Structural considerations for elastic magnetic electron-nucleus scattering

To explain the form factors obtained by magnetic electron scattering on ¹⁷O it is clear that core polarization has to be taken into account. We discuss how this can be done by means of the Hartree-Fock theory. We note that projection from an intrinsic state of definite K gives an incorrect answer for weakly deformed systems such as ¹⁷O and we present an alternative procedure for extracting the moments. Using a purely velocity dependent Skyrme interaction (only t₀ and t₁ are non-zero) a Hartree-Fock calculation is carried out for the seventeen-nucleon system. The calculation leads to a suppression of the M3 form factor. It is emphasized that the suppression arises from the fact that not only is the ¹⁶O core deformed, but that spin-up particles have a different deformation than spin-down particles. This arises from the spin dependence of the interaction and from the Pauli principle. A peculiar feature with the interaction that is used is that the M5 form factor gets enhanced. This is attributed to the repulsive velocity dependent term in the interaction. The origin of the velocity dependence is discussed. The effect of the velocity dependence on the magnetic moment operator is also considered.     

Pion decays and the pion electromagnetic form factor in a σ-model with quarks

We study in the one-loop approximation the decays of the pion and the pion electromagnetic form factor at low q² in the framework of a σ-model with quarks. The theory has a free parameters the quark and σ masses and the results are very insensitive to their values. We get good approximation for all processes considered except for the ratio of the axial to the vector form factor that appear in the electronic radiative decay of the charged pion. The reason is probably that our model includes the pion-pion interaction only in the isoscalar s-wave at the one-loop level.     

The ωDD vertex in a sum rule approach

The study of charmonium dissociation in heavy ion collisions is generally performed in the framework of effective Lagrangians with meson exchange. Some studies are also developed with the intention of calculate form factors and coupling constants related with charmed and light mesons. These quantities are important in the evaluation of charmonium cross sections. In this Letter we present a calculation of the ωDD vertex that is a possible interaction vertex in some meson-exchange models spread in the literature. We used the standard method of QCD sum rules in order to obtain the vertex form factor as a function of the transferred momentum. Our results are compatible with the value of this vertex form factor (at zero momentum transfer) obtained in the vector-meson dominance model.     

Core polarization effects on the Coulomb form factor

Coulomb form factors for the transitions between single-particle (or -hole) states with LS closed shells are investigated. Core polarization effects are calculated by using microscopic models. Information on the effective interaction which cannot be provided by γ-transition data alone is obtained from the momentum transfer dependence of the form factor. An effective interaction is required not to be repulsive in triplet-odd two-particle states in order to explain the experimental data on ¹⁵N. The bare G-matrix gives a reasonable fit to the data while the phenomenological interaction with Rosenfeld mixture does not. It is also shown that the core polarization effect for the Coulomb form factors with high multipolarity is sensitive to the mixing character of the effective interaction.     

Meson properties in an extended non-local NJL model

We consider a non-local version of the NJL model, based on a separable quark-quark interaction. The interaction is extended to include terms that bind vector and axial-vector mesons. The non-locality means that no further regulator is required. Moreover the model is able to confine the quarks by generating a quark propagator without poles at real energies. Working in the ladder approximation, we calculate amplitudes in Euclidean space and discuss features of their continuation to Minkowski energies. Conserved currents are constructed and we demonstrate their consistency with various Ward identities. Various meson masses are calculated, along with their strong and electromagnetic decay amplitudes. We also calculate the electromagnetic form factor of the pion, as well as form factors associated with the processes γγ^* → π⁰ and ω → π⁰γ^*. The results are found to lead to a satisfactory phenomenology and lend some dynamical support to the idea of vector-meson dominance.     

Isoscalar amplitude dominance in e + e − annihilation to N N̄ pair close to the threshold

We use the Paris nucleon-antinucleon optical potential for explanation of experimental data in the process e ⁺ e ^? → p p? near threshold. It turns out that final-state interaction due to Paris optical potential allows us to reproduce available experimental data. It follows from our consideration that the isoscalar form factor is much larger than the isovector one.     

