Strange Form Factors Research Articles

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.

Quark model analysis of strangeness form factors of the proton

The present empirical information on the proton strangeness form factors suggests that the uuds subsystem of the uuds \( \bar s \) configuration has the flavor spin symmetry [4] FS [22] F [22] S , mixed orbital symmetry [31] X and that the \( \bar s \) is in the ground state. This uuds \( \bar s \) configuration yields the empirical signs for both the form factors G E s and G M s . Transition matrix elements between the uud and uuds \( \bar s \) components are significant.

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.

Hadron structure on the lattice and in the continuum

Hadron structure physics has in recent years reached a level of precision which allows for a change of perspective. Model-based arguments are often quite unreliable. However, meanwhile they can be more and more replaced by controlled and systematic QCD approaches. The story of the strange electric form factor, which provided much of the motivation for the PAVI Conference series provides a typical example to illustrate this statement. However, high-precision theory is technically very challenging and progress is, therefore, unpleasantly slow. This fact and the present status in general is illustrated by a few typical examples.

Flavor content of nucleon form factors in a VMD approach

The strange form factors of the nucleon are studied in a two-component model consisting of a three-quark intrinsic structure surrounded by a meson cloud. A comparison with the available experimental world data from the SAMPLE, PVA4, HAPPEX and G0 Collaborations shows a good overall agreement. It is shown that the strangeness contribution to the electric and magnetic form factors is of the order of a few percent. In particular, the strange quark contribution to the charge radius is small E = 0.005 fm2 and to the magnetic moment it is positive μ s = 0.315 μ N .

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

Isospin mixing in the nucleon and He-4 and the nucleon strange electric form factor RID A-7123-2011

Isospin mixing in the nucleon and He-4 and the nucleon strange electric form factor RID A-7123-2011

Jefferson Lab phenomenology: selected highlights

An overview of recent experimental highlights from Jefferson Lab is presented. We review the status of baryon spectroscopy, including the search for pentaquarks, as well as measurements of electromagnetic form factors of the nucleon, featuring the proton G E / G M ratio and the determination of the strangeness form factors. In inclusive scattering, we describe recent studies of quark-hadron duality in structure functions in the resonance–scaling transition region, and outline future physics plans at an energy upgraded 12 GeV facility.

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.

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.

Electric properties of the baryon antidecuplet in the SU(3) chiral quark-soliton model

We investigate the electric form factors and charge radii of the pentaquark baryons within the framework of the chiral quark-soliton model. We consider the rotational $1/{N}_{c}$ and linear ${m}_{s}$ corrections, assuming isospin symmetry and employing the symmetry-conserving quantization. The flavor-decomposed charge densities of the ${\ensuremath{\Theta}}^{+}$ are presented. The electric form factors and charge radii of the charged pentaquark baryons turn out to be very similar to those of the corresponding octet baryons. The charge radii of the neutral pentaquark baryons are obtained to be very tiny and positive. The strange electric form factor of the pentaquark proton is shown to be larger than the corresponding one of the proton by around 20%. We also present the charge radii of the baryon decuplet for comparison.

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 .

Improved analysis ofJ/ψdecays into a vector meson and two pseudoscalars

Recently, the BES collaboration has published an extensive partial-wave analysis of experimental data on $J/\ensuremath{\psi}\ensuremath{\rightarrow}\ensuremath{\phi}{\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}$, $J/\ensuremath{\psi}\ensuremath{\rightarrow}\ensuremath{\omega}{\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}$, $J/\ensuremath{\psi}\ensuremath{\rightarrow}\ensuremath{\phi}{K}^{+}{K}^{\ensuremath{-}}$ and $J/\ensuremath{\psi}\ensuremath{\rightarrow}\ensuremath{\omega}{K}^{+}{K}^{\ensuremath{-}}$. These new results are analyzed here, with full account of detection efficiencies, in the framework of a chiral unitary description with coupled-channel final state interactions between $\ensuremath{\pi}\ensuremath{\pi}$ and $K\overline{K}$ pairs. The emission of a dimeson pair is described in terms of the strange and nonstrange scalar form factors of the pion and the kaon, which include the final state interaction and are constrained by unitarity and by matching to the next-to-leading-order chiral expressions. This procedure allows for a calculation of the $S$-wave component of the dimeson spectrum including the ${f}_{0}(980)$ resonance, and for an estimation of the low-energy constants of Chiral Perturbation Theory, in particular, the large ${N}_{c}$ suppressed constants ${L}_{4}^{r}$ and ${L}_{6}^{r}$. The decays in question are also sensitive to physics associated with OZI violation in the ${0}^{++}$ channel. It is found that the $S$-wave contributions to $\ensuremath{\phi}{\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}$, $\ensuremath{\phi}{K}^{+}{K}^{\ensuremath{-}}$ and $\ensuremath{\omega}{\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}$ given by the BES partial-wave analysis may be very well fitted up to a dimeson center-of-mass energy of $\ensuremath{\sim}1.2\text{ }\text{ }\mathrm{GeV}$, for a large and positive value of ${L}_{4}^{r}$ and a value of ${L}_{6}^{r}$ compatible with zero. An accurate determination of the amount of OZI violation in the $J/\ensuremath{\psi}\ensuremath{\rightarrow}\ensuremath{\phi}{\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}$ decay is achieved, and the $S$-wave contribution to $\ensuremath{\omega}{K}^{+}{K}^{\ensuremath{-}}$ near threshold is predicted.

