Determining the strange and antistrange quark distributions of the nucleon

The difference between the strange and antistrange quark distributions,δs（x） = s（x） ˉs（x）,and the combination of light quark sea and strange quark sea,Δ（x） =ˉ d（x） ＋ uˉ（x） s（x） ˉs（x）,are originated from non-perturbative processes and can be calculated using non-perturbative models of the nucleon.We report calculations of δs（x） and Δ（x） using the meson cloud model.Combining our calculations of Δ（x） with relatively well known light antiquark distributions obtained from global analysis of available experimental data,we estimate the total strange sea distributions of the nucleon.     

Determining the strange and antistrange quark distributions of the nucleon

The difference between the strange and antistrange quark distributions,δs(x) = s(x) ˉs(x),and the combination of light quark sea and strange quark sea,Δ(x) =ˉ d(x) + uˉ(x) s(x) ˉs(x),are originated from non-perturbative processes and can be calculated using non-perturbative models of the nucleon.We report calculations of δs(x) and Δ(x) using the meson cloud model.Combining our calculations of Δ(x) with relatively well known light antiquark distributions obtained from global analysis of available experimental data,we estimate the total strange sea distributions of the nucleon.     

Perturbative and nonperturbative contributions to the strange quark asymmetry in the nucleon

There are two mechanisms for the generation of an asymmetry between the strange and anti-strange quark distributions in the nucleon: nonperturbative contributions originating from nucleons fluctuating into virtual baryon?Cmeson pairs such as ??K and ??K, and perturbative contributions arising from gluons splitting into strange and anti-strange quark pairs. While the nonperturbative contributions are dominant in the large-x region, the perturbative contributions are more significant in the small-x region. We calculate this asymmetry taking into account both nonperturbative and perturbative contributions, thus giving a more accurate evaluation of this asymmetry over the whole domain of x. We find that the perturbative contributions are generally a few times larger in magnitude than the nonperturbative contributions, which suggests that the best region to detect this asymmetry experimentally is in the region 0.02<x<0.03. We find that the asymmetry may have more than one node, which is an effect that should be taken into account, e.g. for parameterizations of the strange and anti-strange quark distributions used in global analysis of parton distributions.     

On the strange quark content in nucleons

We show in many ways that little strange quark axial-vector spin s or quark density can occur in nucleons.     

Stranger still: Kaon loops and strange quark matrix elements of the nucleon

Intrinsic strangeness contributions to lowenergy strange quark matrix elements of the nucleon are modelled using kaon loops and meson-nucleon vertex functions taken from nucleon-nucleon and nucleon-hyperon scattering. A comparison with pion loop contributions to the nucleon electromagnetic (EM) form factors indicates the presence of significantSU(3)-breaking in the mean-square charge radii. As a numerical consequence, the kaon loop contribution to the mean square Dirac strangeness radius is significantly smaller than could be observed with parity-violating elastic p experiments planned for CEBAF, while the contribution to the Sachs radius is large enough to be observed with PV electron scattering from (0+, 0) nuclei. Kaon loops generate a strange magnetic moment of the same scale as the isoscalar EM magnetic moment — a scale large enough to be observed at CEBAF — and a strange axial vector form factor having roughly one-third of the magnitude extracted from elastic scattering. In the chiral limit, the loop contribution to the fraction of the nucleon's scalar density arising from strange quarks has roughly the same magnitude as the value extracted from analyses of _N . The importance of satisfying the Ward-Takahashi Identity, not obeyed by previous calculations, is also illustrated, and the sensitivity of results to input parameters is analyzed.This work is supported in part by funds provided by the U.S. Department of Energy (D.O.E) under contracts #DE-AC02-76ER 03069 and #DE-AC05-84ER 40150     

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.     