On an ideal gas related to the law of corresponding states

By an “ideal gas” we mean a gas which formally does not depend on the form of the interaction between the particles. We construct the thermodynamics (the equation of state) of such a gas, and this thermodynamics depends on three parameters corresponding to the Zeno-line and to the value of the compressibility factor Z at the critical point (a three-parameter family of three-dimensional Lagrangian manifolds).     

CP violation and top quark decays

We investigate how to testCP invariance in the weak decay of top quarks. A prediction ofCP violation from the Standard Model is made by an explicit calculation. The effect is very small as expected. We then use a form factor approach to parametrize possible new interactions ofCP violation. The differences between the form factor approach and an effective Lagrangian approach are discussed. The sensitivity of ourCP-odd observables to the form factors is estimated.     

A Combined Study of the Pion’s Static Properties and form Factors

We study consistently the pion’s static observables and the elastic and γ* γ → π ⁰ transition form factors within a light-front model. Consistency requires that all calculations are performed within a given model with the same and single adjusted length or mass-scale parameter of the associated pion bound-state wave function. Our results agree well with all extent data including recent Belle data on the γ* γ → π ⁰ form factor at large q ², yet the BaBar data on this transition form factor resists a sensible comparison. We relax the initial constraint on the bound-state wave function and show the BaBar data can partially be accommodated. This, however, comes at the cost of a hard elastic form factor not in agreement with experiment. Moreover, the pion charge radius is about 40 % smaller than its experimentally determined value. It is argued that a decreasing charge radius produces an ever harder form factor with a bound-state amplitude difficultly reconcilable with soft QCD. We also discuss why vector dominance type models for the photon-quark vertex, based on analyticity and crossing symmetry, are unlikely to reproduce the litigious transition form factor data.     

Pion-nucleon vertex form factor in a chiral soliton model with vector mesons

We investigate the pion-nucleon vertex from factor in an extended Skyrme soliton model with explicit inclusion of ϱ and ω mesons. We find a pion-nucleon coupling constant quite close to the empirical value and a cutoff mass ⋮ ⋍ 0.9 MeV in a monopole approximation to the πNN form factor at low q². This value of ⋮ is considerably larger than the prediction of the original Skyrme model (⋮⋍0.6 GeV). We also investigate the ϱ-meson-nucleon form factor and present results for the NN isovector tensor potential in comparison with empirical data.     

The deuteron wave function at short range and the triton

It is shown that if one assumes something between zero and the prediction of the scaling model with dipole fit for the neutron electric form factor, then a variety of short-range behaviour for the deuteron wave function is consistent with existing experimental data on the deuteron electric form factor. This still relatively wide latitude for the inner deuteron wave function, consistent with existing experimental electromagnetic data, gives rise to an off-shell variation of approximately 1.2 MeV in the triton binding energy with a fixed ¹S₀ interaction and a P_D varying from 4.5 to 6.5 %. Interactions with greater densities of matter at short range bind the triton more strongly and closer to the experimental value. An off-shell variation of 0.7 MeV is obtained with a fixed p_d and singlet interaction. However, a single measurement of the deuteron tensor polarization at about q² = 20 fm^?2 would severely restrict this variation.     

Light-cone structure of vertex functions for composite fields

Vertex functions for composite fields are defined in a model field theory both on and off mass shell. Light-cone dominance at large momentum transfer is shown to hold, by the compositeness assumption, for the off-shell vertex function. On the other hand, it is in general untrue that the elastic form factor probes light-like distances between the constituents inside the nucleon. The relevant light-cone singularity (in the relative space-time separation x) is less important in this case than the large x₀ behaviour of the wave function at fixed x². It is found however that, under some conditions, the light-cone singularity determines the large x₀ behaviour of the wave function, and therefore the large q² behaviour of the form factor. For composite particles described by a Bethe-Salpeter equation, this result is equivalent to the known fact that at large q² the form factor depends on the binding interaction at small distances. A relation similar to that of Drell-Yan-West is finally established between the asymptotic behaviour of the elastic form factor and the threshold behaviour of the absorptive part of the vertex scaling function.     

