arXiv subjects

E. Oset

Publications and source records attributed to E. Oset.

At least 469 records · Page 26Linked to original sources

arXiv2000

A model for pion Delta electroproduction on the proton

We have extended a model for the $γN\to ππN$ reaction to virtual photons and selected the diagrams which have a $Δ$ in the final state. With this model we have evaluated cross sections for the virtual photon cross section as a function of $Q^2$ for different energies. The agreement found with the $γ_v p\toΔ^0π^+$ and $γ_v p\toΔ^{++}π^-$ reactions is good. The sensitivity of the results to $NΔ$ transition form factors is also studied. The present reaction, selecting a particular final state, is an extra test for models of the $γ_{v} N\to ππN$ amplitude. The experimental measurement of the different isospin channels for this reaction are encouraged as a means to unravel the dynamics of the two pions photoproduction processes.

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arXiv2000

Chiral Unitary approach to meson-meson and meson-baryon interactions and nuclear applications

We report on recent nonperturbative techniques that combine the information of chiral Lagrangians (with and without resonances) with unitarity in coupled channels and other requirements of the S-matrix theory of the strong interactions. As a result, the region of applicability of such techniques is much larger than the one of Chiral Perturbation Theory allowing one to study also resonance physics. Applications to meson-meson and meson-baryon scattering, as well as to problems where pairs of mesons or a meson and a baryon appear in the initial or final state are shown. Implications in several nuclear problems are also discussed.

hep-ph↗

arXiv2000

The phi --> pi^+ pi^- and phi radiative decays within a chiral unitary approach

We report on recent results on the decay of the phi into pi^+ pi^- and phi radiative decays into pi^0 pi^0 gamma and pi^0 eta gamma, which require the consideration of the final state interaction of a pair of mesons in a region inaccessible to Chiral Perturbation Theory. By using nonperturbative chiral unitary methods for the meson meson interaction we can obtain the corresponding decay widths and the results are compared with recent experimental data.

hep-ph↗

arXiv2000

S=I=0 Pion Pairs in the A(pi,pi pi)X Reaction

Recent experimental results show a large enhancement of the 2 pion emission on the scalar isoscalar channel, which had been predicted by some theoretical estimates. We present here a detailed calculation of the A(pi, 2pi)X process incorporating a microscopic model for the elementary reaction in vacuum, pion-pion final state interaction in the nuclear medium, and other nuclear medium effects.

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arXiv2000

Projectile $Δ$ and target-Roper excitation in the p (d, d')X reaction

In this paper we compare a model that contains the mechanisms of $Δ$ excitation in the projectile and Roper excitation in the target with experimental data from two (d, d') experiments on a proton target. The agreement of the theory with the experiment is fair for the data taken at T_d = 2.3 GeV. The $Δ$ excitation in the projectile is predicted close to the observed energy with the correct width. The theory, however, underpredicts by about 40% the cross sections measured at T_d = 1.6 GeV at angles where the cross section has fallen by about two orders of magnitude. The analysis done here allows to extract an approximate strength for the excitation of the Roper [N^*(1440)] excitation and a qualitative agreement with the theoretical predictions is also found.

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arXiv1999

Photoproduction of meson and baryon resonances in a chiral unitary approach

By means of a coupled channel non-perturbative unitary approach, it is possible to extend the strong constrains of Chiral Perturbation Theory to higher energies. In particular, it is possible to reproduce the lowest lying resonances in meson-meson scattering up to 1.2 GeV using the parameters of the O(p^2) and O(p^4) Chiral Lagrangian. The meson baryon sector can also be tackled along similar lines. We report on an update of these results showing some examples of photon induced reactions where the techniques have been recently applied.

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arXiv1999

Radiative decay of rho^0 and phi mesons in a chiral unitary approach

We study the rho^0 and phi decays into pi^+ pi^- gamma, pi^0 pi^0 gamma and phi into pi^0 eta gamma using a chiral unitary approach to deal with the final state interaction of the M M system. The final state interaction modifies only moderately the large momenta tail of the photon spectrum of the rho^0 --> pi^+ pi^- gamma decay. In the case of phi decay the contribution to pi^+ pi^- gamma and pi^0 pi^0 gamma decay proceeds via kaonic loops and gives a distribution of pi pi invariant masses in which the f_0(980) resonance shows up with a very distinct peak. The spectrum found for phi --> pi^0 pi^0 gamma decay agrees with the recent experimental results obtained at Novosibirsk. The branching ratio for phi --> pi^0 eta gamma, dominated by the a_0(980), is also in agreement with recent Novosibirsk results.

