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E. Oset

Publications and source records attributed to E. Oset.

At least 415 records · Page 23Linked to original sources
arXiv2003

Determining the $Θ^+$ quantum numbers through the $K^+p\to π^+K^+n$ reaction

We study the $K^+p\to π^+K^+n$ reaction with some kinematics suited to the production of the $Θ^+$ resonance recently observed. We show that, independently of the quantum numbers of the $Θ^+$, a resonance signal is always observed in the $K^+$ forward direction. In addition, we also show how a combined consideration of the strength at the peak, and the angular dependence of polarization observables can help determine the $Θ^+$ quantum numbers using the present reaction.

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arXiv2003

Mass dependence of inclusive nuclear $ϕ$ photoproduction

Based on a prior determination of the $ϕ$ selfenergy in a nuclear medium we perform a theoretical study of inclusive $ϕ$ photoproduction in nuclei, looking at the $A$ dependence of the cross sections for different $ϕ$ momenta. We find sizeable reductions in the nuclear cross sections with respect to the elementary one, using a $ϕ$ selfenergy which implies a width about six times the free one at normal nuclear density. The calculations are done to match the set up for an ongoing experiment at {\it SPring8/Osaka} which should provide valuable information on the renormalization of the $ϕ$ properties in nuclei.

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arXiv2003

$Λ(1405)$ production in the $π^-p\to K^0πΣ$ reaction

We discuss the mechanisms that lead to $Λ(1405)$ production in the $π^-p\to K^0πΣ$ reaction. The problem has gained renewed interest after different works converge to the conclusion that there are two resonances around the region of 1400 MeV, rather than one, and that they couple differently to the $πΣ$ and $\bar{K}N$ channels. We look at the dynamics of that reaction and find two mechanisms which eventually filter each one of the resonances, leading to very different shapes of the $πΣ$ invariant mass distributions. The combination of the two mechanisms leads to a shape of this distribution compatible with the experimental measurements.

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arXiv2003

Sequential vector and axial-vector meson exchange and chiral loops in radiative phi decay

We study the radiative $ϕ$ decay into $π^0 π^0 γ$ and $π^0 ηγ$ taking into account mechanisms in which there are two sequential vector-vector-pseudoscalar or axial-vector--vector--pseudoscalar steps followed by the coupling of a vector meson to the photon, considering the final state interaction of the two mesons. There are other mechanisms in which two kaons are produced through the same sequential mechanisms or from $ϕ$ decay into two kaons and then undergo final state interaction leading to the final pair of pions or $π^0 η$, this latter mechanism being the leading one. The results of the parameter free theory, together with the theoretical uncertainties, are compared with the latest experimental results of KLOE at Frascati.

hep-ph↗
arXiv2003

$η\to π^0 γγ$ decay within unitarized chiral perturbation theory

We improve the calculations of the $η\to π^0 γγ$ decay within the context of meson chiral lagrangians. We use a chiral unitary approach for the meson-meson interaction, thus generating the $a_0(980)$ resonance and fixing the longstanding sign ambiguity on its contribution. This also allows us to calculate the loops with one vector meson exchange, thus removing a former source of uncertainty. In addition we ensure the consistency of the approach with other processes. First, by using vector meson dominance couplings normalized to agree with radiative vector meson decays. And, second, by checking the consistency of the calculations with the related $γγ\to π^0 η$ reaction. We find an $η\to π^0 γγ$ decay width of $0.47\pm 0.10$ eV, in clear disagreement with published data but in remarkable agreement with the most recent measurement.

hep-ph↗
arXiv2003

Chiral Dynamics of the Two Λ(1405) States

Using a chiral unitary approach for the meson--baryon interactions, we show that two octets of J^π=1/2^- baryon states, which are degenerate in the limit of exact SU(3) symmetry, and a singlet are generated dynamically. The SU(3) breaking produces the splitting of the two octets, resulting in the case of strangeness S=-1 in two poles of the scattering matrix close to the nominal Λ(1405) resonance. These poles are combinations of the singlet state and the octets. We show how actual experiments see just one effective resonance shape, but with properties which change from one reaction to another.

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arXiv2003

The $(π^-,γγ)$ reaction in nuclei and the $σ$ meson in the medium

A theoretical analysis of the $(π^-,γγ)$ reaction in nuclei is made in order to find the viability of this reaction to test modifications of the $σ$ meson mass in nuclear matter. The $ππ$ correlation in the scalar-isoscalar channel in nuclear matter could, in principle, manifest itself in this reaction since it plays an important role in the $ππ\toγγ$ mechanism. But we conclude that this effect is hardly visible in this reaction due to the strong background of the pion-Bremsstrahlung terms. Only with some special cuts and for some polarization states are the effect visible at the cost of a strong reduction in the cross section.

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arXiv2002

The $ρNN$ coupling with direct coupling and loops

Starting from a gauge formalism of $ρ$ mesons, pions and baryons we evaluate the $ρ$ coupling to the nucleon, including the direct coupling provided by the Lagrangians, plus contributions from loops with the virtual pion cloud. We find a contribution to the magnetic $ρ$ coupling to the nucleon from pionic loops of the same size as the direct coupling, which is, however, still small compared to the empirical values. This finding goes in line with chiral formulations of the strong interaction of mesons at low energies where, unlike the scalar mesons which are mostly built of a pion (kaon) cloud, the $ρ$ meson stands as a genuine QCD state with intrinsic properties not tied to those of the pion cloud.