Relationship Between the Proton Charge Form Factor and the Quark-Quark Interaction

We demonstrate how the parameterized proton elastic charge form factor may be used to determine the quark-quark interaction assuming a constituent-quark model of the nucleon and non-relativistic kinematics. The relation between the hypercentral component of the three-quark bound-state wave function and the charge density is obtained by using an Abel-type integral equation. To a high degree of accuracy, this component fixes the hypercentral potential, from which the quark-quark interaction may be determined exactly via another Abel transform. We find that the presence of the meson cloud makes it impossible to determine the long-range behaviour of the quark-quark potential. However, our results indicate that the Gari-Krüpelmann and dipole parametrizations of the form factor both generate potentials that are singular at short distances, indicating a degree of sensitivity of the form factor to the quark-quark interaction at small distances. We also generalize our technique to systems of arbitrarily large number of particles. Received June 21, 1994; revised October 3, 1994; accepted for publication November 24, 1994     

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.     

G-matrix effective interaction with the paris potential

An effective interaction is derived by fitting the oscillator matrix elements of the sum of the OBEP functions to the G-matrix elements derived from the Paris nucleon-nucleon interaction. The functional form, the mass dependence and the ambiguities of the effective interaction are discussed. For the application of the present effective interaction, we study the mass dependence of the G-matrices and estimate the Landau-Migdal parameter g′. We have obtained reasonable results for these cases.     

Double-Λ hypernuclei and the nuclear medium effective ΛΛ interaction

We fit the ¹S₀ ΛΛ interaction in the nuclear medium to the masses of the experimentally known double-Λ hypernuclei: _ΛΛ⁶He, _ΛΛ¹⁰Be and _ΛΛ¹³B. We derive this effective interaction from OBE Jülich ΛΛ-type potentials and using both Hartree-Fock and variational approaches. We find that the inclusion of ΛΛ correlations in the variational scheme leads to significant differences and a better understanding of the dynamical features of the system. We investigate the sensitivity of the binding energies and the mesonic decay widths of the above double-Λ hypernuclei to the ωΛΛ coupling and the form factor at the σΛΛ vertex. We also use this effective interaction to predict binding energies and pionic decay widths of heavier double-Λ hypernuclei, not discovered yet. Finally, we discard the existence of ¹S₀ ΛΛ bound states provided the ΛΛ-ΞN coupling can be neglected.     

Vertex form factors of the rho-meson nucleon interaction at low momentum transfer

We calculated the one pion loop contributions to the form factors of the rho-nucleon interaction within a field theoretical model using effective Lagrangians. The results show that the tensor interaction is likely to be described well within such an approach. We obtain a magnetic coupling in the range of 3.5–7.1 atQ ²=0, depending on the choice of pion-nucleon form factor and a dipole form for theQ ² dependence tensor form factor with a scale of roughly 900 MeV for space-like momentum transfer. The results agree with the findings of the recent analysis of thep-N interaction and support a small scale parameter (0.5 GeV) for the pion-nucleon form factor.This work is supported by BMFT (06BO7027) and COSY-KFA Jülich (41140512)     

Polaronic aspects of spin polarized hydrogen of films of liquid He

We have studied the interaction of a collection of 2-D spin polarized hydrogen (H↓) atoms with the ripplon modes of a superfluid liquid helium film. From the deformation correction to the effective H↓He potential we derive the H↓-ripplon interaction Hamiltonian. We find that a weak coupling polaron state can form and we make specific predictions for the change in H↓ energy and effective mass which arise as a consequence. We also make a direct comparison (in coordinate space) of the ripplon mediated ↓-H↓ interaction, V_R(r), with the usual H↓H↓ interaction V(r).