Strange Form Factors Research Articles

Final state interactions in [formula omitted] decays

Charged B decays to three charged kaons are analysed in the framework of the QCD factorization approach. The strong final state K+K− interactions are described using the kaon scalar and vector form factors. The scalar non-strange and strange form factors at low K+K− effective masses are constrained by chiral perturbation theory and satisfy the two-body unitarity conditions. The latter stem from the properties of the meson–meson amplitudes which describe all possible S-wave transitions between three coupled channels consisting of two kaons, two pions and four pions. The vector form factors are fitted to the data on the electromagnetic kaon interactions. The model results are compared with the Belle and BaBar data. Away from ϕ(1020) resonance, in the S-wave dominated K+K− mass spectra, a possibility for a large CP asymmetry is identified.

CURRENT STATUS OF THE G0 PARITY VIOLATION EXPERIMENT CARRIED OUT AT JEFFERSON LABORATORY

Among experiments looking for strangeness in nuclear systems, electron scattering Parity-Violation (PV) type experiments aim mainly at measuring the strange quark contributions to the form factors of the proton. They can be accessed by measuring the asymmetry in longitudinally polarized electron elastic scattering. The PV asymmetry is measured at the 10-7 level of accuracy, and when combined with the electromagnetic form factors of the nucleon, the strange form factors can be determined. The full separation of form factors according to quark flavour requires measurements at forward angles on hydrogen and at backward angles on hydrogen and deuterium. The experimental setup used for G 0 in the two different configurations is described. The status of the analysis and the current results are presented and discussed in relation with other experiments past and planned. A few comments are also made regarding some other aspects of physics accessible in simultaneous measurements: neutral currents in N Δ transitions, parity violation in inclusive π- production and 2γ contributions to elastic scattering.

Two-boson exchange corrections to parity-violating elastic electron-proton scattering

Details of the calculation of two-boson exchange effects in parity-violating elastic $\mathit{ep}$ scattering within a simple hadronic model, including both the nucleon and the $\ensuremath{\Delta}(1232)$-resonance intermediate states, are presented. We examine the sensitivity of our results with respect to choice of form factors. We emphasize the importance of using correct relations relating $N\ensuremath{\rightarrow}\ensuremath{\Delta}$ and $\ensuremath{\Delta}\ensuremath{\rightarrow}N$ transition vertex functions. The $N\ensuremath{\Delta}$ Coulomb quadrupole transition is found to play an important role at higher ${Q}^{2}\ensuremath{\geqslant}3.0$ GeV${}^{2}$. We also elucidate the relation between our results and the well-known result on the $\ensuremath{\gamma}Z$-boson exchange effect given by Marciano and Sirlin. The effect of the nucleon contribution ${\ensuremath{\delta}}_{N}$ to parity asymmetry ${A}_{\mathrm{PV}}$ is found to be, in general, larger than the corresponding $\ensuremath{\Delta}$ contribution ${\ensuremath{\delta}}_{\ensuremath{\Delta}}$ except at extreme forward angles. Corrections to the extracted values of the strange form factors ${G}_{E}^{s}+\ensuremath{\beta}{G}_{M}^{s}$ from the HAPPEX, A4, and G0 data are also presented. The total TBE corrections to the extracted values of ${G}_{E}^{s}+\ensuremath{\beta}{G}_{M}^{s}$ in recent HAPPEX, G0, and A4 experiments are, depending on kinematics, found to be small except in a few cases where they range from $\ensuremath{-}20.6%$ to $48.3%$.

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 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, ${G}_{M}^{s}=\ensuremath{-}0.034\ifmmode\pm\else\textpm\fi{}0.021 {\ensuremath{\mu}}_{N}$.

Contribution of two-boson exchange withΔ(1232)excitation to parity-violating elastic electron-proton scattering

We study the leading electroweak corrections in the precision measurement of the strange form factors. Specifically, we calculate the two-boson-exchange (TBE), two-photon-exchange (TPE) plus $\gamma Z$-exchange ($\gamma Z$E), corrections with $\Delta(1232)$ excitation to the parity-violating asymmetry of the elastic electron-proton scattering. The interplay between nucleon and $\Delta$ contributions is found to depend strongly on the kinematics, as $\delta_\Delta$ begins as negligible at backward angles but becomes very large and negative and dominant at forward angles, while $\delta_N$ always stays positive and decreases monotonically with increasing $\epsilon$. The total TBE corrections to the extracted values of $G^{s}_{E}+\beta G^{s}_{M}$ in recent experiments of HAPPEX and G0 are, depending on kinematics, found to be large and range between 13% to -75% but small in the case of A4 experiments.

