search for these 1 / 2 − hyperon excited states under the well known Σ ∗ ( 1385 ) , Λ ∗ ( 1520 ) and Ξ ∗ ( 1530 ) peaks in various reactions.

Penta-Quark Components in Baryons and Evidence at BES

Some recent progresses in our understanding of penta-quark components in baryons is briefly reviewed. The diquark cluster picture for the penta-quark components in baryons gives a natural explanation not only to the empirical indications for a positive strangeness magnetic moment μs and positive strangeness radius of the proton but also the longstanding mass-reverse problem of N*(1535), N*(1440) and Λ*(1405) resonances as well as the unusual decay pattern of the N*(1535) resonance. Evidence supporting this picture from available BES data on J/ψ decays is pointed out.

Erratum: Nucleon strangeness and unitarity [Phys. Rev. D55, 2741 (1997)

The strange-quark vector current form factors of the nucleon are analyzed within the framework of dispersion relations. Particular attention is paid to contributions made by $K\bar{K}$ intermediate states to the form factor spectral functions. It is shown that, when the $K\bar{K}\to N\bar{N}$ amplitude is evaluated in the Born approximation, the $K\bar{K}$ contributions are identical to those arising from a one-loop calculation and entail a serious violation of unitarity. The mean square strangeness radius and magnetic moment are evaluated by imposing unitarity bounds on the kaon-nucleon partial wave amplitudes. The impact of including the kaon's form factor in the dispersion integrals is also evaluated.

Strange chiral nucleon form factors

We investigate the strange electric and magnetic form factors of the nucleon in the framework of heavy baryon chiral perturbation theory to third order in the chiral expansion. All counterterms can be fixed from data. In particular, the two unknown singlet couplings can be deduced from the parity-violating electron-scattering experiments performed by the SAMPLE and the HAPPEX Collaborations. Within the given uncertainties, our analysis leads to a small and positive electric strangeness radius, $〈{r}_{E,s}^{2}〉=(0.05\ifmmode\pm\else\textpm\fi{}0.09) {\mathrm{fm}}^{2}.$ We also deduce the consequences for the upcoming MAMI A4 experiment.

Parity violating target asymmetry in electron-proton scattering

We analyze the parity-violating (PV) components of the analyzing power in elastic electron-proton scattering and discuss their sensitivity to the strange quark contributions to the proton weak form factors. We point out that the component of the analyzing power along the momentum transfer is independent of the electric weak form factor and thus compares favorably with the PV beam asymmetry for a determination of the strangeness magnetic moment. We also show that the transverse component could be used for constraining the strangeness radius. Finally, we argue that a measurement of both components could give experimental information on the strangeness axial charge.

KNScattering and the Nucleon Strangeness Radius

The leading nonzero electric moment of the nucleon strange-quark vector current is the mean square strangeness radius $〈{r}_{s}^{2}〉$. We evaluate the lightest Okubo-Zweig-Iizuka--allowed contribution to $〈{r}_{s}^{2}〉$, arising from the kaon cloud, using dispersion relations. Drawing upon unitarity constraints as well as ${K}^{+}N$ scattering and ${e}^{+}{e}^{\ensuremath{-}}\ensuremath{\rightarrow}K\overline{K}$ cross section data, we find the structure of this contribution differs significantly from that suggested by a variety of QCD-inspired model calculations. In particular, we find evidence for a strong $\ensuremath{\varphi}$-meson resonance which may enhance the scale of kaon cloud contribution to an observable level.

K * Loops and the Strangeness Radius and Magnetic Moment of the Nucleon

In this work we extend previous kaon loop calculations, including the vector K* meson and estimate the strangeness radius of the nucleon and its strange magnetic moment. We find out that, in contrast to naive expectations, this contribution is very important and gives large values for both the radius and the magnetic moment

Dispersion analysis of the strange vector form factors of the nucleon

We analyze the nucleon matrix element of the strange quark vector current in a nucleon-model-independent dispersive approach with input from the current world data set for the isoscalar electromagnetic form factors. The update of Jaffe's minimal three-pole Ansatz for the spectral functions yields a 40% larger (Sachs) strangeness radius ${(r}_{s}^{2}{)}_{\mathrm{Sachs}}=0.20 {\mathrm{fm}}^{2}$ and a by 20% reduced magnitude of the strangeness magnetic moment ${\ensuremath{\mu}}_{s}=\ensuremath{-}0.26$. In the pole approximation these values are shown to be upper bounds. After extending the Ansatz in order to implement the asymptotic QCD momentum dependence (which the three-pole form factors cannot reproduce), we find the magnitude of the three-pole results reduced by up to a factor of 2.5. The signs of the leading moments originate primarily from the large $\ensuremath{\varphi}$-meson couplings and are generic in the pole approximation.

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.