hep-ph↗

arXiv1999

Resonances, Chiral Symmetry, Coupled Channel Unitarity and Effective Lagrangians

By means of a coupled channel non-perturbative unitary approach, it is possible to extend the strong constrains of Chiral Perturbation Theory to higher energies. In particular, it is possible to reproduce the lowest lying resonances in meson-meson scattering up to 1.2 GeV using the parameters of the O(p^2) and O(p^4) Chiral Lagrangian. We report on an update of these results examining their possible relevance for meson spectroscopy.

hep-ph↗

arXiv1999

SU(3) Chiral approach to meson and baryon dynamics

We report on recent progress on the chiral unitary approach, which is shown to have a much larger convergence radius than ordinary chiral perturbation theory, allowing one to reproduce data for meson meson interaction up to 1.2 GeV and meson baryon interaction up to the first baryonic resonances. Applications to physical processes so far unsuited for a standard chiral perturbative approach are presented, concretely the K^- p\toΛ(1405)γreaction and the N^\ast (1535)N^\ast(1535)πand ηcouplings.

hep-ph↗

arXiv1999

Two Meson Scattering Amplitudes and their Resonances from Chiral Symmetry and the N/D Method

We study the vector and scalar meson-meson amplitudes up to \sqrt{s}\lesssim 1.4 GeV and their associated spectroscopy. The study has been done considering jointly the N/D method, Chiral Symmetry and implications from large N_c QCD. The N/D method provides us with the way to unitarize the tree level amplitudes constructed in agreement with Chiral Symmetry and its breaking (explicit and spontaneous). These amplitudes are calculated making use of the lowest order Chiral Perturbation Theory (\chiPT) Lagrangians and the exchanges of resonances compatible with Chiral Symmetry as given in. On the other hand the large N_c considerations allow us to distinguish between elementary (as elementary as the pions, for instance) and compound (meson-meson) states. Making use of this formalism one observes that the σ, κand a_0(980) resonances are meson-meson states originating from the unitarization of the lowest order \chiPT amplitudes. On the other hand, the f_0(980) is a combination of a strong S-wave meson-meson unitarity effect and of a preexisting singlet resonance with a mass around 1 GeV.

hep-ph↗

arXiv1999

A Non-Perturbative Chiral Approach for Meson-Meson Interactions

A non-perturbative method which combines constraints from chiral symmetry breaking and coupled channel unitarity is used to describe meson-meson interactions up to \sqrt{s}\lesssim 1.2 GeV, extending in this way the range of applicability of the information contained in Chiral Perturbation Theory (χPT), since this perturbative series is typically restricted to \sqrt{s}\lesssim 500 MeV. The approach uses the O(p^2) and O(p^4) \chiPT Lagrangians. The seven free parameters resulting from the O(p^4) Lagrangian are fitted to the experimental data. The approach makes use of the expansion of T^{-1} instead of the amplitude itself as done in \chiPT. The former expansion is suggested by analogy with the effective range approximation in Quantum Mechanics and it appears to be very useful. The results, in fact, are in good agreement with a vast amount of experimental analyses.

hep-ph↗

arXiv1999

Charged current weak production of the Delta resonance

The charge changing weak production of Delta in nucleons and nuclei is studied. The reactions e^{-} p -> Delta^{0} ν_{e} and e^{+} p -> Delta^{++} \barν_{e} are considered as a possible source of information about the weak N-Delta transition form factors. The low q^2 BNL data on neutrino production of Delta are used to extract the axial vector N-Delta coupling, taking into account the deuteron structure and the Delta width. Finally, pion production induced by neutrinos in ^{16}O in the Delta region, relevant to atmospheric neutrino experiments, is investigated.

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arXiv1999

Chiral symmetry in electromagnetically induced resonant two hadron production

We report on recent progress on the chiral unitary approach, analogous to the effective range expansion in Quantum Mechanics, which is shown to have a much larger convergence radius than ordinary chiral perturbation theory, allowing one to reproduce data for meson meson interaction up to 1.2 GeV. Applications to physical processes so far unsuited for a standard chiral perturbative approach are presented, in which two hadrons, either two mesons or a meson and a baryon, in a resonant state are produced in photoproduction processes.

