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E. Tomasi-Gustafsson

Publications and source records attributed to E. Tomasi-Gustafsson.

87 records · Page 5Linked to original sources
arXiv2006

Open questions and perspectives for hadron electromagnetic form factors in space-like and time-like regions

An overview of recent data on electromagnetic hadron form factors is presented and discussed in terms of selected nucleon models in space-like and time-like regions. Model independent properties connecting scattering and annihilation channels are presented and give a deeper understanding of the underlying physics. Radiative corrections are discussed. The interest of polarization observables with respect to model predictions and reaction mechanism (one photon versus two photon exchange) is highlighted.

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arXiv2006

High energy inelastic electron hadron scattering, in peripheral kinematics. Sum rules for hadron form factors

Relations between differential cross section for inelastic scattering of electrons on hadrons and hadron form factors (sum rules) are derived on the basis of analytical properties of heavy photon forward Compton scattering on hadrons. Sum rules relating the slope of form-factors at zero momentum transfer and anomalous magnetic moments of hadrons with some integrals on photo-production on a hadrons is obtained as well. To provide the convergence of these integrals we construct differences of individual sum rules for different hadrons. Universal interaction of Pomeron with nucleons is assumed. We derive the explicit formulae for processes of electro-production on proton and light isobar nuclei. Sudakov's parametrization of momenta, for peripheral kinematics relevant here, is used. The light-cone form of differential cross sections is also discussed. The accuracy of sum rules estimated in frames of point-like hadrons and it is shown to be at the level of precision achievable by experiments. Suggestions and predictions for future experiments are also given.

hep-ph↗
arXiv2005

Two-component model for the deuteron electromagnetic structure

We suggest a simple phenomenological parametrization for all three deuteron electromagnetic form factors, and show that a good fit on the available data, with a minimal number of parameters, can be obtained. The present description of the deuteron electromagnetic structure is based on two components with different radii, one corresponding to two nucleons separated by $\simeq $2 fm, and a standard isoscalar contribution, saturated by $ω$ and $ϕ$ mesons, only.

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arXiv2005

General analysis of polarization phenomena in $ e^++e^-\to N+\bar N$ for axial parametrization of two--photon exchange

General properties of the differential cross section and of the polarization observables are derived for the $e^++e^-\to N+\bar N$ reaction, in presence of two--photon exchange. Polarization effects are investigated for longitudinally polarized electron and polarized antinucleon and/or polarized nucleon in the final state. The single--spin asymmetry induced by the transverse polarization of the electron beam is also discussed.

hep-ph↗
arXiv2005

Asymptotic behavior of nucleon electromagnetic form factors in time-like region

We study the asymptotic behavior of the ratio of Pauli and Dirac electromagnetic nucleon form factors, $F_2/F_1$, in time-like region for different parametrizations built for the space-like region. We investigate how fast the ratio $F_2/F_1$ approaches the asymptotic limits according to the Phragmèn-Lindelöf theorem. We show that the QCD-inspired logarithmic behavior of this ratio results in very far asymptotics, experimentally unachievable. This is also confirmed by the normal component of the nucleon polarization, $P_y$, in $e^++e^-\to N+\bar{N}$ (in collisions of unpolarized leptons), which is a very interesting observable, with respect to this theorem. Finally we observe that the 1/Q parametrization of $F_2/F_1$ contradicts this theorem.

hep-ph↗
arXiv2005

Search for evidence of two photon contribution in elastic electron proton data

We reanalyze the most recent data on elastic electron proton scattering. We look for a deviation from linearity of the Rosenbluth fit to the differential cross section, which would be the signature of the presence of two photon exchange. The two photon contribution is parametrized by a one parameter formula, based on symmetry arguments. The present data do not show evidence for such deviation.

hep-ph↗
arXiv2005

$Θ^+$-pentaquark parity from associative baryon production in $πD$ collisions, $π^{\pm}+D\to Θ^+ +Σ^{\pm}$

We derive, in model independent way, the spin structure of the matrix element for the reaction of associative $Θ^+$ pentaquark production, $π^{\pm}+D\to Θ^+ +Σ^{\pm}, $ in the threshold region and in collinear kinematics. The expressions for the polarization observables in this reaction are found assuming spin 1/2 and different parities for $Θ^+ .$ We have proved that such reaction can be used for a model independent determination of the P-parity of $Θ^+$ only by measuring the $Θ^+$ polarization. Other polarization observables, such as the dependence of the $Σ^{\pm}$ polarization on the vector and tensor deuteron polarizations, are insensitive to the $Θ^+$ parity in the considered kinematical conditions. All linear and quadratic relations between polarization observables in $π^{\pm}+\vec D\to Θ^+ +\vecΣ^{\pm}(Θ^+ $ is unpolarized) do not depend on the parity of the $Θ^+$ pentaquark.

hep-ph↗
arXiv2004

Further Evidence of Narrow Baryonic Structures with Hadronic as Well as Leptonic Probes

Although extracted from several experiments using hadronic probes \cite{bor1}, narrow baryonic structures have been sometimes met with disbelief. New signatures are presented, which appear from already published data, obtained with hadronic probes as well as with leptonic probes. The authors of these results did not take into account the possibility to associate the discontinuities of their spectra with the topic of narrow baryonic low mass structures. The stability of the observed narrow structure masses, represents a confirmation of their genuine existence.

