Extended Vector Meson Dominance Research Articles

E+e−→Λc+Λ¯c− Cross Sections and the Λc+ Electromagnetic Form Factors within the Extended Vector Meson Dominance Model

Within the extended vector meson dominance model, we investigate the e+e−→Λc+Λ¯c− reaction and the electromagnetic form factors of the charmed baryon Λc+ . The model parameters are determined by fitting them to the cross sections of the process e+e−→Λc+Λ¯c− and the magnetic form factor |G M| of Λc+ . By considering four charmonium-like states, called ψ(4500), ψ(4660), ψ(4790), and ψ(4900), we can well describe the current data on the e+e−→Λc+Λ¯c− reaction from the reaction threshold up to 4.96 GeV. In addition to the total cross sections and |G M|, the ratio |G E/G M| and the effective form factor |G eff| for Λc+ are also calculated, and found that these calculations are consistent with the experimental data. Within the fitted model parameters, we have also estimated the charge radius of the charmed Λc+ baryon.

Electromagnetic form factors of nucleons in the extended vector meson dominance model

Extended vector meson dominance model, that allows to describe the electromagnetic form factors of nucleons obeying the asymptotic quark counting rule prescriptions and contains the minimal number of free parameters, is presented. We get a reasonable fit of form factors over experimentally available space-like region of momentum transfer and get also reasonable results in the time-like region.

Dilepton production in heavy-ion collisions with in-medium spectral functions of vector mesons

The in-medium spectral functions of \ensuremath{\rho} and \ensuremath{\omega} mesons and the broadening of nucleon resonances at finite baryon density are calculated self-consistently by combining a resonance dominance model for the vector meson production with an extended vector meson dominance model. The influence of the in-medium modifications of the vector meson properties on the dilepton spectrum in heavy-ion collisions is investigated. The dilepton spectrum is generated for the $\mathrm{C}+\mathrm{C}$ reaction at 2.0$A$ GeV and compared with recent HADES Collaboration data. The collision dynamics is then described by the T\ubingen relativistic quantum molecular dynamics transport model. We find that an iterative calculation of the vector meson spectral functions that takes into account the broadening of the nucleon resonances due to their increased in-medium decay branchings is convergent and provides a reasonable description of the experimental data in the mass region $0.45\ensuremath{\leqslant}M\ensuremath{\leqslant}0.75$ GeV. On the other side, the theoretical calculations slightly underestimate the region ${m}_{\ensuremath{\pi}}\ensuremath{\leqslant}M\ensuremath{\leqslant}0.4$ GeV. Popular in-medium scenarios such as a schematic collisional broadening and dropping vector mesons masses are discussed as well.

Influence of In-Medium Properties of Vector Mesons on Dilepton Production in Heavy Ion Collisions at SIS Energies

Dileptons represent a unique probe for nuclear matter under extreme conditions reached in heavy ion collisions. They allow the study of meson properties, like mass and decay width, at various density and temperature regimes. Up to now, in the Tubingen model for dilepton production, modification of meson properties in nuclear medium has been accounted for by allowing a density dependence of the mass (Brown-Rho scaling) together with an ad hoc dependence of the meson decay widths on the same variable. We use the extended vector meson dominance (eVMD) model to extract meson properties in nuclear matter by computing the in-medium meson spectral functions. Dilepton spectra for C+C at 1.0 and 2.0 AGeV are calculated and compared with previous results.

Dilepton production in elementary and in heavy ion reactions

We present a unified description of the vector meson and dilepton production in elementary and in heavy ion reactions. The production of vector mesons (ρ,ω,φ) is described via the excitation of nucleon resonances (R). The theoretical framework is an extended vector meson dominance model (eVMD) for resonance decays RNV with arbitrary spin which is covariant and kinematically complete. The eVMD includes thereby excited vector meson states in the transition form factors. The model has successfully been applied to ω and φ production in p+p reactions. The same model is used to describe the dilepton production in elementary reactions where corresponding data are well reproduced. However, when the model is applied to heavy ion reactions in the BEVALAC/SIS energy range the experimental dilepton spectra measured by the DLS Collaboration are significantly underestimated at small invariant masses. In view of this fact we discuss further medium effects: one is a substantial collisional broadening of the ρ and in particular of the ω meson in the vicinity of the ρ/ω peak. The second medium effect is the destruction of quantum interference in a dense medium. A decoherent dilepton emission through vector mesons decays enhances the corresponding low mass dilepton yield in heavy ion reactions and improves the agreement with existing data.

