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E. Oset

Publications and source records attributed to E. Oset.

At least 433 records · Page 24Linked to original sources
arXiv2002

The sigma meson in a nuclear medium through two pion photoproduction

We present theoretical results for (gamma,pi0 pi0) and (gamma,pi+- pi0) production on nucleons and nuclei in the kinematical region where the scalar isoscalar pi pi amplitude is influenced by the sigma pole. The final state interaction of the pions modified by the nuclear medium produces a spectacular shift of strength of the pi0 pi0 invariant mass distribution induced by the moving of the sigma pole to lower masses and widths as the nuclear density increases.

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arXiv2002

$η$ bound states in nuclei

The energies and widths of bound states of the $η$ meson in different nuclei are obtained using the results for its selfenergy in a nuclear medium, which is evaluated in a selfconsistent manner using techniques of unitarized chiral perturbation theory. We find bound states in all studied nuclei (from $^{12}{C}$ on) and the half widths obtained are larger than the separation of the levels, what makes the experimental observation of peaks unlikely. We have paid a special attention to the region of nuclei where only the $1s$ state appears and the binding energies are of the order of magnitude of the half width, which would magnify the chances that some broad peak could be observed. This is found in the region of $^{24}{Mg}$ with a binding energy around 12.6 MeV and half width of 16.7 MeV. In heavy nuclei like $^{208}{Pb}$ there are many bound states which would be difficult to disentangle and the deepest state has a binding energy about 21 MeV and half width around 16 MeV. Such an overlapping accumulation of states could be seen as an extension of the continuum of $η$ strength into the bound region in $η$ production experiments.

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arXiv2002

eta in the nuclear medium within a chiral unitary approach

The self-energy of an eta meson in the nuclear medium is calculated in a chiral unitary approach. A coupled channel Bethe-Salpeter equation is solved to obtain the effective eta-N interaction in the medium. The base model reproduces well the free space pi-N elastic and inelastic scattering at the eta-N threshold or N^*(1535) region. The Pauli blocking on the nucleons, binding potentials for the baryons and self-energies of the mesons are incorporated, including the eta self-energy in a self-consistent way. Our calculation predicts about -54 -i29 MeV for the optical potential at normal nuclear matter for an eta at threshold but also shows a strong energy dependence of the potential.

hep-ph↗
arXiv2002

Chiral loops and VMD in the V->PPgamma decays

We evaluate radiative decays of $ρ$ and $ω$ going to two neutral mesons, $π^{0} π^{0}$ and $π^{0} η$. We use the sequential vector decay mechanisms in addition to chiral loops and $ρ$-$ω$ mixing. The chiral loops are obtained using elements of $UχPT$ successfully applied in the study of meson-meson interactions up to 1.2 GeV. The chiral loops are found very important in the case of the $ρ\to π^{0} π^{0} γ$ decay and small in the other cases. A good agreement with present measurements of $ρ\to π^{0} π^{0} γ$ and $ω\to π^{0} π^{0}γ$ is obtained and predictions are made for the other decays where the rates obtained are rather small.

hep-ph↗
arXiv2002

On the spin, parity and nature of the $Ξ(1620)$ resonance

Using a unitary extension of chiral perturbation theory with a lowest-order s-wave SU(3) chiral Lagrangian we study low-energy meson-baryon scattering in the strangeness $S=-2$ sector. A scattering-matrix pole is found around 1606 MeV which corresponds to an s-wave $Ξ$ resonance with $J^P=1/2^-$. We identify this resonance with the $Ξ(1620)$ state, quoted in the Particle Data Book with $I=1/2$ but with unknown spin and parity. The addition of the $S=-2$ state to the recently computed $Λ(1670)$, $Σ(1620)$ and N(1535) states completes the octet of $J^P=1/2^-$ resonances dynamically generated in this chiral unitary approach.

