arXiv subjects

E. Oset

Publications and source records attributed to E. Oset.

At least 505 records · Page 28Linked to original sources

arXiv1997

Delta excitation in K^+-nucleus collisions

We present calculations for Δexcitation in the (K^+,K^+) reaction in nuclei. The background from quasielastic K^+ scattering in the Δregion is also evaluated and shown to be quite small in some kinematical regions, so as to allow for a clean identification of the Δexcitation strength. Nuclear effects tied to the Δrenormalization in the nucleus are considered and the reaction is shown to provide new elements to enrich our knowledge of the Δproperties in a nuclear medium.

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arXiv1997

Theoretical Study of the γγ-->meson-meson Reaction

We present a unified picture which studies simultaneously the gamma gamma-->pi^+ pi^-, pi^0 pi^0, K^+ K^-, K^0 bar{K}^0, pi^0 eta reactions up to about sqrt(s)=1.4 GeV reproducing the experimental cross sections. The present work implements in an accurate way the final state interactions of the meson-meson system, which is shown to be essential in order to reproduce the data, particularly the L=0 channel. This latter channel is treated here following a recent theoretical work in which the meson-meson amplitudes are well reproduced and the f_0, a_0, sigma resonances show up clearly as poles of the t matrix. The present work, as done in earlier ones, also incorporates elements of chiral symmetry and exchange of vector and axial resonances in the crossed channels, as well as a direct coupling to the f_2(1270) and a_2(1320) resonances. We also evaluate the decay width of the f_0(980) and a_0(980) resonances into the gamma-gamma channel.

hep-ph↗

arXiv1997

Non perturbative chiral approach to s-wave \bar{K}N interactions

The s-wave meson-nucleon interaction in the $S = -1$ sector is studied by means of coupled-channel Lippmann Schwinger equations, using the lowest order chiral Lagrangian and a cut off to regularize the loop integrals. The method reproduces succesfully the $Λ(1405)$ resonance and the $K^- p \to K^- p, \bar{K}^0 n, π^0 Λ, π^0 Σ, π^+ Σ^-, π^- Σ^+$ cross sections at low energies. The inclusion of the $ηΛ, ηΣ^0$ channels in the coupled system is found very important and allows a solution in terms of only the lowest order Lagrangian.

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arXiv1997

Many Body approach to the inclusive $(e,e')$ reaction from the quasielastic to the $Δ$ excitation region

We have performed a many body calculation of the inclusive (e,e') cross section which runs over the three traditional regions at intermediate energies: the quasielastic peak, the dip region and the delta region. The longitudinal and transverse response functions in the quasielastic peak have also been evaluated. Traditional effects like polarization, meson exchange currents, final state interaction and delta renormalization in the nuclear medium have been included. Meson exchange currents are generated from a model of pion electroproduction on the nucleon which reproduces accurately the experimental data. The inclusive cross section accounts for 1N,2N,3N mechanisms of virtual photon absorption and one pion production. Meson exchange currents associated to the (γ^*,2π) reaction are also accounted for. We obtain good results for the (e,e') cross sections in the whole energy range and for different nuclei. The response functions are also in good agreement with the latest experimental analysis. On the other hand, the method provides the separation of the contribution to the inclusive cross section from different physical channels which is a necessary input to evaluate cross sections like (e,e' N), (e,e' NN), (e,e' π) etc.

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arXiv1997

Chiral symmetry amplitudes in the S-wave isoscalar and isovector channels and the sigma, f_0(980), a_0(980) scalar mesons (HADRON'97)

We use a non-perturbative approach which combines coupled channel Lippmann-Schwinger equations with meson-meson potentials provided by the lowest order chiral Lagrangian. By means of one parameter, a cut off in the momentum of the loop integrals, which results of the order of 1 GeV, we obtain singularities in the S-wave amplitudes corresponding to the sigma, f_0 and a_0 resonances. The ππ--> ππ, ππ--> K bar(K) phase shifts and inelasticities in the T=0 scalar channel are well reproduced as well as the π^0 eta and K bar(K) mass distributions in the T=1 channel. Furthermore, the total and partial strong decay widths of the f_0 and a_0 resonances are properly reproduced.

hep-ph↗

arXiv1997

Theoretical Study of the γγ-->meson-meson Reaction (HADRON'97)

We present a unified picture which studies simultaneously the gamma gamma--> pi^+ pi^-, pi^0 pi^0,K^+ K^-, K^0 bar{K}^0, pi^0 eta reactions up to about sqrt(s)=1.4 GeV reproducing the experimental cross sections. The present work implements in an accurate way the final state interactions of the meson-meson system, which is shown to be essential in order to reproduce the data, particularly the L=0 channel. This latter channel is treated here following a recent theoretical work in which the meson-meson amplitudes are well reproduced and the f_0, a_0, sigma resonances show up clearly as poles of the t matrix. The present work, as done in earlier ones, also incorporates elements of chiral symmetry and exchange of vector and axial resonances in the crossed channels, as well as a direct coupling to the f_2(1270) and a_2(1320) resonances. We also evaluate the decay width of the f_0(980) and a_0(980) resonances into the gamma-gamma channel.

