Taming the pion cloud of the nucleon

We present a light-front determination of the pionic contribution to the nucleon self-energy, Σ(π), to second order in pion-baryon coupling constants that allows the pion-nucleon vertex function to be treated in a model-independent manner constrained by experiment. The pion mass μ dependence of Σ(π) is consistent with chiral perturbation theory results for small values of μ and is also linearly dependent on μ for larger values, in accord with the results of lattice QCD calculations. The derivative of Σ(π) with respect to μ(2) yields the dominant contribution to the pion content, which is consistent with the d[over ˉ]-u[over ˉ] difference observed experimentally in the violation of the Gottfried sum rule.     

A coherent interpretation of the form factors of the nucleon in terms of a pion cloud and constituent quarks

The recent measurements of the electric form factor of the neutron suggest that its shape may be interpreted as a smooth broad distribution with a bump at 0.3 (GeV/c)² superimposed. As a consequence the corresponding charge distribution in the Breit frame shows a negative charge extending as far out as 2 fm. It is natural to identify this charge with the pion cloud. This realisation is then used to reanalyse all old and new data of the electric and magnetic from factors of the proton and the neutron by a phenomenological fit and by a fit based on the constituent quark model. It is shown that it is possible to fit all form factors coherently with both ansaetzen and that they all show the signal of the pion cloud.Received: 6 March 2003, Published online: 17 June 2003PACS: 14.20.Dh Protons and neutrons - 13.40.Gp Electromagnetic form factors - 21.10.Ft Charge distribution     

The pion nucleon coupling constant

The latest elastic scattering data are re-analysed to determine the coupling constant g_c of the charged pion, using the dispersion relation for the invariant amplitude B^{(+ )}. Depending on the choice of data-base, values to 13.65 are obtained with errors of . The mass difference between charge states of is MeV, close to twice the mass difference between neutron and proton. The difference in widths on resonance is MeV. One may account for a width difference of 4.5 MeV from phase space for decays and the extra channel .Received: 21 January 2004, Published online: 3 March 2004     

Spin structure of the pion in a light-cone representation

The spin structure of the pion is discussed by transforming the wave function for the pion in the naive quark model into a light-cone representation. It is shown that there are higher helicity (λ ₁+λ ₂=±1) states in the full light-cone wave function for the pion besides the ordinary helicity (λ ₁+λ ₂=0) component wave functions as a consequence from the Melosh rotation relating spin states in light-front dynamics and those in instantform dynamics. Some low energy properties of the pion, such as the electromagnetic form factor, the charged mean square radius, and the weak decay constant, could be interrelated in this representation with reasonable parameters.     

pi-Mesonic decay of hypernuclei and pion wavefunction

The pi-mesonic decay of hypernuclei is studied by using the pion distorted waves which are the solutions of the Klein-Gordon equation with the optical potential. The distortions of the pion waves give rise to significant enhancement of theπ-decay rate compared with the pion free-wave case. Theπ-decay rates are very sensitive to the behavior of the pion wave deep inside the nucleus and therefore to the chosen pion optical potentials. There is a tendency that the enhancement is larger for theπ ^?-decay than for theπ ⁰-decay due to the combined effects of the Coulomb and optical potentials.     

Pions in the light-cone nucleon and electromagnetic form factors

The light-cone Fock-state decomposition of the nucleon involves pion-quark core (π′N′ andπ′Δ′) components. The size of the quark core is at present of special importance for the analysis of the onset of colour transparency in the A(e,e′p) reactions. With the light-coneNπ′N′ andNπ′Δ′ vertex functions deduced from experimental data on high-energy proton fragmentation into nucleons and isobars, we evaluate the effect of the pionic cloud on the electromagnetic form factors of nucleons. We find the quark core radius to be about 10% smaller than the nucleon's radius. Analysis of the renormalization of the total nucleon-nucleon cross section for the presence of the pionic cloud suggests a similar core radius.     

Direct measurement of the pion valence-quark momentum distribution, the pion light-cone wave function squared

We present the first direct measurements of the pion valence-quark momentum distribution which is related to the square of the pion light-cone wave function. The measurements were carried out using data on diffractive dissociation of 500 GeV/c pi(-) into dijets from a platinum target at Fermilab experiment E791. The results show that the /q&q> light-cone asymptotic wave function describes the data well for Q2 approximately 10 (GeV/c)(2) or more. We also measured the transverse momentum distribution of the diffractive dijets.     