Dilepton production in elementary and in heavy ion reactions

We present a unified description of the vector meson and dilepton production in elementary and in heavy ion reactions. The production of vector mesons ( ρ, ω, φ) is described via the excitation of nucleon resonances ( R). The theoretical framework is an extended vector meson dominance model (eVMD) for resonance decays R ▪ NV with arbitrary spin which is covariant and kinematically complete. The eVMD includes thereby excited vector meson states in the transition form factors. The model has successfully been applied to ω and φ production in p+ p reactions. The same model is used to describe the dilepton production in elementary reactions where corresponding data are well reproduced. However, when the model is applied to heavy ion reactions in the BEVALAC/SIS energy range the experimental dilepton spectra measured by the DLS Collaboration are significantly underestimated at small invariant masses. In view of this fact we discuss further medium effects: one is a substantial collisional broadening of the ρ and in particular of the ω meson in the vicinity of the ρ/ ω peak. The second medium effect is the destruction of quantum interference in a dense medium. A decoherent dilepton emission through vector mesons decays enhances the corresponding low mass dilepton yield in heavy ion reactions and improves the agreement with existing data.

Dilepton production in heavy-ion collisions at intermediate energies

We present a unified description of the vector meson and dilepton production in elementary and in heavy ion reactions. The production of vector mesons ($\rho,\omega$) is described via the excitation of nuclear resonances ($R$). The theoretical framework is an extended vector meson dominance model (eVMD). The treatment of the resonance decays $R\longmapsto NV$ with arbitrary spin is covariant and kinematically complete. The eVMD includes thereby excited vector meson states in the transition form factors. This ensures correct asymptotics and provides a unified description of photonic and mesonic decays. The resonance model is successfully applied to the $\omega$ production in $p+p$ reactions. The same model is applied to the dilepton production in elementary reactions ($p+p, p+d$). Corresponding data are well reproduced. However, when the model is applied to heavy ion reactions in the BEVALAC/SIS energy range the experimental dilepton spectra measured by the DLS Collaboration are significantly underestimated at small invariant masses. As a possible solution of this problem the destruction of quantum interference in a dense medium is discussed. A decoherent emission through vector mesons decays enhances the corresponding dilepton yield in heavy ion reactions. In the vicinity of the $\rho/\omega$-peak the reproduction of the data requires further a substantial collisional broadening of the $\rho$ and in particular of the $\omega$ meson.

Electromagnetic Transition Form Factors and Dilepton Decay Rates of Nucleon Resonances

Relativistic, kinematically complete phenomenological expressions for the dilepton decay rates of nucleon resonances with arbitrary spin and parity are derived in terms of the magnetic, electric, and Coulomb transition form factors. The dilepton decay rates of the nucleon resonances with masses below 2 GeV are estimated using the extended vector meson dominance (VMD) model for the transition form factors. The model provides a unified description of the photo- and electroproduction data, γ(γ*)N→N*, the vector meson decays, N*→Nϕ(ω), and the dilepton decays, N*→Nℓ+ℓ−. The constraints on the transition form factors from the quark counting rules are taken into account. The parameters of the model are fixed by fitting the available photo- and electroproduction data and using results of the multichannel partial-wave analysis of the π N scattering. Where experimental data are not available, predictions of the non-relativistic quark models are used as an input. The vector meson coupling constants of the magnetic, electric, and Coulomb types are determined. The dilepton widths and the dilepton spectra from decays of nucleon resonances with masses below 2 GeV are calculated.

Extended vector meson dominance model for the baryon octet electromagnetic form factors.