hep-ph↗

arXiv1997

Inclusive $(e,e^\prime N)$, $(e,e^\prime NN)$, $(e,e^\prime π)$ ... reactions in nuclei

We study the inclusive $(e,e^\prime N)$, $(e,e^\prime NN)$, $(e,e^\prime π)$, $(e,e^\prime πN)$ reactions in nuclei using a Monte Carlo simulation method to treat the multichannel problem of the final state. The input consists of reaction probabilities for the different steps evaluated using microscopical many body methods. We obtain a good agreement with experiment in some channels where there is data and make predictions for other channels which are presently under investigation in several electron laboratories. The comparison of the theoretical results with experiment for several kinematical conditions and diverse channels can serve to learn about different physical processes ocurring in the reaction. The potential of this theoretical tool to make prospections for possible experiments, aiming at pinning down certain reaction probabilities, is also emphasized.

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arXiv1997

Coherent two pion photoproduction on 12C

We develop the formalism for coherent two pion photoproduction in nuclei and perform actual calculations of cross sections for $π^-π^+$ and $π^0π^0$ photoproduction on $^{12}C$. We find that due to the isospin symmetry the cross section for $π^0π^0$ production is very small and has a maximum when the pions propagate together. However, the kinematical region where the energies and polar angles of the two $π^0$ mesons are equal and their relative azimuthal angle $ϕ=180^0$ is forbidden. Conversely in the $π^-π^+$ production the pions prefer to have a relative azimuthal angle 180$^0$ and the production of the pions propagating together is suppressed. The dominant one-body mechanism in both channels is related to the excitation of the $Δ$ isobar. Hence the reaction can serve as a source of information about $Δ$'s properties in nucleus. We have found that the reaction is sensitive to effects of the pion and $Δ$ renormalization in the nuclear medium, similar to those found in the coherent $(γ,π^0)$ reaction, but magnified because of the presence of the two pions.

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arXiv1997

Quasielastic K-nucleus scattering

Quasielastic K^+ - nucleus scattering data at q=290, 390 and 480 MeV/c are analyzed in a finite nucleus continuum random phase approximation framework, using a density-dependent particle-hole interaction. The reaction mechanism is consistently treated according to Glauber theory, keeping up to two-step inelastic processes. A good description of the data is achieved, also providing a useful constraint on the strength of the effective particle-hole interaction in the scalar-isoscalar channel at intermediate momentum transfers. We find no evidence for the increase in the effective number of nucleons participating in the reaction which has been reported in the literature.

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arXiv1997

Chiral symmetry amplitudes in the s-wave isoscalar and isovector channels and the $σ, f_0(980), a_0 (980)$ scalar mesons

We use a nonpertubative approach which combines coupled channel Lippmann Schwinger equations with meson-meson potentials provided by the lowest order chiral Lagrangian. By means of one parameter, a cut off in the momentum of the loop integrals, which results of the order of 1 GeV, we obtain singularities in the S-wave amplitudes corresponding to the $σ$, f_0 and a_0 resonances. The $ππ\to ππ, ππ\to K \bar{K}$ phase shifts and inelasticities in the T = 0 scalar channel are well reproduced as well as the $π^0 η$ and $K \bar{K}$ mass distributions in the T = 1 channel. Furthermore, the total and partial decay widths of the f_0 and a_0 resonances are properly reproduced including also the decay into the $γγ$ channel. The results seem to indicate that chiral symmetry constraints at low energy and unitarity in coupled channels is the basic information contained in the meson-meson interaction below $\sqrt{s} \simeq 1.2 GeV$.

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arXiv1997

Quasielastic Versus Inelastic and Deep Inelastic Lepton Scattering in Nuclei at x > 1

We have made a thorough investigation of the nuclear structure function W_2A in the region of 0.8 < x < 1.5 and Q^2 < 20 GeV^2, separating the quasielastic and inelastic plus deep inelastic contributions. The agreement with present experimental data is good giving support to the results for both channels. Predictions are made in yet unexplored regions of x and Q^2 to assert the weight of the quasielastic or inelastic channels. We find that at Q^2 < 4 GeV^2 the structure function is dominated by the quasielastic contributions for x < 1.5, while for values of Q^2 > 15 GeV^2 and the range of x studied the inelastic channels are over one order of magnitude bigger than the quasielastic one. The potential of the structure function at x > 1 as a source of information on nuclear correlations is stressed once more.

