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E. Tomasi-Gustafsson

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

At least 55 records · Page 3Linked to original sources
arXiv2010

Feasibility studies of the time-like proton electromagnetic form factor measurements with PANDA at FAIR

The possibility of measuring the proton electromagnetic form factors in the time-like region at FAIR with the \PANDA detector is discussed. Detailed simulations on signal efficiency for the annihilation of $\bar p +p $ into a lepton pair as well as for the most important background channels have been performed. It is shown that precision measurements of the differential cross section of the reaction $\bar p +p \to e^++ e^-$ can be obtained in a wide angular and kinematical range. The individual determination of the moduli of the electric and magnetic proton form factors will be possible up to a value of momentum transfer squared of $q^2\simeq 14$ (GeV/c)$^2$. The total $\bar p +p\to e^++e^-$ cross section will be measured up to $q^2\simeq 28$ (GeV/c)$^2$. The results obtained from simulated events are compared to the existing data. Sensitivity to the two photons exchange mechanism is also investigated.

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arXiv2010

Radiative proton-antiproton annihilation to a lepton pair

The annihilation of proton and antiproton to electron-positron pair, including radiative corrections due to the emission of virtual and real photons is considered. The results are generalized to leading and next-to leading approximations. The relevant distributions are derived and numerical applications are given in the kinematical range accessible to the PANDA experiment at the FAIR facility.

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arXiv2010

Recoil Polarization Measurements of the Proton Electromagnetic Form Factor Ratio to Q^2 = 8.5 GeV^2

Among the most fundamental observables of nucleon structure, electromagnetic form factors are a crucial benchmark for modern calculations describing the strong interaction dynamics of the nucleon's quark constituents; indeed, recent proton data have attracted intense theoretical interest. In this letter, we report new measurements of the proton electromagnetic form factor ratio using the recoil polarization method, at momentum transfers Q2=5.2, 6.7, and 8.5 GeV2. By extending the range of Q2 for which GEp is accurately determined by more than 50%, these measurements will provide significant constraints on models of nucleon structure in the non-perturbative regime.

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arXiv2010

Exotic low mass narrow baryons extracted from charge exchange reactions

This paper aims to give further evidence for the existence of low mass exotic baryons. Narrow structures in baryonic missing mass or baryonic invariant mass were observed during the last twelve years. Since their evidence is still under debate, various data, measured with incident hadrons, by different collaborations, are reanalyzed to bring evidence on these narrow exotic baryonic resonances excited in charge-exchange reactions. These structures are clearly exotic as there is no room for them in the $qqq$ configurations: their width is smaller than the widths of "classical" baryonic resonances, moreover some of the masses lie below the pion threshold mass.

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arXiv2009

Bremsstrahlung and pair production processes at low energies, multi-differential cross section and polarization phenomena

Radiative electron-proton scattering is studied in peripheral kinematics, where the scattered electron and photon move close to the direction of the initial electron. Even in the case of unpolarized initial electron the photon may have a definite polarization. The differential cross sections with longitudinally or transversal polarized initial electron are calculated. The same phenomena are considered for the production of an electron-positron pair by the photon, where the final positron (electron) can be also polarized. Differential distributions for the case of polarized initial photon are given. Both cases of unscreened and completely screened atomic targets are considered.

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arXiv2009

Measuring the deviation from the Rutherford formula

Modern experiments with heavy ion-leptons collisions open the possibility to measure the deviation of cross section of small angles electron(positron)-ion elastic scattering from the Rutherford formula due to multiple virtual photons exchange. The charge asymmetry and the polarization of the scattered leptons are calculated and numerical predictions are given. A generalization to elastic proton-nucleus scattering is discussed.

