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Egle Tomasi-Gustafsson

Publications and source records attributed to Egle Tomasi-Gustafsson.

At least 55 records · Page 3Linked to original sources
arXiv2002

Polarization phenomena for heavy meson production in NN-collisions near threshold

Polarization phenomena are essential for the understanding of elementary processes, and are typically large in the threshold region. The selection rules with respect to P-parity, angular momentum, isotopic spin and the Pauli principle allow to parametrize in a model independent way, the spin structure of the matrix elements for the near-threshold heavy meson production in NN-collisions in terms of a small number of partial amplitudes. Such parametrization is very powerful for the analysis of polarization phenomena and possible $t-$exchange mechanisms.

nucl-th↗
arXiv2002

Determination of the $g_{ρσγ}$ coupling constant through the process $γ+N\to N+V$ with circularly polarized photons

We suggest a method to determine the product of the coupling constants $g_{ρσγ}g_ {σNN}$, in absolute value and relative sign, through the $t$-dependence of the two possible T-even asymmetries for the photoproduction of vector mesons, $γ+p\to p+V^0$, $V^0=ρ$ or $ω$, in the collision of circularly polarized photons with a polarized proton target. In the framework of a $π+σ$-model, these asymmetries are very sensitive to the relative role of $π$- and $σ$-exchanges, in the near threshold region.

hep-ph↗
arXiv2002

P-parity of charmed particles from associative photoproduction of D and D^*-mesons

We suggest to measure the triple polarization correlations in the exclusive associative charm particle photoproduction, vector gamma + vector p --> vector Lambda_c^{++} bar{D^0} with linearly polarized photons, as a method to determine the P-parity of the charmed D-meson. The dependence of these correlations on the parity P(N Lambda_c D) can be predicted in model independent way. The t-dependence of the differential cross section for vector meson photoproduction, gamma + p --> Lambda_c^{++} bar{D^{*0}}, in a model based on D-exchange, is also sensitive to P(N Lambda_c D).

hep-ph↗
arXiv2002

Associative charm photoproduction with circularly polarized photons

We calculate the asymmetries for the processes $\vecγ+\vec p\to Λ_c^+ (Σ_c^+)+\bar{D^0}$ and $\vecγ+\vec p\to Λ_c^+ (Σ_c^+)+\bar{D^*}$, induced by circularly polarized photons on a polarized proton target in the framework of an effective Lagrangian model. We found large and positive values for the asymmetry induced by the $z-$component of the proton polarization, in a wide energy region, from threshold up to $E_γ=70$ GeV. In the case of pseudoscalar meson photoproduction, the predictions are qualitatively model-independent. For $D^*$-photoproduction, there is a strong dependence of the asymmetries on $NΛ_c D(D^*)$-coupling constants.

hep-ph↗
arXiv2002

Considerations on rescattering effects for threshold photo- and electro-production of $π^0$ on deuteron

We show that for the S-state $π^0$-production in processes $γ+d\to d+π^0$ and $e^-+d\to e^-+d+π^0$ the rescattering effects due to the transition: $ γ+d\to p+p+π^-$ (or $n+n+π^+)\to d+π^0$ are cancelled out due to the Pauli principle. The large values for these effects predicted in the past may result from the fact that the spin structure of the corresponding matrix element and the necessary antisymmetrization induced by the presence of identical protons (or neutrons) in the intermediate state was not taken into account accurately. One of the important consequences of these considerations is that $π^0$ photo- and electro-production on deuteron near threshold can bring direct information about elementary neutron amplitudes.

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arXiv2002

The reaction $Δ+N\to N+N+ϕ$ in ion-ion collisions

We study the threshold $ϕ$-meson production in the process $Δ+N\to N+N+ϕ$, which appears as a possible important mechanism in high energy nuclei-nuclei collisions. The isotopic invariance of the strong interaction and the selection rules due to P-parity and total angular momentum result in a general and model independent parametrization of the spin structure of the matrix element in terms of three partial amplitudes. In the framework of one-pion exchange model these amplitudes can be derived in terms of the two threshold partial amplitudes for the process $π+N\to N+ϕ$. We predict the ratio of cross sections for $ϕ-$meson production in $pp$- and $ΔN$-collisions and the polarization properties of the $ϕ$-meson, in $Δ+N\to N+N+ϕ$, as a function of a single parameter, which characterizes the relative role of transversal and longitudinal $ϕ$-meson polarizations in the process $π+N\to N+ϕ$.