Strange baryon electromagnetic form factors and SU(3) flavor symmetry breaking

We study the nucleon, Sigma and cascade octet baryon electromagnetic form factors and the effects of SU(3) flavor symmetry breaking from $2+1$-flavor lattice calculations. We find that electric and magnetic radii are similar; the maximum discrepancy is about 10%. In the pion-mass region we explore, both the quark-component and full-baryon moments have small SU(3) symmetry breaking. We extrapolate the charge radii and the magnetic moments using three-flavor heavy-baryon chiral perturbation theory. The systematic errors due to chiral and continuum extrapolations remain significant, giving rise to charge radii for $p$ and ${\ensuremath{\Sigma}}^{\ensuremath{-}}$ that are 3--4 standard deviations away from the known experimental ones. Within these systematics the predicted ${\ensuremath{\Sigma}}^{+}$ and ${\ensuremath{\Xi}}^{\ensuremath{-}}$ radii are 0.67(5) and $0.306(15)\text{ }\text{ }{\mathrm{fm}}^{2}$, respectively. When the next-to-next-to-leading order of heavy-baryon chiral perturbation theory is included, the extrapolated magnetic moments are less than 3 standard deviations away from Particle Data Group values, and the discrepancy is possibly due to remaining chiral and continuum extrapolation errors.

Measurement of Strange Quark Contributions to the Vector Form Factors of the Proton atQ2=0.22 (GeV/c)2

A new measurement of the parity violating asymmetry in elastic electron scattering on hydrogen at backward angles and at a four momentum transfer of Q;{2} = 0.22 (Ge V / c);{2} is reported here. The measured asymmetry is A_{LR} = (-17.23 +/- 0.82_{stat} +/- 0.89_{syst}) x 10;{-6}. The standard model prediction assuming no strangeness is A_{0} = (-15.87 +/- 1.22) x 10;{-6}. In combination with previous results from measurements at forward angles, it is possible to disentangle for the first time the strange form factors at this momentum transfer, G_{E};{s} = 0.050 +/- 0.038 +/- 0.019 and G_{M};{s} = -0.14 +/- 0.11 +/- 0.11.

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.

Strange quark contribution to the vector and axial form factors of the nucleon: Combined analysis of data from the G0, HAPPEx, and Brookhaven E734 experiments

The strange quark contribution to the vector and axial form factors of the nucleon has been determined for momentum transfers in the range $0.45<{Q}^{2}<1.0$ GeV${}^{2}$. The results are obtained via a combined analysis of forward-scattering, parity-violating elastic $\stackrel{\ensuremath{\rightarrow}}{e}p$ asymmetry data from the G0 and HAPPEx experiments at Jefferson Lab and elastic $\ensuremath{\nu}p$- and $\overline{\ensuremath{\nu}}p$-scattering data from Experiment 734 at Brookhaven National Laboratory. The parity-violating asymmetries measured in elastic $\stackrel{\ensuremath{\rightarrow}}{e}p$ scattering at forward angles establish a relationship between the strange vector form factors ${G}_{E}^{s}$ and ${G}_{M}^{s}$, with little sensitivity to the strange axial form factor ${G}_{A}^{s}$. However, elastic neutrino scattering at low ${Q}^{2}$ is dominated by the axial form factor, with some significant sensitivity to the vector form factors as well. Combination of the two data sets allows the simultaneous extraction of ${G}_{E}^{s},{G}_{M}^{s}$, and ${G}_{A}^{s}$ over a significant range of ${Q}^{2}$ for the very first time. The ${Q}^{2}$ dependence of the strange axial form factor suggests that the strange quark contribution to the proton spin, $\ensuremath{\Delta}s$, is negative.

Ambiguities of neutrino (antineutrino) scattering on the nucleon due to the uncertainties of relevant strangeness form factors

Strange quark contributions to the neutrino (antineutrino) scattering on the nucleon level are investigated in the quasi-elastic region. Incident energy is used at 500 MeV for the scattering. All of the physical observables by the scattering are investigated within available experimental and theoretical results for strangeness form factors of the nucleon. Specifically, a newly combined datum of the parity violating electron scattering and the neutrino scattering is exploited for the strangeness form factors. Feasible quantities to be explored for the strangeness contents are discussed for the application to the neutrino–nucleus scattering.