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arXiv1997

Chiral Symmetry Amplitudes in the S-Wave Isoscalar and Isovector Channels and the σ, f_0(980), a_0(980) Scalar Mesons

We use a nonperturbative approach which combines coupled channel Lippmann Schwinger equations with meson-meson potentials provided by the lowest order chiral Lagrangians. By means of one parameter, a cut off in the momentum of the loop integrals, which results of the order of 1 GeV, we obtain singularities in the S-wave amplitudes corresponding to the σ,f_0 and a_0 resonances. The ππ-->ππ, ππ-->K\bar{K} phase shifts and inelasticities in the T=0 scalar channel are well reproduced as well as the π^-ηand K^-K^0 mass distributions in the T=1 channel. Furthermore, the total and partial decay widths of the f_0 and a_0 resonances are properly reproduced including also the decay into the γγchannel. The results seem to indicate that chiral symmetry constraints at low energy and unitarity in coupled channels is the basic information contained in the meson-meson interaction below \sqrt(s)=1.2 GeV.

hep-ph↗

arXiv1997

Meson Exchange Currents in Kaon Scattering on the Lightest Nuclei

The K^+ scattering on the lightest nuclei, d, 3He and 4He is studied in the framework of multiple scattering theory. Effects from MEC tied to the K^+N-->KNpi reaction are evaluated. We found that at momentum transfers Q^2<0.5 (GeV/c)^2 contributions from MEC are much smaller than kaon rescattering corrections. This makes the conventional multiple scattering picture a reliable tool to study these reactions in this kinematical domain and to extract the K^+n scattering amplitude from the K^+d data. At larger transferred momentum MEC can become more relevant.

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arXiv1996

Deep Inelastic Lepton Scattering in Nuclei at x > 1 and the Nucleon Spectral Function

The nuclear structure function F_2A(x) has been studied in the Bjorken limit for (l, l') scattering on nuclei in the region of x > 1 and was found to be very sensitive to the information contained in the nucleon spectral function in nuclei, particularly the correlations between momenta and energies in the region of large momenta. Calculations were done in a local density approximation using two different spectral functions for nuclear matter. Results are compared to those obtained for a spectral function which has been evaluated directly for the finite nucleus, ^{16}O, under consideration. For values of x around 1.5 and larger the quasiparticle contribution is negligible, thus stressing the sensitivity of the present reaction to the dynamical properties of nuclei beyond the shell model approach. Several approximations which are usually employed in studies of the EMC effect have been analyzed and their inaccuracy in this region is demonstrated. The results stress the fact that the nuclear structure function contains important information on nuclear dynamical correlations. Therefore further measurements of F_2A(x) in that region and for many nuclei would be most welcome.

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arXiv1996

Mesonic and Binding Contributions to the EMC Effect in a Relativistic Many Body Approach

We revise the conventional nuclear effects of Fermi motion, binding and pionic effects in deep inelastic lepton scattering using a relativistic formalism for an interacting Fermi sea and the local density approximation to translate results from nuclear matter to finite nuclei. In addition we also consider effects from rho-meson renormalization in the nucleus. The use of nucleon Green's functions in terms of their spectral functions offers a precise way to account for Fermi motion and binding. On the other hand the use of many body Feynman diagrams in a relativistic framework allows one to avoid using prescriptions given in the past to introduce relativistic corrections in a non relativistic formalism. We show that with realistic nucleon spectral functions and meson nucleus selfenergies one can get a reasonable description of the EMC effect for x > 0.15, outside the shadowing region.

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arXiv1996

Mesonic and Binding Contributions to the EMC Effect in a Relativistic Many-body Approach

Starting from a covariant relativistic formalism which uses nucleon and pion propagators in a nuclear medium, we revise the traditional effects of Fermi motion, binding and mesonic corrections to the EMC effect. The calculations are done in a very accurate way, using precise nucleon spectral functions and meson self-energies and a good reproduction of the EMC data is obtained outside the shadowing region which is not explored.

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arXiv1996

Lepton Scattering on Nuclei at x > 1 and the Nucleon Spectral Function

We analyze the processes of deep inelastic and quasielastic scattering in the region of x > 1. These processes are found to be very sensitive to the information contained in the nucleon spectral function in nuclei, particularly to the correlations between energy and momentum. Calculations are done using two spectral functions for nuclear matter and one for finite nuclei, ^{16}O. Several approximations are also analyzed and are shown to be inaccurate in this region. These results stress the fact that the region of x > 1 contains important information on nuclear dynamical correlations.

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arXiv1996

The Imaginary Part of Nucleon Self-energy in hot nuclear matter

A semiphenomenological approach to the nucleon self-energy in nuclear matter at finite temperatures is followed. It combines elements of Thermo Field Dynamics for the treatment of finite temperature with a model for the self-energy, which evaluates the second order diagrams taking the needed dynamics of the NN interaction from experiment. The approach proved to be accurate at zero temperature to reproduce Im(Sigma) and other properties of nucleons in matter. In the present case we apply it to determine Im(Sigma) at finite temperatures. An effective NN cross section is deduced which can be easily used in analyses of heavy ion reactions.

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