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arXiv2002

The phi NN coupling from chiral loops

Starting from effective Lagrangians which combine a gauge formulation of Vector Meson Dominance with Chiral Lagrangians, the coupling of the phi to the nucleon, which is zero at tree level due to the OZI rule, is calculated perturbatively considering loop contributions to the electric and magnetic form factors. We obtain reasonably smaller values for both form factors than those for rho NN and consistent with the expected order of magnitude of the OZI rule violation.

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arXiv2002

Unitarized chiral perturbation theory of hadrons

An exposition is made of recent developments using techniques of unitary chiral perturbation theory, $U χP T$, which allows one to extend predictions using chiral Lagrangians to higher energies than ordinary chiral perturbation theory, including the region of low lying mesonic and baryonic resonances, some of which are dynamically generated in the approach. Results for meson meson scattering, pion and kaon form factors and meson baryon scattering are shown. Applications are done for nuclear problems showing the results for the kaon and eta selfenergies, phi renormalization in a nuclear medium and $σ$ renormalization in the medium, comparing results with recent experiments.

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arXiv2002

eta nucleus optical potential in a chiral unitary approach

The self-energy of an eta in a nuclear medium is calculated in a chiral unitary model, and applied to eta states in nuclei. Our calculation predicts an attractive eta nucleus optical potential which can accommodate many eta bound states in different nuclei.

hep-ph↗
arXiv2002

Chiral dynamics in systems with strangeness

In this talk a brief review of several problems involving systems with strangeness is made. In the first place one shows how the $Λ(1405)$, $Λ(1670)$ and $Σ(1620)$ states, for $S = -1$, and the $Ξ(1620)$ for $S= -2$ are generated dynamically in the context of unitarized chiral perturbation theory. The results for the $\bar{K}N$ interaction are then used to evaluate the $K^- d$ scattering length. Results obtained for the kaon selfenergy in a nuclear medium within this approach, with application to $K^-$ atoms, are also mentioned. Finally a few words are said about recent developments in the weak decay of $Λ$ hypernuclei and the puzzle of the $Γ_n/Γ_p$ ratio.

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arXiv2002

The two pion decay of the Roper resonance

We evaluate the two pion decay of the Roper resonance in a model where explicit re-scattering of the two final pions is accounted for by the use of unitarized chiral perturbation theory. Our model does not include an explicit $ε$ or $σ$ scalar-isoscalar meson decay mode, instead it generates it dynamically by means of the pion re-scattering. The two ways, explicit or dynamically generated, of introducing this decay channel have very different amplitudes. Nevertheless, through interference with the other terms of the model we are able to reproduce the same phenomenology as models with explicit consideration of the $ε$ meson.

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arXiv2002

Chiral unitary approach to hadron spectroscopy

The s-wave meson-baryon interaction in the $S = -1$, $S= 0$ and $S= -2$ sectors is studied by means of coupled channels, using the lowest-order chiral Lagrangian and the N/D method or equivalently the Bethe-Salpeter equation to implement unitarity. This chiral approach leads to the dynamical generation of the $Λ(1405)$, $Λ(1670)$ and $Σ(1620)$ states for $S = -1$, the $N^*(1535)$ for $S= 0$ and the $Ξ(1620)$ for $S= -2$. We look for poles in the complex plane and extract the couplings of the resonances to the different final states. This allows identifying the $Λ(1405)$ and the $Λ(1670)$ resonances with $\bar{K}N$ and $KΞ$ quasibound states, respectively. Our results are found to be incompatible with the measured properties of the $Ξ(1690)$ resonance, thus ruling this state out as the remaining member of this octet of dynamically generated resonances. We therefore assign $1/2^-$ for the spin and parity of the $Ξ(1620)$ resonance as the $S=-2$ member of the lowest-lying $1/2^-$ octet.

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arXiv2002

The $σ$ meson in a nuclear medium through two pion photoproduction

We present theoretical results for $(γ, π^0 π^0)$ production on nucleons and nuclei in the kinematical region where the scalar isoscalar $ππ$ amplitude is influenced by the $σ$ pole. The final state interaction of the pions modified by the nuclear medium produces a spectacular shift of strength of the two pion invariant mass distribution induced by the moving of the $σ$ pole to lower masses and widths as the nuclear density increases.

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arXiv2002

Chiral dynamics of p-wave in K^- p and coupled states

We perform an evaluation of the p-wave amplitudes of meson-baryon scattering in the strangeness S=-1 sector starting from the lowest order chiral Lagrangians and introducing explicitly the Sigma^* field with couplings to the meson-baryon states obtained using SU(6) symmetry. The N/D method of unitarization is used, equivalent, in practice, to the use of the Bethe-Salpeter equation with a cut-off. The procedure leaves no freedom for the p-waves once the s-waves are fixed and thus one obtains genuine predictions for the p-wave scattering amplitudes, which are in good agreement with experimental results for differential cross sections, as well as for the width and partial decay widths of the Sigma^*(1385).

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arXiv2002

Chiral restoration from pionic atoms?

We evaluate widths and shifts of pionic atoms using a theoretical microscopical potential in which the pion decay constant $f_π$ is changed by an in--medium density dependent one ($f_π(ρ)$), predicted by different partial Chiral restoration calculations. We show that the results obtained for shifts and widths are worse than if this modification were not implemented. On the other hand, we argue that in microscopic many body approaches for the pion selfenergy, based on effective Lagrangians, the mechanisms responsible for the change of $f_π$ in the medium should be automatically incorporated. Therefore, the replacement of $f_π$ by $f_π(ρ)$ in the many body derivation of the microscopic potential would be inappropriate.

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