Electromagnetic radii of nucleon and pion

A new determination of the spectral functions of nucleon form factors according to the Frazer-Fulco method leads to reliable results up to t ?1GeV² and to improved values for the radii.     

Parity violating pion electroproduction off the nucleon

Parity violating (PV) contributions due to interference betweenγ and Z⁰ exchange are calculated for pion electroproduction off the nucleon. A phenomenological model with effective Lagrangians is used to determine the resulting asymmetry for the energy region between threshold andΔ(1232) resonance. TheΔ resonance is treated as a Rarita-Schwinger field with phenomenological NΔ transition currents. The background contributions are given by the usual Born terms using the pseudovectorπN Lagrangian. Numerical results for the asymmetry are presented.     

Space- and time-like electromagnetic pion form factors in light-cone pQCD

We present a combined analysis of the space- and time-like electromagnetic pion form factors in light-cone perturbative QCD with transverse momentum dependence and Sudakov suppression. Including the genuine non-perturbative “soft” QCD and the power suppressed twist-3 corrections to the standard twist-2 perturbative QCD result, the experimental form factor data available at moderate energies/momentum transfers can be explained reasonably well. To this end, the bulk of the existing discrepancy between the space- and time-like experimental data is ostensibly reconciled.     

The applicability of perturbative QCD to the pion form factor and the pionic wavefunction

We reanalyse the pionic form factor by using perturbative QCD theory and contributions from endpoint regions. We find that the perturbative QCD can be applied to the pionic form factor asQ ²>4 GeV² and they become unreliable asQ ²≦4 GeV². Therefore the applicability of perturbative QCD to the form factor is questionable only asQ ²≦4 GeV².     

Non-locality effects on the pion nucleon form factor

We investigate effects of the non-locality in the elementary pion quark coupling on theπNN form factor in the framework of the chiral bag model. We show that this nonlocality leads to shorter ranged form factors thereby reducing the discrepancy between large bag-model confinement radii and empirically found small-ranged cut-off parameters.     

Magnetic moments of the nucleon octet in a relativistic light-cone quark model

The magnetic moments of the baryon octet are calculated in a relativistic constituent quark model formulated in the light-cone Fock approach. Invoking a natural idea of strangeness-dependent hadron size we find very good agreement (to an accuracy of 1σ) for the recent precision hyperon magnetic-moment data and reveal 10–15% discrepancy for the Σ⁻ and nucleon. It suggests pions as a missing ingredient in the baryon magnetic moment calculations.     

N¯N annihilation into two mesons — The effect of the pion radial wavefunction

N¯ N (S-wave) annihilation into twos-wave mesons is treated as a two step process. The first stage involves a singleq¯ q ³P₀-vertex to give a two meson state (ones-wave and onep-wave meson in a relatives-state). The second stage is a meson-meson scattering mechanism producing the final twos-wave mesons in a relativep-wave. The new feature in this work is to study the effect of using a pion wavefunction whose radial form is different to that of the others- andp-wave mesons. This modification over earlier work results in a better understanding of the experimental branching ratios.     

Effective Lagrangian Approach to pion photoproduction from the nucleon

We present a pion photoproduction model on the free nucleon based on an Effective Lagrangian Approach (ELA) which includes the nucleon resonances (Δ(1232), N(1440), N(1520), N(1535), Δ(1620), N(1650), and Δ(1700)), in addition to Born and vector meson exchange terms. The model incorporates a new theoretical treatment of spin-3/2 resonances, first introduced by Pascalutsa, avoiding pathologies present in previous models. Other main features of the model are chiral symmetry, gauge invariance, and crossing symmetry. We use the model combined with modern optimization techniques to assess the parameters of the nucleon resonances on the basis of world data on electromagnetic multipoles. We present results for electromagnetic multipoles, differential cross-sections, asymmetries, and total cross-sections for all one pion photoproduction processes on free nucleons. We find overall agreement with data from threshold up to 1 GeV in laboratory frame.     

Pauli principle and renormalization of the pion nucleon interaction in nuclei

We show that the exclusion principle provides an alternative mechanism to the Lorentz-Lorenz effect for the renormalization of the pion-nucleon coupling constant in nuclei. This mechanism proves to be less sensitive to the range of the π - N forces. The implications for pion-nucleus scattering are discussed.     

An application of the quasipotential method to pion nucleon scattering

The quasipotential method developed in Dubna Laboratory was applied to pion-nucleon scattering processes up to the energies 500 MeV, for obtaining the resonance behaviour of te P₃₃-phase shift. Although numerical calculations have not yet been completed, the existence of a resonance seems very probable.