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arXiv2008

Relativistically invariant analysis of $Δ$--isobar production in deuteron electrodisintegration: $e^-+d\to e^-+Δ+N:$ general analysis of polarization effects

The differential cross section and the polarization observables for $Δ$--isobar production in the deuteron electrodisintegration process, $e^-+d\to e^-+Δ+N$, are calculated in a general formalism based on structure functions. The obtained expressions have a general nature, hold for one--photon--exchange, assuming P--invariance of the electromagnetic interaction and the conservation of the hadron electromagnetic current. The dependence of the differential cross section of the $e^-+d\to e^- +Δ+N$ reaction on the vector and tensor polarizations of the deuteron target with unpolarized and longitudinally polarized electrons is considered. The general dependence of the asymmetries on two of five kinematic variables, the azimuthal angle $ϕ$ and $ε$ (linear polarization of the virtual photon) is calculated. A similar analysis is performed for the polarization of the nucleon produced in $γ^*d \to ΔN$ reaction provided the electron beam is unpolarized or longitudinally polarized. Polarization effects, which are due to the strong $ΔN-$ interaction in the final state are calculated. The photoproduction of the $Δ$--isobar on the deuteron target has been considered in detail, as a particular case. The differential cross section and various polarization observables have been derived in terms of the reaction amplitudes. The polarization observables due to the linear and circular polarizations of the photon, when the deuteron target is arbitrarily polarized have been derived in terms of the reaction amplitudes. The polarization of the final nucleon is also considered.

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arXiv2008

The two photon exchange amplitude in $ep$ and $eμ$ elastic scattering: a comparison

In this note we give arguments in favor of the statement that the contribution of the box diagram calculated for electron muon elastic scattering can be considered an upper limit to electron proton scattering. As an exact QED calculation can be performed, this statement is useful for constraining model calculations involving the proton structure.

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arXiv2008

Two-component model for the axial form factor of the nucleon

The axial form factor of the nucleon is studied in a two-component model consisting of a three-quark intrinsic structure surrounded by a meson cloud. The experimental data in the space-like region are well reproduced with a minimal number of parameters. The results are similar to those obtained from a dipole fit up to 1 GeV$^2$, but they surprisingly show a zero between 2 and 3 GeV$^2$. Such behavior, predicted by the present model completely disagrees with the dipole parametrization. The theoretical expression for the form factor is then analytically extrapolated to the time-like region, giving the first predictions in this regime. This kinematical region is especially interesting for present and future colliders.

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arXiv2008

Cross section of the processes $e^++e^-\to e^++e^-(γ)$, $\to π^++π^-(γ)$, $ μ^++μ^-(γ)$, $ γ+γ(γ)$ in the energy region 200 MeV $\le 2E\le$ 3 GeV

The cross section for different processes induced by $e^+e^-$ annihilation, in the kinematical limit $β_μ\approxβ_π=(1-m_π^2/ε^2)^{1/2}\sim 1$, is calculated taking into account first order corrections to the amplitudes and the corrections due to soft emitted photons, with energy $ω\leΔE\le ε$ in the center of mass of the $e^+e^-$ colliding beams. The results are given separately for charge--odd and charge--even terms in the final channels $π^+π^-(γ)$ and $μ^+μ^-(γ)$. In case of pions, form factors are taken into account. The differential cross sections for the processes: $e^++e^-\to e^++e^-(+γ)$, $\to π^++π^-(γ)$, $\to μ^++μ^-(γ),\to γγ(γ)$ have been calculated and the corresponding formula are given in the ultrarelativistic limit $\sqrt{s}/2= ε\gg m_μ\sim m_π$ . For a quantitative evaluation of the contribution of higher order of the perturbation theory, the production of $π^+π^-$, including radiative corrections, is calculated in the approach of the lepton structure functions. This allows to estimate the precision of the obtained results as better than 0.5% outside the energy region corresponding to narrow resonances. A method to integrate the cross section, avoiding the difficulties which arise from singularities is also described.