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arXiv2002

Threshold $J/ψ-$ production in nucleon-nucleon collisions

We analyze $J/ψ-$ production in nucleon-nucleon collisions near threshold in the framework of a general model independent formalism, which can be applied to any reaction $N+N\to N+N+V^0$, where $V^0=ω$, $ϕ$, or $J/ψ$. Such reactions show large isotopic effects: a large difference for $pp$- and $pn$-collisions, which is due to the different spin structure of the corresponding matrix elements. The analysis of the spin structure and of the polarization observables is based on symmetry properties of the strong interaction. Using existing experimental data on the different decays of $J/ψ-$meson, we suggest a model for $N+N\to N+N+J/ψ$, based on $t-$channel $η+π$-exchanges. We predict polarization phenomena for the $n+p\to n+p+J/ψ$-reaction and the ratio of cross sections for $np$ and $pp$-collisions. For the processes $η(π)+N\to N+J/ψ$ we apply two different approaches: vector meson exchange and local four-particle interaction. In both cases we find larger $J/ψ$-production in $np$-collisions, with respect to $pp$-collisions.

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arXiv2002

Polarization effects in elastic electron-proton scattering

In these notes we present a detailed and complete scheme for the calculation of polarization phenomena in elastic electron-nucleon scattering. The longitudinal and transversal components of the polarization for the scattered nucleon, induced by the scattering of longitudinally polarized electrons and the asymmetries for the collision of polarized electrons with a polarized nucleon target are included. The calculations are done in the framework of the one-photon mechanism, in the Breit system, which is the most convenient for the analysis of polarization effects. We briefly discuss the region of applicability of such model and other mechanisms (radiative and electroweak corrections) which may come into this idealized picture.

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arXiv2001

Recent results on hadron form factors

We discuss the recent data on the electric proton form factor, obtained at JLab, which show a spectacular deviation from the commonly assumed dipole behavior. We discuss the implication of these results on the deuteron structure and on the neutron electric form factor: at relatively large $Q^2$ a revision of the deuteron models may be required, and the neutron electric form factor might become even larger than the proton electric form factor.

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arXiv2001

Polarization phenomena for meson production in nucleon-nucleon collisions

We analyze polarization phenomena for pseudoscalar and vector mesons production in nucleon-nucleon collisions. We identify three energy regions corresponding to different physics and different approaches in the analysis of polarization effects. In the threshold region, characterized by the S-wave production for all final particles, the general symmetry properties of strong interaction can be applied. The region of intermediate energies, T=2-4 GeV, is characterized by the essential role of central i.e. non-peripheral collisions, where only a small number of s-channel states with definite quantum numbers, ${\cal J}^P=1^-$ and $2^+$, contribute. At higher energies, T $\ge $10 GeV, the leading mechanism is the diffractive dissociation and it is especially interesting for baryon spectroscopy. The transition to this region is an open field for experimental research at the Nuclotron.

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arXiv2001

Photoproduction of charmed vector mesons, $γ+N\to {\cal B}_c +\overline{D^*}$, with ${\cal B}_c=Λ_c$ or $Σ_c$

The energy and the angular dependence of the associative vector charmed ${D^*}$-meson photoproduction, $γ+N\to {\cal B}_c+ \overline{D^*}$, with ${\cal B}_c=Λ_c$ or $Σ_c$, has been predicted in framework of pseudoscalar D-meson exchange model. The behavior of the cross section is driven by phenomenological form factors, which can be parametrized in terms of two independent parameters. The predicted values of the cross section are sizeable enough to be measured in the near threshold region.

hep-ph↗
arXiv2001

Neutron electric form factor at large momentum transfer

Based on the recent, high precision data for elastic electron scattering from protons and deuterons, at relatively large momentum transfer $Q^2$, we determine the neutron electric form factor up to $Q^2=3.5$ GeV$^2$. The values obtained from the data (in the framework of the nonrelativistic impulse approximation) are larger than commonly assumed and are in good agreement with the Gari-Krümpelmann parametrization of the nucleon electromagnetic form factors.

