The neutral pion photoproduction on the proton near threshold has a very small scattering cross section when compared to the charged channels, which in ChPT is explained by strong cancellations between the lowest order pieces. Therefore it is very sensitive to higher-order corrections of chiral perturbation theory. We perform a fully covariant calculation up to chiral order p 3 and we investigate the effect of the inclusion of the ∆(1232) resonance as an explicit degree of freedom. We show that the convergence improves, leading to a much better agreement with data at a wide range of energies.
Meson-Baryon Interactions can be successfully described using both Chiral
Symmetry and Unitarity. The $s-$wave meson-baryon scattering amplitude is
analyzed in a Bethe-Salpeter coupled channel formalism incorporating Chiral
Symmetry in the potential. Two body coupled channel unitarity is exactly
preserved. The needed two particle irreducible matrix amplitude is taken from
lowest order Chiral Perturbation Theory in a relativistic formalism. Off-shell
behavior is parameterized in terms of low energy constants. The relation to the
heavy baryon limit is discussed. The position of the complex poles in the
second Riemann sheet of the scattering amplitude determine masses and widths
baryonic resonances of the N(1535), N(1670), $\Lambda (1405)$ and
$\Lambda(1670)$ resonances which compare well with accepted numbers.Comment: 14 pages, 3 figures; Talk at 2nd International Workshop On Hadron
Physics: Effective Theories Of Low-Energy QCD, Coimbra, Portugal, 25-29 Sep
2002. (To appear in the proceedings.) Presented by E. Ruiz Arriol
Abstract. The s−wave meson-baryon scattering amplitude is analyzed for the strangeness S = −1 and isospin I = 0 sector in a Bethe-Salpeter coupled channel formalism incorporating Chiral Symmetry. Four two-body channels have been considered:KN , πΣ, ηΛ, KΞ. The needed two particle irreducible matrix amplitude is taken from lowest order Chiral Perturbation Theory in a relativistic formalism. Off-shell behaviour is parameterized in terms of low energy constants, which outnumber those assumed in previous works and provide a better fit to the data. The position of the complex poles in the second Riemann sheet of the scattering amplitude determine masses and widths of the Λ(1405) and Λ(1670) resonances which compare well with accepted numbers.
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