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arXiv2008

Cross section and polarization observables for the reaction $e^++e^-\to a_1(1260)+π$

A model independent formalism for the electron positron annihilation reaction $e^++e^-\to a_1(1260)+π$ has been derived. The differential and total cross sections and the elements of the spin--density matrix of the $a_1$-meson were calculated in terms of the electromagnetic form factors of the corresponding $γ^*a_1π$ current. Simple models of $a_1$ form factors have been fitted to the available cross section data and they allow to give numerical predictions for the different observables.

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arXiv2008

High order radiative corrections for unpolarized electron proton elastic scattering at low $Q^2$

We investigate the effect of high order radiative corrections in unpolarized electron proton elastic scattering and compare with the calculations at lowest order, which are usually applied to experimental data. We show that higher order terms play a role, starting from values of the momentum transfer squared, $Q^2$, larger than the electron mass. Particular attention is devoted to the $ε$ dependence of radiative corrections.

hep-ph↗
arXiv2008

Contribution to the study of narrow low mass hadronic structures

New data are presented, concerning narrow exotic structures in mesons, baryons and dibaryons. The sequence of narrow baryons is quite well described starting from the sequence of narrow mesons. In the same way, the sequence of narrow dibaryons is rather well described starting from the sequence of narrow baryons. Lastly it is shown that the masses of these narrow hadronic structures lie on straight line Regge-like trajectories.

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arXiv2007

Sum rule for the double virtual Compton scattering

The two photon exchange amplitude is investigated in frame of analytic properties of the virtual Compton scattering amplitude as a function of the invariant mass squared of the intermediate hadronic state. A sum rule is built, based on arguments from analyticity. It relates the differential cross section of elastic electron-proton scattering including form factors, and the cross section of inelastic scattering channel, with a contribution of nucleon anti-nucleon pair production arising from the Fermi statistics. The last term is calculated in frame of a simple model of nucleon-pion interaction.

hep-ph↗
arXiv2007

Charge asymmetry for electron (positron)-proton elastic scattering at large angle

Charge asymmetry in electron (positron) scattering arises from the interference of the Born amplitude and the box-type amplitude corresponding to two virtual photons exchange. It can be extracted from electron proton and positron proton scattering experiments, in the same kinematical conditions. Considering the virtual photon Compton scattering tensor, which contributes to the box-type amplitude, we separate proton and inelastic contributions in the intermediate state and parametrize the proton form-factors as the sum of a pure QED term and a strong interaction term. Arguments based on analyticity are given in favor of cancellation of contributions from proton strong interaction form factors and of inelastic intermediate states in the box type amplitudes. In frame of this model, with a realistic expression for nucleon form-factors, numerical estimations are given for moderately high energies.

hep-ph↗
arXiv2007

Search for Low Mass Exotic mesonic structures. Part I: experimental results

Recently, several papers discussed on the existence of a low mass new structure at a mass close to M=214.3 MeV. It was suggested that the $Σ^{+}$ disintegration: $Σ^{+}\to$pP$^{0}$, P$^{0}\toμ^{-}μ^{+}$ proceeds through an intermediate particle P$^{0}$ having such mass. The present work intends to look at other new or available data, in order to observe the eventual existence of small narrow peaks or shoulders in very low mesonic masses. Indeed narrow structures were already extracted from various data in dibaryons, baryons and mesons (at larger masses that those studied here).

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arXiv2007

Search for Low Mass Exotic mesonic structures. Part II: attempts to understand the experimental results

Our previous paper, part I of the same study, shows the different experimental spectra used to conclude on the genuine existence of narrow, weakly excited mesonic structures, having masses below and a little above the pion (M=139.56 MeV) mass. This work \cite{previous} was instigated by the observation, in the $Σ^{+}$ disintegration: $Σ^{+}\to$pP$^{0}$, P$^{0}\toμ^{-}μ^{+}$ \cite{park}, of a narrow range of dimuon masses. The authors conclude on the existence of a neutral intermediate state P$_{0}$, with a mass M=214.3 MeV $\pm$ 0.5 MeV. We present here some attempts to understand the possible nature of the structures observed in part I.

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