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Register a new userOn the Existence of $N^*(890)$ Resonance in $S_{11}$ Channel of $pi N$ Scatterings
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Low-energy partial-wave $pi N$ scattering data is reexamined with the help of the production representation of partial-wave $S$ matrix, where branch cuts and poles are thoroughly under consideration. The left-hand cut contribution to the phase shift is determined, with controlled systematic error estimates, by using the results of $mathcal{O}(p^3)$ chiral perturbative amplitudes obtained in the extended-on-mass-shell scheme. In $S_{11}$ and $P_{11}$ channels, severe discrepancies are observed between the phase shift data and the sum of all known contributions. Statistically satisfactory fits to the data can only be achieved by adding extra poles in the two channels. We find that a $S_{11}$ resonance pole locates at $sqrt{z_{r}}=(0.895pm0.081)-(0.164pm0.023)i$ GeV, on the complex $s$-plane. On the other hand, a $P_{11}$ virtual pole, as an accompanying partner of the nucleon bound-state pole, locates at $sqrt{z_{v}}=(0.966pm0.018)$ GeV, slightly above the nucleon pole on the real axis below threshold. Physical origin of the two newly established poles is explored to the best of our knowledge. It is emphasized that the $mathcal{O}(p^3)$ calculation greatly improves the fit quality comparing with the previous $mathcal{O}(p^2)$ one.
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We present a dispersive representation of the $gamma Nrightarrow pi N$ partial-wave amplitude based on unitarity and analyticity. In this representation, the right-hand-cut contribution responsible for $pi N$ final-state-interaction effect are taken into account via an Omnes formalism with elastic $pi N$ phase shifts as inputs, while the left-hand-cut contribution is estimated by invoking chiral perturbation theory. Numerical fits are performed in order to pin down the involved subtraction constants. It is found that good fit quality can be achieved with only one free parameter and the experimental data of the multipole amplitude $E_{0}^+$ in the energy region below the $Delta(1232)$ are well described. Furthermore, we extend the $gamma Nrightarrow pi N$ partial-wave amplitude to the second Riemann sheet so as to extract the couplings of the $N^ast(890)$. The modulus of the residue of the multipole amplitude $E_{0}^+$ ($S_{11pE}$) is $2.41rm{mfmcdot GeV^2}$ and the partial width of $N^*(890)togamma N$ at the pole is about $0.369 {rm MeV}$, which is almost the same as the one of $N^*(1535)$, indicating that $N^ast(890)$ strongly couples to $pi N$ system.
A new unitarization approach incorporated with chiral symmetry is established and applied to study the $pi K$ elastic scatterings. We demonstrate that the $kappa$ resonance exists, if the scattering length parameter in the I=1/2, J=0 channel does not deviate much from its value predicted by chiral perturbation theory. The mass and width of the $kappa$ resonance is found to be $M_kappa=594pm 79MeV$, $Gamma_kappa=724pm 332MeV$, obtained by fitting the LASS data up to 1430MeV. Better determination to the pole parameters is possible if the chiral predictions on scattering lengths are taken into account.
We have developed a model for the N N --> N N pi pi reaction and evaluated cross sections for the different charged channels. The low energy part of those channels where the pions can be in an isospin zero state is dominated by N* excitation, driven by an isoscalar source recently found experimentally, followed by the decay N* --> N (pi pi, T=0, s-wave). At higher energies, and in channels where the pions are not in T=0, Delta excitation mechanisms become relevant. A rough agreement with the experimental data is obtained in most channels. Repercussions of the present findings for the ABC effect and the p p --> p p pi0 reaction close to threshold are also suggested.
We present in this talk a recent investigation on $phi$ photoproduction, emphasizing the rescattering effects of the $KLambda^*$ channel near the threshold region. We discuss the results of the differential cross section and the angular distributions.
We study the pi N --> phi N reaction close to the phi N threshold within the chiral unitary approach, by combining the pi^- p --> K^+ Sigma^-, pi^- p --> K^0 Sigma^0 and pi^- p --> K^0 Lambda amplitudes with the coupling of the phi to the K components of the final states of these reactions via quantum loops. We obtain a good agreement with experiment when the dominant pi^- p --> K^0 Lambda amplitude is constrained with its experimental cross section. We also evaluate the coupling of the N*(1535) to phi N and find a moderate coupling as a consequence of partial cancellation of the large KY components of the N*(1535). We also show that the N*(1535) pole approximation is too small to reproduce the measured cross section for the pi N --> phi N reaction.
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