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arXiv1999

Scalar Isoscalar pion pairs in nuclei and the A(pi,pipi)X reaction

The reaction A(pi,pi pi)X has been studied at low energies, paying particular attention to the interaction of the two final pions in the scalar isoscalar (I=J=0) channel. We have developed a microscopic model for the pion production, and then implemented the two pion final state interaction by using the results of a non-perturbative unitary coupled-channels method based in the standard chiral Lagrangians. The resulting model, describes well the reaction on the nucleon for all different isospin channels. Finally, we have considered the reaction in nuclei. Our calculation takes into account Fermi motion, Pauli blocking, pion absorption, and also the strong modification of the pi-pi interaction in the nuclear medium.

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arXiv1999

The properties of \bar{K} in the nuclear medium

The self-energy of the K^- meson in nuclear matter is calculated in a self-consistent microscopic approach, using a \bar{K}N interaction obtained from the lowest-order meson-baryon chiral Lagrangian. The effective \bar{K}N interaction in the medium is derived by solving the coupled-channel Bethe-Salpeter equation including Pauli blocking on the nucleons, mean-field binding potentials for the baryons and the self-energy of the πand \bar{K} mesons. The incorporation of the self-consistent {\bar K} self-energy in the description, in addition to the Pauli blocking effects, yields a weaker attractive in-medium {\bar K}N interaction and a Λ(1405) which dissolves faster with increasing matter density, as a result of the {\bar K} spectral function being spread out over a wide range of energies. These effects are further magnified when the intermediate pions are dressed.

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arXiv1999

N/D Description of Two Meson Amplitudes and Chiral Symmetry

The most general structure of an elastic partial wave amplitude when the unphysical cuts are neglected is deduced in terms of the N/D method. This result is then matched to lowest order, ${\mathcal{O}}(p^2)$, Chiral Perturbation Theory($χ$PT) and to the exchange (consistent with chiral symmetry) of resonances in the s-channel. The extension of the method to coupled channels is also given. Making use of the former formalism, the $ππ$ and $Kπ$(I=1/2) P-wave scattering amplitudes are described without free parameters when taking into account relations coming from the 1/$N_c$ expansion and unitarity. Next, the scalar sector is studied and good agreement with experiment up to $\sqrt{s}=1.4$ GeV is found. It is observed that the $a_0(980)$, $σ$ and $κ(900)$ resonances are meson-meson states originating from the unitarization of the ${\mathcal{O}}(p^2)$ $χ$PT amplitudes. On the other hand, the $f_0(980)$ is a combination of a strong S-wave meson-meson unitarity effect and of a preexisting singlet resonance with a mass around 1 GeV. We have also studied the size of the contributions of the unphysical cuts to the $ππ$(I=0) and $Kπ$(I=1/2) elastic S-wave amplitudes from $χ$PT and the exchange of resonances in crossed channels up to $\sqrt{s}\approx 800$ MeV. The loops are calculated as in $χ$PT at next to leading order. We find a small correction from the unphysical cuts to our calculated partial waves.

hep-ph↗

arXiv1999

Photoproduction of scalar mesons on protons and nuclei

We study the photoproduction of scalar mesons close to the threshold of f_0(980) and a_0(980) using a unitary chiral model. Peaks for both resonances show up in the invariant mass distributions of pairs of pseudoscalar mesons. A discussion is made on the photoproduction of these resonances in nuclei, which can shed light on their nature, a subject of continuous debate.

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arXiv1999

Radiative production of the Lambda(1405) resonance in K collisions on protons and nuclei

We have carried a theoretical study of the K^- p\to M B γreaction with M B = K^-p, \bar{K}^0 n, π^- Σ^+, π^+ Σ^-, π^0 Σ^0, π^0 Λ, for K^- lab. momenta between 200 and 500 MeV/c, using a chiral unitary approach for the strong K^-p interaction with its coupled channels. The Λ(1405) resonance, which is generated dynamically in this approach, shows up clearly in the dσ/dM_I spectrum, providing new tests for chiral symmetry and the unitary approach, as well as information regarding the nature of the resonance. The photon detection alone, summing all channels, is shown to reproduce quite accurately the strength and shape of the Λ(1405) resonance. Analogous reactions in nuclei can provide much information on the properties of this resonance in a nuclear medium.

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