Deuteron electroweak disintegration

We study the deuteron electrodisintegration with inclusion of the neutral currents focusing on the helicity asymmetry of the exclusive cross section in coplanar geometry $\mathcal{A}({\ensuremath{\vartheta}}_{\mathrm{c}.\mathrm{m}.})$. We stress that a measurement of $\mathcal{A}({\ensuremath{\vartheta}}_{\mathrm{c}.\mathrm{m}.})$ in the quasielastic region is of interest for an experimental determination of the weak form factors of the nucleon, allowing one to obtain the parity-violating electron neutron asymmetry. Numerically, we consider the reaction at low-momentum transfer and discuss the sensitivity of $\mathcal{A}({\ensuremath{\vartheta}}_{\mathrm{c}.\mathrm{m}.})$ to the strangeness radius and magnetic moment. The problems coming from the finite angular acceptance of the spectrometers are also considered.

Nucleon strangeness and unitarity

The strange-quark vector current form factors of the nucleon are analyzed within the framework of dispersion relations. Particular attention is paid to contributions made by $K\mathrm{K\ifmmode\bar\else\textasciimacron\fi{}}$ intermediate states to the form factor spectral functions. It is shown that, when the $K\mathrm{K\ifmmode\bar\else\textasciimacron\fi{}}\ensuremath{\rightarrow}N\mathrm{N\ifmmode\bar\else\textasciimacron\fi{}}$ amplitude is evaluated in the Born approximation, the $K\mathrm{K\ifmmode\bar\else\textasciimacron\fi{}}$ contributions are identical to those arising from a one-loop calculation and entail a serious violation of unitarity. The mean square strangeness radius and magnetic moment are evaluated by imposing unitarity bounds on the kaon-nucleon partial wave amplitudes. The impact of including the kaon's strangeness vector current form factor in the dispersion integrals is also evaluated.

Strangeness in the nucleon on the light cone

Strange matrix elements of the nucleon are calculated within the light-cone formulation of the meson cloud model. The ${Q}^{2}$ dependence of the strange vector and axial vector form factors is computed, and the strangeness radius and magnetic moment extracted, both of which are found to be very small and slightly negative. Within the same framework one finds a small but nonzero excess of the antistrange distribution over the strange at large $x$. Kaon loops are unlikely, however, to be the source of a large polarized strange quark distribution.

The strangeness radius and magnetic moment of the nucleon revisited

We update Jaffe's estimate of the strange isoscalar radius and magnetic moment of the nucleon. We make use of a recent dispersion-theoretical fit to the nucleon electromagnetic form factors and an improved description of symmetry breaking in the vector nonet. We find μ s = −0.24±0.03 n.m. and r s 2 = 0.21±0.03 fm 2. The strange formfactor F 2 s ( t) follows a dipole with a cut-off mass of 1.46 GeV, F 2 s(t) = μ s(1 − t 2.14 GeV 2) −2 . We also discuss the theoretical uncertainties entering this approach.

Locality of the strange sea in the nucleon

We introduce the concept of “locality” for the strange sea in the nucleon, which measures the proximity of the strange and anti-strange quarks in the momentum and coordinate spaces. The CCFR data for the strange and anti-strange distributions imply a “local” strange sea in momentum space, which is unexpected in QCD and is at variance with the simple meson-cloud model where the strangeness is generated from the virtual transition of the nucleon to a hyperon plus a kaon. We present a simple model to interpret the CCFR data and to correlate momentum and coordinate space locality, yielding an upper bound of 0.005 fm 2 on the strange radius. We also discuss the significance of locality for other charge-conjugation-odd observables.

Isospin-breaking corrections to nucleon electroweak form factors in the constituent quark model.

We examine isospin breaking in the nucleon wave functions due to the {ital u}{minus}{ital d} quark mass difference and the Coulomb interaction among the quarks, and their consequences on the nucleon electroweak form factors in a nonrelativistic constituent quark model. The mechanically induced isospin breaking in the nucleon wave functions and electroweak form factors are exactly evaluated in this model. We calculate the electromagnetically induced isospin admixtures by using first-order perturbation theory, including the lowest-lying resonance with nucleon quantum numbers but isospin 3/2. We find a small ({le} 1%), but finite correction to the anomalous magnetic moments of the nucleon stemming almost entirely from the quark mass difference, while the static nucleon axial coupling remains uncorrected. Corrections of the same order of magnitude appear in charge, magnetic, and axial radii of the nucleon. The correction to the charge radius in this model is primarily isoscalar, and may be of some significance for the extraction of the strangeness radius from, e.g., elastic forward angle parity violating electron-proton asymmetries, or elastic {sup 4}He({bold e},{ital e}{prime}) experiments.

Strange vector form factors of the nucleon.

A model combining vector meson dominance, $\omega - \phi$ mixing and a kaon cloud contribution is used to estimate the strangeness vector form factors of the nucleon in the momentum range of the planned measurements at MIT-Bates and CEBAF. We compare our results with some other theoretical estimates and discuss the nucleon strangeness radius in models based on dynamically dressed, extended constituent quarks. For a quantitative estimate in the latter framework we use the Nambu-Jona-Lasinio model.