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arXiv2004

Polarization phenomena in elastic $e^{\mp}N$ scattering, for axial parametrization of two-photon exchange

We analyze polarization phenomena for elastic lepton-nucleon scattering, parametrizing the $2γ$-exchange contribution, as the product of lepton and nucleon axial vector currents, that correspond to $C=+1$ exchange in the annihilation channel. We found two combinations of polarization observables (one for three T-odd observables and another one - for five T-even observables), which allow to measure the ratio $G_E(Q^2)/G_M(Q^2)$ of nucleon electromagnetic form factors in model independent way, without any specific assumption about the $2γ$-exchange mechanism. Both these combinations have a general character and do not depend on the choice of the spin structure for the $2γ$-exchange. We show the inconsistency of other approximations recently used in the literature.

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arXiv2003

The complete experiment for $e^{\mp}N$ elastic scattering in presence of two-photon exchange

We propose possible methods to measure the nucleon electromagnetic form factors in presence of two-photon exchange. Using a general parametrization of the spin structure of the matrix element for elastic $eN$-scattering, in terms of three independent complex amplitudes, we show that the measurements of the differential cross section for electron and positron nucleon elastic scattering, in the same kinematical conditions, allows to extract the nucleon electromagnetic form factors. The same is correct for the polarization method, with the measurement of the $P_{x,z}$ components of the final nucleon polarization (for the scattering of longitudinally polarized electrons and positrons). An alternative way, in absence of positron beam, is to measure a definite set of T-odd polarization observables, which includes three different experiments or only T-even observables, with five independent experiments. In both cases, the ratio $G_E(Q^2)/G_M(Q^2)$ is related to quantities of the order of $α$, which requires different polarization experiments of very high accuracy.

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arXiv2003

Precise method for the determination of the neutron electric form factor based on a relativistic analysis of the process $d(e,e'n)p

We generalize the recoil polarization method for the determination of the proton form factor to the case of the disintegration of vector polarized deuterons by longitudinally polarized electrons, $\vec d(\vec e, e'n)p$. We suggest to measure for this reaction, in the kinematics of quasi-elastic $en$-scattering, the ratio $R_{xz}=A_x/A_z$ of the asymmetries induced by the $x$- and $z$-components of the deuteron vector polarization. In the framework of the relativistic impulse approximation the ratio $R_{xz}$ is sensitive to $G_{En}$ in a wide interval of momentum transfer squared, whereas it depends weakly on the details of the $np$-interaction and on the choice of the deuteron wave function. Moreover, in the range $0.5\le Q^2\le$1.5 GeV$^2$, the ratio $R_{xz}$ shows a smooth dependence on $Q^2$, making the analysis simpler.

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arXiv2003

Observation of coherent $π^0$ electroproduction on deuterons at large momentum transfer

The first experimental results for coherent $π^0$-electroproduction on the deuteron, $e+d\to e+d +π^0$, at large momentum transfer, are reported. The experiment was performed at Jefferson Laboratory at an incident electron energy of 4.05 GeV. A large pion production yield has been observed in a kinematical region for 1.1$<Q^2<$1.8 GeV$^2$, from threshold to 200 MeV excitation energy in the $dπ^0$ system. The $Q^2$-dependence is compared with theoretical predictions.

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arXiv2001

Radiative corrections to polarization observables for elastic $e+A$-scattering. Part I: Virtual Compton Scattering

We calculate polarization phenomena for virtual Compton scattering on protons, at relatively large momentum transfer 1 GeV$^2$ $\le -q^2\le$ 5 GeV$^2$ on the basis of a model for $γ^*+ p\to γ+p$ with two main contributions: $π^0$-exchange in $t-$channel and $Δ$-excitation in $s$-channel. This model applies from threshold to $Δ$ region. The parameters entering in this model, such as coupling constants and electromagnetic form factors are well known. The analyzing powers for $γ^*+\vec p\to γ+ p$ and the components of the final proton polarization in $γ^*+ p\to γ+ \vec p$ are large in absolute value and show strong sensitivity to $π\bigotimesΔ$ interference. These results can be applied to the calculation of radiative corrections to polarization phenomena in elastic $ep$-scattering.

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arXiv1999

Sensitivity of tensor analyzing power in the process $d+p\to d+X$ to the longitudinal isoscalar form factor of the Roper resonance electroexcitation

The tensor analyzing power of the process $d + p \to d + X$, for forward deuteron scattering in the momentum interval 3.7 to 9 GeV/c, is studied in the framework of $ω$ exchange in an algebraic collective model for the electroexcitation of nucleon resonances. We point out a special sensitivity of the tensor analyzing power to the isoscalar longitudinal form factor of the Roper resonance excitation. The main argument is that the $S_{11}(1535)$, $D_{13}(1520)$ and $S_{11}(1650)$ resonances have only isovector longitudinal form factors. It is the longitudinal form factor of the Roper excitation, which plays an important role in the $t-$dependence of the tensor analyzing power. We discuss possible evidence of swelling of hadrons with increasing excitation energy.

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