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arXiv2001

Polarization Phenomena in Hadronic and Nuclear Processes in Threshold Regime

The threshold region for processes of hadronic and nuclear interactions is very interesting for a theoretical as well as an experimental point of view. In this region one can apply different physical methods, starting from classical current algebra methods - for processes invloving soft pions, through effective Lagrangian approaches - or perturbative chiral symmetry theory (ChPT). The essential simplification of the spin structure of matrix elements for threshold regime results in better understanding of the underlying mechanisms and allows the analysis of polarization phenomena. A dedicated formalism is developed here for a wide class of processes including even such complicated processes as production of pseudoscalar and vector mesons in nucleon-nucleon collisions. Many interesting physical problems are discussed: hidden strangeness of nucleons and OZI-violation, $ηN$ -and $ωN$ interactions in S-state, the determination of P-parity of strange particles. Special attention is devoted to the analysis of the spin structure and polarization phenomena for nuclear processes with light nuclei which have important applications in fundamental astrophysics and in nuclear fusion.

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arXiv2000

Nuclear processes in magnetic fusion reactors with polarized fuel

We consider the processes $d +d \to n +{^3He}$, $d +{^3He} \to p +{^4He}$, $d +{^3H} \to n +{^4He}$, ${^3He} +{^3He}\to p+p +{^4He}$, ${^3H} +{^3He}\to d +{^4He}$, with particular attention for applications in fusion reactors. After a model independent parametrization of the spin structure of the matrix elements for these processes at thermal colliding energies, in terms of partial amplitudes, we study polarization phenomena in the framework of a formalism of helicity amplitudes. The strong angular dependence of the final nuclei and of the polarization observables on the polarizations of the fuel components can be helpful in the design of the reactor shielding, blanket arrangement etc..We analyze also the angular dependence of the neutron polarization for the processes $\vec d +\vec d \to n +{^3He}$ and $\vec d +\vec {^3H} \to n +{^4He}$.

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arXiv2000

Asymptotic Behavior of Nucleon Electromagnetic Form Factors in Space- and Time-Like Regions

We compare the existing data about electromagnetic form factors of the proton in the time-like region of momentum transfer (up to $q^2\simeq$ 14 (GeV/c)$^2$), with the corresponding data in the space-like region. From the constrains given by the Phragmèn-Lindelöf theorem, it turns out that the asymptotic regime can not be reached simultaneously for both form factors (electric and magnetic) in the considered region of momentum transfer. The angular dependence for the annihilation processes, such as $\bar{p}+p\leftrightarrow e^++e^-$, is sensitive to the asymptotic properties of form factors and to the two-photon physics in the time-like region.

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arXiv2000

Neutral weak currents in pion electroproduction on the nucleon

Parity violating asymmetry in inclusive scattering of longitudinally polarized electrons by unpolarized protons with $π^0$ or $π^+$ meson production, is calculated as a function of the momentum transfer squared $Q^2$ and the total energy $W$ of the $πN$-system. This asymmetry, which is induced by the interference of the one-photon exchange amplitude with the parity-odd part of the $Z^0$-exchange amplitude, is calculated for the $γ^*(Z^*)+p\to N+π$ processes ($γ^*$ is a virtual photon and $Z^*$ a virtual Z-boson) considering the $Δ$-contribution in the $s-$channel, the standard Born contributions and vector meson ($ρ$ and $ω$) exchanges in the $t-$channel. Taking into account the known isotopic properties of the hadron electromagnetic and neutral currents, we show that the P-odd term is the sum of two contributions. The main term is model independent and it can be calculated exactly in terms of fundamental constants. It is found to be linear in $Q^2$. The second term is a relatively small correction which is determined by the isoscalar component of the electromagnetic current. Near threshold and in the $Δ$-region, this isoscalar part is much smaller (in absolute value) than the isovector one: its contribution to the asymmetry depend on the polarization state (longitudinal or transverse) of the virtual photon.

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arXiv2000

Associative photoproduction of charmed particles near threshold

We calculate the cross section and the beam asymmetry for exclusive photoproduction of charmed particles near threshold ($γ+p\to Λ_c^+ +\bar{D^0}$), in the framework of an effective Lagrangian model. We discuss the sensitivity of these observables to the magnetic moment of the $Λ_c$ baryon and of the coupling constant in the $pΛ_c D$ vertex. We show that exclusive measurements allow, in principle, to determine the magnetic moments of charmed baryons.

hep-ph↗