Veneziano model for the ρ contribution to the nucleon form factors

The Igi-Veneziano model forπN scattering, combined with the experimental pion form factor is used to study theρ contribution to the isovector electromagnetic nucleon form factors.     

Two-photon-exchange contribution to proton form factors in both space-like and time-like regions

The two two-photon exchange corrections to the unpolarized cross section and polarized observable PT,PL in elastic ep scattering are discussed in a simple hadronic model.Comparing with previous results,theΔ contribution are re-analysed.And the similar corrections in e+e-→ p(p-)are also discussed.     

Two-photon-exchange contribution to proton form factors in both space-like and time-like regions

The two two-photon exchange corrections to the unpolarized cross section and polarized observable PT, PL in elastic ep scattering are discussed in a simple hadronic model. Comparing with previous results, the △ contribution are re-analysed. And the similar corrections in e^＋e^- → pp are also discussed.     

Twist-3 contribution to the pion electromagnetic form factor

Non-leading contribution to the pion electromagnetic form factor which comes from the pion twist-3 wave function is analyzed in the modified hard scattering approach (MHSA) proposed by Li and Sterman. This contribution is enhanced significantly due to bound state effect (the twist-3 wave function is independent of the fractional momentum carried by the parton and has a large factor with being the pion meson mass and being the mean u- and d-quark masses). Consequently, although it is suppressed by the factor , the twist-3 contribution is comparable with and even larger than the leading twist (twist-2) contribution at intermediate energy region of being . Received: 23 March 1998 / Published online: 3 November 1999     

τ-d overlap function and the τ charge form factor

An Eckart-form parameterization of the τ-d overlap function is shown to give excellent fits to the τ charge form factor and radius. The function is constrained to have the correct asymptotic behavior and succesfully reproduces the diffraction dip at 11.6 fm^?2.     

Logarithmic corrections and soft photon phenomenology in the multipole model of the nucleon form factors

We analyzed the presently available experimental data on nucleon electromagnetic form factors within a multipole model based on dispersion relations. A good fit of the data is achieved by considering the coefficients of the multipole expansions as logarithmic functions of the momentum transfer squared. The superconvergence relations, applied to this coefficients, makes the model agree with unitary constraints and pQCD asymptotics for the Dirac and Pauli form factors. The soft photon emission is proposed as a mechanism responsible for the difference between the Rosenbluth, polarization and beam-target asymmetry data. It is shown that the experimentally measured cross-sections depend not only on the Dirac and Pauli form factors, but also on the average number of the photons emitted. For the proton this number is shown to be different for different types of experimental measurements and then estimated phenomenologically. For the neutron the same mechanism predicts that the data form different types of experiments must coincide with high accuracy. A joint fit of all the experimental data reproduce the Q²-dependence with the accuracy χ²/dof = 0.86 . Predictions of the model, that 1) the ratios of the proton form factors G _E/G _M are different for Rosenbluth, polarization and beam-target asymmetry experiments and 2) similar ratios are nearly the same for neutron, can be used for experimental verification of the model.     

Center-of-mass corrections to overlap factors for two-nucleon transfer

The following simple recipe is presented for making c.m. corrections to the overlap factors involved in two-nucleon transfer: Expand the shell-model overlap in products of harmonic oscillator wave functions of the relative and c.m. coordinates of the two nucleons. Change the oscillator ν-value to $\text{Aν}\text{(A + 2)}$ in the c.m. functions, and multiply each term in the sum by [$\text{(A + 2)}\text{A}\text{]}\text{1}\text{2}{}^{\mathrm{N}}$, where N is the energy quantum number associated with the c.m. state of that term. The recipe depends on the value of ν used in the expansion. The proper choice of ν is discussed, and numerical examples are presented which indicate the magnitude of the correction. The zero-range form factor for two-neutron stripping on ¹⁶O is increased by a factor of about two, or more, in the surface region, resulting in a factor of about five in the cross section. The correction is greater at larger distances, implying that heavy-ion reactions will be effected more than light ion reactions.     

Nonperturbative contribution to the quark form factor within the instanton model

The nonperturbative effects in the quark form factor are considered in the Wilson loop formalism, within the framework of the instanton liquid model. For the integration path corresponding to this form factor, the explicit expression for the vacuum expectation value of the Wilson operator is found to the leading order. It is shown that the instantons produce the power-like corrections to the perturbative result, which are comparable in magnitude with the perturbative part at the scale of order of the inverse average instanton size. It is demonstrated that the instanton contributions to the quark form factor are exponentiated to high orders in the small instanton density parameter.Received: 30 September 2002, Published online: 22 October 2003PACS: 12.38.-t Quantum chromodynamics - 11.15.Tk Other nonperturbative techniques     

Factorization of soft and collinear divergences in non-equilibrium quantum field theory

Proof of factorization of soft and collinear divergences in non-equilibrium QCD may be necessary to study hadronic signatures of quark-gluon plasma at RHIC and LHC. In this paper we prove factorization of soft and collinear divergences in non-equilibrium QED by using Schwinger-Keldysh closed-time path integral formalism in the background field method in pure gauge.     

Center-of-mass corrections to overlap factors in the theory of nucleon transfer reactions

Form factors for direct nuclear reactions involving the transfer of one or more nucleons are examined for the consequences of the unphysical c.m. motion inherent in the shell model. The standard procedure employed in one-nucleon transfer is shown to be properly translation invariant. In the case of the transfer of two nucleons, it is pointed out that c.m. corrections are needed but that no tractable procedure exists for making them. Recent proposals of recoil corrections in transfer reactions are critically examined.     

Dependence of the scattering amplitude on the form of the overlap function

On the basis of the unitarity condition we examine the behavior of the elastic scattering amplitude as a function of the form of the overlap function and the phase shift.     

Strangeness contributions to nucleon form factors

We review a recent theoretical determination of the strange quark content of the electromagnetic form factors of the nucleon. These are compared with a global analysis of current experimental measurements in parity-violating electron scattering.     

An algebraic approach to form factors

An associative *-algebra is introduced (containing a TTR-algebra as a subalgebra) that implements the form factor axioms, and hence indirectly the Wightman axioms, in the following sense: Each T-invariant linear functional over the algebra automatically satisfies all the form factor axioms. It is argued that this answers the question (posed in the functional Bethe ansatz) how to select the dynamically correct representations of the TTR-algebra. Applied to the case of integrable QFTs with diagonal factorized scattering theory a universal formula for the eigenvalues of the conserved charges emerges.     

A contribution to the global formulation of the higher-order Poincaré-Cartan form

The global formulation of the higher-order Poincaré-Cartan form for Lagrangian field theories in the calculus of variations is re-examined in terms of the theory of lifts of tensor fields on a manifold to its higher-order prolongated jet bundles.Partially supported by CNPq, Brazil, Proc. MA 30.1115/79.     

Bag-model form factors and Hartree-Fock approximation to the σωπ model

We evaluate form factors for the scattering ofσ, ω andπ mesons from nucleons described by the MIT bag model. These form factors are then written covariantly and used for nuclear matter in the Hartree-Fock approximation to theσω model of quantum hadrodynamics with pions. For fixed parameters (meson couplings and masses) the inclusion of these form factors results in more binding at a higher saturation density. After refitting the parameters to reproduce the empirical saturation properties, the result is very close to that without form factors.     

Isovector form factors and the rho-prime

A simple but successful pole model for the low q² behaviour of the pion, ΔNγ and nucleon isovector form factors is given, using the Dirac form factors in the latter case. The rho-prime required decouples completely from charge form factors, so that its only two body decay mode is ωπ.