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arXiv2002

Magnetic moments of the Lambda(1405) and Lambda(1670) resonances

By using techniques of unitarized chiral perturbation theory, where the $Λ(1405)$ and $Λ(1670)$ resonances are dynamically generated, we evaluate the magnetic moments of these resonances and their transition magnetic moment. The results obtained here differ appreciably from those obtained with existing quark models. The width for the $Λ(1670) \to Λ(1405) γ$ transition is also evaluated leading to a branching ratio of the order of $2 \times 10^{-6}$.

hep-ph↗
arXiv2002

Low lying S=-1 excited baryons and chiral symmetry

The s-wave meson-baryon interaction in the $S = -1$ sector is studied by means of coupled-channels, using the lowest-order chiral Lagrangian and the N/D method to implement unitarity. The loops are regularized using dimensional renormalization. In addition to the previously studied $Λ(1405)$, employing this chiral approach leads to the dynamical generation of two more s-wave hyperon resonances, the $Λ(1670)$ and $Σ(1620)$ states. We make comparisons with experimental data and look for poles in the complex plane obtaining the couplings of the resonances to the different final states. This allows us to identify the $Λ(1405)$ and the $Λ(1670)$ resonances with $\bar{K}N$ and $KΞ$ quasibound states, respectively.

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arXiv2002

Chiral approach to the rho meson in nuclear matter

In this work, the properties of the $ρ$ meson at rest in cold symmetric nuclear matter are studied. We make use of a chiral unitary approach to pion-pion scattering in the vector-isovector channel, calculated from the lowest order Chiral Perturbation Theory ($χPT$) lagrangian including explicit resonance fields. Low energy chiral constraints are considered by matching our expressions to those of one loop $χPT$. To account for the medium corrections, the $ρ$ couples to $ππ$ pairs which are properly renormalized in the nuclear medium, accounting for both $p-h$ and $Δ-h$ excitations. The terms where the $ρ$ couples directly to the hadrons in the $p-h$ or $Δ-h$ excitations are also accounted for. In addition, the $ρ$ is also allowed to couple to $N^{*}(1520)-h$ components.

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arXiv2002

Deeply bound pionic atoms from the $(γ,p)$ reaction in nuclei

We study the $(γ,p)$ reaction on $^{208}Pb$ leading to $^{207}Pb$ with a bound pion attached to it in the lowest 1s or 2p pionic levels. The reaction can be made recoilless to optimize the production cross section but we must choose a bit higher photon energy to overcome the Coulomb barrier in the proton emission. The cross sections obtained are easily measurable and can be larger than 50 per cent of the background from inclusive ($γ$,p). This makes it a clear case for the detection of the pionic atom signals, converting this reaction into a practical tool to produce deeply bound pionic atoms.

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arXiv2001

Nuclear medium effects on the sigma mass and width

The meson meson interaction in the scalar isoscalar channel at finite baryonic density is studied in the framework of a chiral unitary approach which generates the f0 and sigma resonances and reproduces well the meson meson phase shifts in vacuum. We investigate the sigma spectral function, its mass and its width as a function of the baryon density and discuss possible experimental signatures in the (pi,2pi) and the (gamma,2pi) reactions.

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arXiv2001

Two pion photoproduction on nucleons and nuclei in the rho and sigma regions

In this talk we report on two physical processes, the photoproduction of $ρ$ mesons in nuclei and the $π^0 π^0$ photoproduction in nuclei at low energies. In the first case the aim is to observe experimentally the theoretically predicted changes on the $ρ$ properties in the nuclear medium. In the second case one wishes to investigate the modifications of the $ππ$ interaction in the nuclear medium in the region of the $σ$ meson. In the first case we observe that it is quite difficult to see large effects due to the $ρ$ modification in the medium because the detection of the $ρ$ is done through the two pions which are produced in the $Δ$ region and are largely distorted in the medium. In the second case, when the I=0 part of the $ππ$ interaction is substituted by the in medium $ππ$ amplitude, we observe a very large shift of strength in the invariant mass distribution to small values of the mass in $^{12}C$ and $^{208}Pb$ with respect to the distribution of the elementary reaction. This spectacular shift appears to be corroborated by recent experiments at Mainz reported in this same Workshop.