Probing the strange quark condensate by di-electrons from φ-meson decays in heavy-ion collisions at SIS energies

QCD sum rules predict that the change of the strange quark condensate 〈ˉss〉 in hadron matter at finite baryon density causes a shift of the peak position of the di-electron spectra from φ-meson decays. Due to the expansion of hadron matter in heavy-ion collisions, the φ peak suffers a smearing governed by the interval of density in the expanding fireball, which appears as an effective broadening of the di-electron spectrum in the φ region. The emerging broadening is sensitive to the in-medium change of 〈ˉss〉. This allows to probe directly in-medium modifications of 〈ˉss〉 via di-electron spectra in heavy-ion collisions at SIS energies with HADES. Received: 22 November 2002 / Accepted: 30 January 2003 / Published online: 29 April 2003     

Determination of the strange quark content of the nucleon from a next-to-leading-order QCD analysis of neutrino charm production

We present the first next-to-leading-order QCD analysis of neutrino charm production, using a sample of 6090 ^– and -induced opposite-sign dimuon events observed in the CCFR detector at the Fermilab Tevatron. We find that the nucleon strange quark content is suppressed with respect to the non-strange sea quarks by a factor =0.477 _–0.053 ^+0.063 , where the error includes statistical, systematic and QCD scale uncertainties. In contrast to previous leading order analyses, we find that the strange seax-dependence is similar to that of the non-strange sea, and that the measured charm quark mass,m _c =1.70±0.19 GeV/c², is larger and consistent with that determined in other processes. Further analysis finds that the difference inx-distributions betweenxs(x) and is small. A measurement of the Cabibbo-Kobayashi-Maskawa matrix element |V _cd |=0.232 _–0.020 ^+0.018 is also presented.     

Properties of color-flavor locked strange quark matter and strange stars in a new quark mass scaling

Considering the effect of one-gluon-exchange interaction between quarks,the color-flavor locked strange quark matter and strange stars are investigated in a new quark mass density-dependent model.It is found that the color-flavor locked strange quark matter can be more stable if the one-gluon-exchange effect is included.The lower density behavior of the sound velocity in this model is different from the previous results.Moreover,the new equation of state leads to a heavier acceptable maximum mass,supporting the recent observation of a compact star mass as large as about 2 times the solar mass.     

Thermodynamical information on quark matter from the nucleon valence quark distribution

If the valence quarks in the nucleon are considered as a gas of the non-interacting massless particles, an expression of the xF₃(x) structure function is obtained which reproduces very well the experimental $\text{v(}\text{v}\text{)}$N data in the range 0.1?x?0.8. The effective temperature of such a quark gas is found to be of the order of 50 MeV.     

The quark spin structure of the nucleon

Some non-perturbative aspects of the nucleon quark spin structure are reviewed. The first part is a brief summary of early theoretical developments in the field of polarized deep inelastic scattering of electrons on polarized nucleons and an illustration of the non-perturbative power of lattice QCD 25 years later. The second part is a short pedagogical introduction to the analysis of high energy scattering in the complex angular momentum plane, with particular emphasis on spin-dependent deep inelastic electron-nucleon scattering. The third part comprises a brief introduction to lattice QCD and its applications in the non-perturbative determination of the spin-dependent structure functions.     

Isospin mixing in the nucleon and 4He and 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.     

Two quark excitations in two nucleon system

The unambiguously determined bases of the finite dimensional irreducible representation of the unitary and orthogonal groups are constructed. It is shown that the bases can be successfully applied in finding decomposition coefficients when the system of physical particles is divided into two subsystems. The general forms for some of these coefficients are presented.Presented at the symposium Mesons and Light Nuclei, Bechyn, Czechoslovakia, May 27–June 1, 1985.     

Bulk viscosity of strange quark matter

We rederive the bulk viscosity of strange quark matter from the dominant reactionu + s ?d + u by taking the effect of temperature and quark-gluon coupling perturbatively to first order in the chemical composition of the quark matter. We also calculate the contribution from the β-decay processes s(d) →u + e + ¯v andu + e → s(d)+ v and show that this contribution has different temperature dependence and can even be larger than the contribution from the former reaction at temperatures of the order of the electron Fermi energy.     

Phenomenological determination of polarized quark distributions in the nucleon

We present a fit to spin asymmetries which gives polarized quark distributions. These functions are closely related to the ones given by the Martin, Roberts and Stirling fit for unpolarized structure functions. The integrals of polarized distributions are discussed and compared with the corresponding quantities obtained from neutron and hyperonβ-decay data. We use the combination of proton, neutron and deuteron spin asymmetries in order to determine the coefficients of our polarized quark distributions. Our fit shows that phenomenologically there is no need for taking polarized gluons into account.