An unresolved issue in the present understanding of nucleon structure is the effect of hidden strangeness on electromagnetic observables such as ${\mathit{G}}_{\mathit{E}}^{\mathit{n}}$(${\mathit{q}}^{2}$). Previously, we have shown that ${\mathit{G}}_{\mathit{E}}^{\mathit{n}}$(${\mathit{q}}^{2}$) is sensitive to small \ensuremath{\varphi}NN couplings. A complementary approach for understanding effects due to strangeness content and the Okubo-Zweig-Iizuka (OZI) rule is to investigate the electromagnetic structure of hyperons. We apply Sakurai's universality limit of the SU(3${)}_{\mathit{F}}$ symmetry relations and a prescription based on the OZI rule to calculate the electromagnetic form factors of the baryon octet states (p,n,\ensuremath{\Lambda},${\mathrm{\ensuremath{\Sigma}}}^{+}$,${\mathrm{\ensuremath{\Sigma}}}^{0}$,${\mathrm{\ensuremath{\Sigma}}}^{\mathrm{\ensuremath{-}}}$,${\mathrm{\ensuremath{\Xi}}}^{0}$,${\mathrm{\ensuremath{\Xi}}}^{\mathrm{\ensuremath{-}}}$) within the framework of an extended vector meson dominance model. To provide additional motivation for experimental investigation, we discuss the possibility of extracting the ratio ${\mathit{G}}_{\mathit{M}}^{\mathrm{\ensuremath{\Lambda}}}$(${\mathit{q}}^{2}$)/${\mathit{G}}_{\mathit{M}}^{\mathrm{\ensuremath{\Sigma}}\mathrm{\ensuremath{\Lambda}}}$(${\mathit{q}}^{2}$) from the \ensuremath{\Lambda}/\ensuremath{\Sigma} polarization ratio in kaon electroproduction experiments. \textcopyright{} 1996 The American Physical Society.

U-spin considerations to guess the unknown time-like neutron form factors

The neutron time-like form factors have never been measured. Perturbative QCD, extended vector meson dominance and other models give very different predictions. The available data on Λ form factor and\({J \mathord{\left/ {\vphantom {J \Psi }} \right. \kern-\nulldelimiterspace} \Psi } \to B\bar B\) allow an estimation of the neutron time-like form factors according to theU-spin symmetry. At the threshold a large neutron magnetic form factor comes out, likely falling steeper than the proton one.

Experimental test of the PCAC-hypothesis in charged current neutrino and antineutrino interactions on protons

Data obtained with the bubble chamber BEBC at CERN are used for the first significant test of Adler's prediction for the neutrino and antineutrino-proton scattering cross sections at vanishing four-momentum transfer squaredQ 2. An Extended Vector Meson Dominance Model (EVDM) is applied to extrapolate Adler's prediction to experimentally accessible values ofQ 2. The data show good agreement with Adler's prediction forQ 2→0 thus confirming the PCAC hypothesis in the kinematical region of high leptonic energy transfer ν>2 GeV. The good agreement of the data with the theoretical predictions also at higherQ 2, where the EVDM terms are dominant, also supports this model. However, an EVDM calculation without PCAC is clearly ruled out by the data.

WKB solutions to relativistic quarkonium model with the potentialV(r)=μ 2 r+βlnr/r 0

Weapply WKB techniques to a Schrodinger equation modified for relativistic kinematics. We show that a potential consisting of linear and logarithmic terms accounts for different aspects of quarkonium phenomenology. Using an Extended Vector Meson Dominance model we calculateR e + e− and we derive the Quark Parton Model result.

Electromagnetic form factor of the pion: Vector mesons or quarks?

An updated analysis of the pion form-factor data in the spacelike region up to ${q}^{2}=\ensuremath{-}10$ Ge${\mathrm{V}}^{2}$ is performed using the extended vector-meson-dominance (EVMD) model, with parameters fixed by the dual model, as well as asymptotic quantum-chromodynamics (QCD) expressions. EVMD provides a good fit to the data and at the same time it predicts a deviation from universality in very good agreement with experiment. Perturbative QCD results to first order in ${\ensuremath{\alpha}}_{s}$ do very poorly while the nonperturbative QCD prediction of Pagels and Stokar gives an excellent fit to the data in the entire region $1\ensuremath{\le}\ensuremath{-}{q}^{2}\ensuremath{\le}10$ Ge${\mathrm{V}}^{2}$.

Vector dominance, q2 dependence and radial excitations

An extended vector meson dominance model which includes both radial excitations of the vector mesons, and q 2 dependence of the photon-vector meson coupling is applied to radiative decays and photoproduction of the vector mesons. The parameters estimated from Ψ radiative decay, define a phenomenological prescription in which the radiative excitations play a minor role compared with the ground state vector mesons. Various predictions are made for cross sections and decay widths.

Radiative decays of mesons and an extended vector-meson dominance model

A model describing the radiative P→γγ, V→Pγ, P→PPγ and the strong V→PPP decays is proposed. The amplitudes for processes with a three-particle final state are approximated by finite dispersion relations and the coupling constants are related by the quark model. Predictions derived from usual VMD are only in qualitative agreement with experiments. Better agreement is achieved by extending VMD with the inclusion of further vector mesons.