Effect of Two-Boson Exchange on Parity-Violatinge−pScattering

We compute the corrections from two-photon and gamma-Z exchange in parity-violating elastic electron-proton scattering, used to extract the strange form factors of the proton. We use a hadronic formalism that successfully reconciled the earlier discrepancy in the proton's electron to magnetic form factor ratio, suitably extended to the weak sector. Implementing realistic electroweak form factors, we find effects of the order 2%-3% at Q(2) less, similar0.1 GeV2, which are largest at backward angles and have a strong Q2 dependence at low Q2. Two-boson contributions to the weak axial current are found to be enhanced at low Q2 and for forward angles. We provide corrections at kinematics relevant for recent and upcoming parity-violating experiments.

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.

Two-Photon-Exchange andγZ-Exchange Corrections to Parity-Violating Elastic Electron-Proton Scattering

Leading electroweak corrections play an important role in precision measurements of the strange form factors. We calculate the two-photon-exchange (TPE) and gammaZ-exchange corrections to the parity-violating asymmetry of the elastic electron-proton scattering in a simple hadronic model including the finite size of the proton. We find both can reach a few percent and are comparable in size with the current experimental measurements of strange-quark effects in the proton neutral weak current. The effect of gammaZ exchange is in general larger than that of TPE, especially at low momentum transfer Q2</=1 GeV2. Their combined effects on the values of G(E)s + beta G(M)s [corrected] extracted in recent experiments can be as large as -40% in certain kinematics.

Sum rules for baryon decuplet magnetic moments

In chiral models with SU(3) group structure, baryon decuplet, and octet magnetic moments are evaluated by constructing their sum rules to yield theoretical predictions. In these sum rules we exploit six experimentally known baryon magnetic moments. Sum rules for flavor components and strange form factors of the octet and decuplet magnetic moments and decuplet-to-octet transition magnetic moments are also investigated.

Strangeness content of the nucleon in quasielastic neutrino-nucleus reactions

We present a systematic study of the sensitivity of quasielastic neutrino-nucleus cross sections at intermediate energies to the strange quark sea of the nucleon. To this end, we investigate the impact of the weak strangeness form factors on the ratio of proton-to-neutron knockout, the ratio of neutral-to-charged current cross sections, on the Paschos-Wolfenstein relation, and on the longitudinal helicity asymmetry. The influence of axial as well as vector strangeness effects is discussed. For the latter, we introduce strangeness parameters from various hadron models and from a recent fit to data from parity-violating electron scattering. In our model, the nuclear target is described in terms of a relativistic mean-field approach. The effects of final-state interactions on the outgoing nucleon are quantified within a relativistic multiple-scattering Glauber approach. Our results are illustrated with cross sections for the scattering of 1-GeV neutrinos and antineutrinos off a {sup 12}C target. Folding with a proposed FINeSSE (anti-)neutrino energy distribution has no qualitative influence on the overall sensitivity of the cross-section ratios to strangeness mechanisms. We show that vector strangeness effects are large and strongly Q{sup 2} dependent.

Isospin Mixing in the Nucleon andHe4and the Nucleon Strange Electric Form Factor

In order to isolate the contribution of the nucleon strange electric form factor to the parity-violating asymmetry measured in 4He(e-->],e')4He experiments, it is crucial to have a reliable estimate of the magnitude of isospin-symmetry-breaking (ISB) corrections in both the nucleon and 4He. We examine this issue in the present Letter. Isospin admixtures in the nucleon are determined in chiral perturbation theory, while those in 4He are derived from nuclear interactions, including explicit ISB terms. A careful analysis of the model dependence in the resulting predictions for the nucleon and nuclear ISB contributions to the asymmetry is carried out. We conclude that, at the low momentum transfers of interest in recent measurements reported by the HAPPEX Collaboration at Jefferson Lab, these contributions are of comparable magnitude to those associated with strangeness components in the nucleon electric form factor.

Precise determination of low-Qnucleon electromagnetic form factors and their impact on parity-violatinge-pelastic scattering

The extraction of the strangeness form factors from parity-violating elastic electron-proton scattering is sensitive to the electromagnetic form factors at low ${Q}^{2}$. We provide parametrizations for the form factors and uncertainties, including the effects of two-photon exchange corrections to the extracted electromagnetic form factors. We study effect of the correlations between different form factors, in particular as they impact the parity-violating asymmetry and the extraction of the strangeness form factors. We provide a prescription to extract the strangeness form factors from the asymmetry that provides an excellent approximation of the full two-photon correction. The corrected form factors are also appropriate as input for other low-$Q$ analyses, although the effects of correlations and two-photon exchange corrections may be different.

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.

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.