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arXiv2001

Chiral dynamics and pion-nucleon scattering around the N*(1535) resonance

We study the S-wave interaction of mesons with baryons in the strangeness S=0 sector in a coupled channel unitary approach. The basic dynamics is drawn from the lowest order meson baryon chiral Lagrangians. Small modifications inspired by models with explicit vector meson exchange in the t-channel are also considered. In addition the pi pi N channel is included and shown to have an important repercussion in the results, particularly in the isospin 3/2 sector.

hep-ph↗
arXiv2001

Chiral unitary approach to S-wave meson baryon scattering in the strangeness S=0 sector

We study the S-wave interaction of mesons with baryons in the strangeness S=0 sector in a coupled channel unitary approach. The basic dynamics is drawn from the lowest order meson baryon chiral Lagrangians. Small modifications inspired by models with explicit vector meson exchange in the t-channel are also considered. In addition the pi pi N channel is included and shown to have an important repercussion in the results, particularly in the isospin 3/2 sector.

hep-ph↗
arXiv2001

Non-localities and Fermi motion corrections in $K^-$ atoms

We evaluate the p-wave $K^-N$ amplitudes from the chiral Lagrangians and from there construct the p-wave part of the $K^-$ nucleus optical potential plus a small s-wave part induced from the elementary p-wave amplitude and the nuclear Fermi motion. Simultaneously, the momentum and energy dependence of the s-wave optical potential, previously developed, are taken into account and shown to generate a small p-wave correction to the optical potential. All the corrections considered are small compared to the leading s-wave potential, and lead to changes in the shifts and widths which are smaller than the experimental errors. A thorough study of the threshold region and low densities is conducted, revealing mathematical problems for which a physical solution is given.

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arXiv2001

Optical Potentials in Kaonic Atoms

A microscopic optical potential based on a chiral model is used as a starting point for studying kaonic atoms levels. We add to this potential a phenomenological part fitted to the experimentally known shifts and widths of kaonic levels. This fitted potential is used to predict deeply bound atomic levels, as well as nuclear levels. Comparison with the predictions of other optical models found in the literature is done. Also the effects on the kaonic atoms levels of certain known non-local contributions to the optical potential are also analyzed.

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arXiv2001

Chiral dynamics and the reactions pp \to d K^+ \bar{K}^0 and pp \to d π^+ η

We perform a study of the final state interactions of the K^+ \bar{K^0} and the \bar{K^0}d systems in the reactions pp \to d K^+ \bar{K^0} and pp \to d π^+ η. Since the two-meson system couples strongly to the a_0(980) resonance, these reactions are expected to be an additional source of information about the controversial scalar sector. We also show that these reactions present peculiar features which can shed additional light on the much debated meson-baryon scalar sector with strangeness -1. We deduce the general structure of the amplitudes close to the d K^+\bar{K}^0 threshold, allowing for primary K^+ \bar{K^0} as well as π^+ηproduction with the two mesons in relative S- or P-wave. The interactions of the mesons are accounted for by using chiral unitary techniques, which generate dynamically the a_0(980) resonance, and the \bar{K^0}d interaction is also taken into account. General formulae are derived that allow to incorporate the final state interactions in these systems for any model of the production mechanism. We illustrate this approach by considering two specific production mechanisms based on three flavor meson-baryon chiral perturbation theory. It is demonstrated that in this scenario the \bar{K^0}d interactions are very important and can change the cross section by as much as one order of magnitude. The amount of π^+ηversus K^+ \bar{K^0} production is shown to depend critically on the primary mixture of the two mechanisms, with large interference effects due to final state interactions. These effects are also shown to occur in the event distributions of invariant masses which are drastically modified by the final state interactions of the two-meson or the \bar{K}d system.

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arXiv2001

Dynamical meson-baryon resonances with chiral Lagrangians

The s-wave meson-baryon interaction is studied using the lowest-order chiral Lagrangian in a unitary coupled-channels Bethe-Salpeter equation. In the strangeness $S=-1$ sector the low-energy $K^- p$ dynamics leads to the dynamical generation of the $Λ(1405)$ as a ${\bar K}N$ state, along with a good description of the $K^- p$ scattering observables. At higher energies, the $Λ(1670)$ is also found to be generated dynamically as a $K Ξ$ quasibound state for the first time. For strangeness S=0, it is the $S_{11}(1535)$ resonance that emerges from the coupled-channels equations, leading to a satisfactory description of meson-baryon scattering observables in the energy region around the $S_{11}(1535)$. We speculate on the possible dynamical generation of $Ξ$ resonances within the chiral $S=-2$ sector as ${\bar K} Λ$ or ${\bar K} Σ$ quasibound states.

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