Dispersion-theoretical analysis of the nucleon electromagnetic formfactors

Dispersion relations allow for a coherent description of the nucleon electromagnetic form factors measured over a large range of momentum transfer, Q^2 \simeq 0 \ldots 35 GeV^2. Including constraints from unitarity and perturbative QCD, we present a novel parametrisation of the absorptive parts of the various isoscalar and isovector nucleon form factors. Using the current world data, we obtain results for the electromagnetic form factors, nucleon radii and meson couplings. We stress the importance of measurements at large momentum transfer to test the predictions of perturbative QCD

The ωNN\omega NN couplings derived from QCD sum rules

The light cone QCD sum rules are derived for $\omega NN$ vector and tensor couplings simultaneously. The vacuum gluon field contribution is taken into account. Our results are $g_\omega =(18\pm 8), \kappa_\omega=(0.8\pm 0.4)$.Comment: To appear in Phys. Rev. C (Brief Report

Updated dispersion-theoretical analysis of the nucleon electromagnetic form factors

In the light of the new data on the various neutron and proton electromagnetic form factors taken in recent years, we update the dispersion-theoretical analysis of the nucleon electromagnetic form factors from the mid-nineties. The parametrization of the spectral functions includes constraints from unitarity, perturbative QCD, and recent measurements of the neutron charge radius. We obtain a good description of most modern form factor data, with the exception of the Jefferson Lab data on G_E^p/G_M^p in the four-momentum transfer range Q^2=3...6 GeV^2. For the magnetic radii of the proton and the neutron we find r_M^p = 0.857 fm and r_M^n = 0.879 fm, which is consistent with the recent determinations using continued fraction expansions.Comment: 5 pages, 3 ps figures, final version, exp. errors in Figs. 1 and 3 correcte

Proton charge and magnetic rms radii from the elastic epep scattering data

The elastic electron-proton scattering data are analysed in order to determine proton charge and magnetic rms radii, r_E and r_M. Along with the usual statistical error, we try to estimate a systematic error in the radii, caused by the inadequacy of particular form factor parameterization employed. The range of data to use in the analysis is chosen so as to minimize the total (statistical + systematic) error. We obtain r_E = 0.912 +- 0.009 (stat) +- 0.007 (syst) fm, and r_M = 0.876 +- 0.010 (stat) +- 0.016 (syst) fm. The cross-section data were corrected for two-photon exchange. We found that without such corrections obtained r_E and r_M are somewhat smaller while the quality of fit is worse.Comment: 6 pages, 4 figures. Numbers slightly changed due to discovered error in minimization program. Sec.III revised, discussion of G_E behaviour added

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 $\mu_s = -0.24 \pm 0.03$~n.m. and $r_s^2 = 0.21 \pm 0.03$~fm$^2$. The strange formfactor $F_2^s (t)$ follows a dipole with a cut--off mass of 1.46~GeV, $F_2^s (t)= \mu_s (1-t/2.14 \, {\rm GeV}^2 )^{-2}$. These numbers should be considered as upper limits on the strange vector current matrix--elements in the nucleon.Comment: 8 pp, LaTeX, uses epsf, 1 figure in separate fil

The Role of 5-quark Components on the Nucleon Form Factors

The covariant quark model is shown to allow a phenomenological description of the neutron electric form factor, G_E^n(Q^2), in the impulse approximation, provided that the wave function contains minor (~ 3 %) admixtures of the lowest sea-quark configurations. While that form factor is not very sensitive to whether the \bar q in the qqqq\bar q component is in the P-state or in the S-state, the calculated nucleon magnetic form factors are much closer to the empirical values in the case of the former configuration. In the case of the electric form factor of the proton, G_E^p(Q^2), a zero appears in the impulse approximation close to 9 GeV^2, when the \bar q is in the P-state. That configuration, which may be interpreted as a pion loop ("cloud") fluctuation, also leads to a clearly better description of the nucleon magnetic moments. When the amplitude of the sea-quark admixtures are set so as to describe the electric form factor of the neutron, the qqqq\bar q admixtures have the phenomenologically desirable feature, that the electric form factor of the proton falls at a more rapid rate with momentum transfer than the magnetic form factor.Comment: To appear in Nuclear Physics

Dispersion-theoretical analysis of the nucleon electromagnetic form factors: Inclusion of time-like data

We update a recent dispersion--theoretical fit to the nucleon electromagnetic form factors by including the existing data in the time--like region. We show that while the time--like data for the proton can be described consistently with the existing world space--like data, this is not the case for the neutron. Another measurement of the process $e^+ e^- \to \bar n n$ is called for. We furthermore sharpen the previous estimate of the separation between the perturbative and the non--perturbative regime, which is characterized by a scale parameter $\Lambda^2 \simeq 10\,$GeV$^2$.Comment: 7 pp, LaTeX, uses epsf, 2 figures in separate file, four data points changed, slight changes in the fits, conclusions unchange

Fluctuating semiflexible polymer ribbon constrained to a ring

Twist stiffness and an asymmetric bending stiffness of a polymer or a polymer bundle is captured by the elastic ribbon model. We investigate the effects a ring geometry induces to a thermally fluctuating ribbon, finding bend-bend coupling in addition to twist-bend coupling. Furthermore, due to the geometric constraint the polymer's effective bending stiffness increases. A new parameter for experimental investigations of polymer bundles is proposed: the mean square diameter of a ribbonlike ring, which is determined analytically in the semiflexible limit. Monte Carlo simulations are performed which affirm the model's prediction up to high flexibility.Comment: 6 pages, 3 figures, Version as published in Eur. Phys. J.

Nucleon form factors: From the space-like to the time-like region

I discuss how dispersion relations can be used to analyse the nucleon electromagnetic form factors, with particular emphasis on the constraints from unitarity and pQCD. Results for nucleon radii, vector-meson couplings, the onset of pQCD and bounds on the strangeness form factors are presented. The em form factors in the time-like region reveal some interesting physics which is not yet understood in full detail. The need for a better data basis at low, intermediate and large momentum transfer and also in the time-like region is stressed.Comment: 11 pp, LaTeX, uses epsf and espcrc1.sty, 6 figures, invited talk, DAPHCE 96, Frascati, November 1996, to appear in the proceedings (Nucl. Phys. A

Aspects of chiral pion-nucleon physics

The next-to-leading order chiral pion-nucleon Lagrangian contains seven finite low-energy constants. Two can be fixed from the nucleon anomalous magnetic moments and another one from the quark mass contribution to the neutron-proton mass splitting. We find a set of nine observables, which to one loop order do only depend on the remaining four dimension two couplings. These are then determined from a best fit. We also show that their values can be understood in terms of resonance exchange related to $\Delta$ excitation as well as vector and scalar meson exchange. In particular, we discuss the role of the fictitious scalar-isoscalar meson. We also investigate the chiral expansion of the two P-wave scattering volumes $P_1^-$ and $P_2^+$ as well as the isovector S-wave effective range parameter $b^-$. The one-loop calculation is in good agreement with the data. The difference $P_1^- - P_2^+$ signals chiral loop effects in the $\pi N$ P-waves. The calculated D- and F-wave threshold parameters compare well with the empirical values.Comment: 19 pp, LaTeX, enlarged and revised version, title changed, minor corrections in section 4, more detailed discussion in section 5, additional results in sec. 6 and the appendix, accepted for publication in Nucl. Phys.