Nucleon electromagnetic form factors from lattice QCD

We evaluate the strange nucleon electromagnetic form factors using an ensemble of gauge configurations generated with two degenerate maximally twisted mass clover-improved fermions with mass tuned to approximately reproduce the physical pion mass. In addition, we present results for the disconnected light quark contributions to the nucleon electromagnetic form factors. Improved stochastic methods are employed leading to high-precision results. The momentum dependence of the disconnected contributions is fitted using the model-independent z-expansion. We extract the magnetic moment and the electric and magnetic radii of the proton and neutron by including both connected and disconnected contributions. We find that the disconnected light quark contributions to both electric and magnetic form factors are nonzero and at the few percent level as compared to the connected. The strange form factors are also at the percent level but more noisy yielding statistical errors that are typically within one standard deviation from a zero value.

Isospin violation in the vector form factors of the nucleon

A quantitative understanding of isospin violation is an increasingly important ingredient for the extraction of the nucleon's strange vector form factors from experimental data. We calculate the isospin violating electric and magnetic form factors in chiral perturbation theory to leading and next-to-leading order respectively, and we extract the low-energy constants from resonance saturation. Uncertainties are dominated largely by limitations in the current knowledge of some vector meson couplings. The resulting bounds on isospin violation are sufficiently precise to be of value to on-going experimental studies of the strange form factors.

Parity-violating electron scattering at the MAMI facility in Mainz

A measurement of the weak form factor of the proton allows a separation of the strangeness contribution to the electromagnetic form factors. The weak form factor is accessed experimentally by the measurement of a parity violating (PV) asymmetry in the scattering of polarized electrons on unpolarized protons. We performed such measurements with the setup of the A4-experiment at the MAMI accelerator facility in Mainz. The role of strangeness in low energy nonperturbative QCD is discussed. The A4-experiment is presented as well as the results on the strangeness form factors which have been measured at two Q2-values. The plans for backward angle measurements at the MAMI facility are presented.

Parity violation in electron scattering

Parity-violating electron scattering has been a very useful tool for probing the structure of neutral currents and providing detailed information on electroweak form factors. A pioneering SLAC measurement in the mid-70s provided an important early test of the Standard Model. Modern electron accelerators provide high-intensity (> 100 µA), CW beams with polarizations as high as 85%. Experiments such as SAMPLE, A4, HAPPEX and G0 have exploited these capabilities and obtained new information on electroweak strange form factors in the Q 2 range of 0.1–1.0 (GeV/c)2. That activity continues. Other experiments are designed to provide stringent tests of the Standard Model. E-158 at SLAC recently measured the weak charge of the electron. Q weak is a challenging new experiment at JLAB which is designed to measure the weak charge of the proton. This will probe for physics beyond the Standard Model corresponding to energy scales of more than 5TeV.

The strangeness form factors of the proton

The present empirical information on the strangeness form factors indicates that the corresponding uudss¯ component in the proton is such that the uuds subsystem has the flavor spin symmetry [4]FS[22]F[22]S and mixed orbital symmetry [31]X. This uudss¯ configuration leads to the empirical signs of all the form factors GEs,GMs and GAs. An analysis with simple quark model wave functions for the preferred configuration shows that the qualitative features of the empirical strangeness form factors may be described with a ∼15% admixture of uudss¯ with a compact wave function in the proton. Transition matrix elements between the uud and uudss¯ components give significant contributions.

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.