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.
The Λ(1405) baryon resonance plays an outstanding role in various aspects in hadron and nuclear physics. It has been considered that the Λ(1405) resonance is generated by the attractive interaction of the antikaon and the nucleon as a quasi-bound state below its threshold decaying into the πΣ channel. Thus, the structure of Λ(1405) is closely related to theKN interaction which is the fundamental ingredient to study few-body systems with antikaon. In this paper, after reviewing the basic properties of the Λ(1405) resonance, we introduce the dynamical coupled-channel model which respects chiral symmetry of QCD and the unitarity of the scattering amplitude. We show that the structure of the Λ(1405) resonance is dominated by the meson-baryon molecular component and is described as a superposition of two independent states. The meson-baryon nature of Λ(1405) leads to various hadronic molecular states in few-body systems with strangeness which are hadron composite systems driven by the hadronic interactions. We summarize the recent progress in the investigation of the Λ(1405) structure and future perspective of the physics of the Λ(1405) resonance.
We study the origin of the resonances associated with pole singularities of the scattering amplitude in the chiral unitary approach. We propose a "natural renormalization" scheme using the low-energy interaction and the general principle of the scattering theory. We develop a method to distinguish dynamically generated resonances from genuine quark states [Castillejo-Dalitz-Dyson (CDD) poles] using the natural renormalization scheme and phenomenological fitting. Analyzing physical meson-baryon scatterings, we find that the Lambda(1405) resonance is largely dominated by the meson-baryon molecule component. In contrast, the N(1535) resonance requires a sizable CDD pole contribution, while the effect of the meson-baryon dynamics is also important.Comment: RevTeX4, 14 pages, 4 figures, 4 tables, title changed by editors, final version to appear in Phys. Rev.
We examine flavor SU(3) breaking effects on meson-baryon scattering amplitudes in the chiral unitary model. It turns out that the SU(3) breaking, which appears in the leading quark mass term in the chiral expansion, can not explain the channel dependence of the subtraction parameters of the model, which are crucial to reproduce the observed scattering amplitudes and resonance properties.PACS numbers: 12.39. Fe, 11.80.Gw, 14.20.Gk, 14.20.Jn, 11.30.Hv Keywords: chiral unitary approach, meson-baryon scatterings, flavor SU(3) breaking Properties of baryonic excited states are investigated with great interest both theoretically and experimentally. Recently, the chiral unitary model has been successfully applied to this problem, especially to the first excited states of negative parity (J P = 1/2 − ) such as Λ(1405) and N (1535) [1,2,3,4,5,6,7]. In this method, based on the leading order interactions of the chiral Lagrangian and the unitarization of the S-matrix, the baryon resonances are dynamically generated as quasi-bound states of ground state mesons and baryons. It reveals the importance of chiral dynamics not only in the threshold but also in the resonance energy region.In the chiral unitary model for the meson-baryon scattering, we consider the coupled channel scatterings of the octet mesons and baryons. Imposing the unitarity condition on the scattering amplitudes T ij in the N/D method, we obtain the scattering equation in the matrix form Refs. [3,8]:where V ij denotes the elementary tree level interaction derived from the chiral Lagrangian. This equation can be solved algebraically. The loop integral G i is the fundamental building block in the chiral unitary model and are regularized by the dimensional regularization;with Ln ±± ≡ ln(±s ± (M 2 i − m 2 i ) + 2 √ sq i )), the masses of baryon and meson M i and m i , the three-momentum of the mesonq i , the total energy in the center of mass system
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