arXiv subjects

Takuma Nishibuchi

Publications and source records attributed to Takuma Nishibuchi.

5 recordsLinked to original sources

arXiv2025

Effect of eigenstates on spectra in coupled-channel scattering with the chiral unitary model

In recent years, as experimental data on the excited $Ξ(1620)$ and $Ξ(1690)$ states have accumulated, theoretical analyses based on chiral dynamics have also been actively pursued. In this study, we construct theoretical models within the chiral unitary approach to reproduce recent experimental data, and perform a model interpolation to examine the relationship between the poles appearing in different models. We then calculate the invariant mass spectra of the $π^+Ξ^-$ system in the $Ξ_c \to ππΞ$ decay, using the scattering amplitudes obtained from each model, to investigate how the distinct pole structures influence the observable spectrum.

hep-ph↗

arXiv2025

Eigenstates in coupled-channel scattering amplitude and their effects on spectrum

In general, discrete eigenstates such as resonances are represented by poles of the scattering amplitude, analytically continued to the complex energy plane. In multi-channel scattering, however, the Riemann surface becomes more complicated, leading to the emergence of various types of poles with distinct characteristics. In this study, we investigate the relationship between poles located on different Riemann sheets and analyze how they influence the observable spectra. In particular, we clarify the effect of the decay channel on the pole trajectory, where an $s$-wave bound state evolves into a resonance via a virtual state. It is shown that the quasibound state pole below the threshold does not continuously connected to the resonance pole above the threshold, and a kind of interchange of poles occurs. As a concrete example, we consider several models based on the chiral unitary approach that describe meson-baryon scattering amplitudes involving the $Ξ(1620)$ and $Ξ(1690)$ resonances. We examine their impact on the $πΞ$ invariant mass distributions in the $Ξ_{c} \to ππΞ$ decay, discussing how the pole structure manifests itself in experimental observables.

hep-ph↗

arXiv2023

Analysis of $Ξ(1620)$ resonance with chiral unitary approach

Recently, the $Ξ(1620)$ resonance has attracted much attention, thanks to detailed experimental data by the Belle and ALICE collaborations. This experimental progress has prepared us to conduct theoretical analyses based on experimental data. In this study, we analyze the $Ξ(1620)$ resonance using the chiral unitary model and discuss the properties of the $Ξ(1620)$ resonance and the $K^-Λ$ scattering length.

hep-ph↗

arXiv2023

Analysis of $Ξ(1620)$ resonance and $\bar{K}Λ$ scattering length with chiral unitary approach

We study the $Ξ(1620)$ resonance near the $\bar{K}Λ$ threshold in the light of the recent experimental constraints. The Belle collaboration have found a resonance peak of $Ξ(1620)$ slightly below the $\bar{K}^{0}Λ$ threshold in the $π^{+}Ξ^{-}$ invariant mass spectrum, and the ALICE collaboration have determined the $K^{-}Λ$ scattering length from the measurement of the momentum correlation functions in the heavy ion collisions. Using the effective range expansion, we classify the nature of the pole of the near-threshold eigenstate in terms of the scattering length, in the presence of the decay channel. It is shown that the quasibound state below the threshold can be described by only the scattering length, while the description of the resonance above the threshold requires the contribution from the effective range. Based on the chiral unitary approach, we construct a theoretical model which generates the pole of $Ξ(1620)$ below the $\bar{K}Λ$ threshold with relatively narrow width, as reported by the Belle collaboration. It is quantitatively demonstrated that the spectrum of the $Ξ(1620)$ quasibound state is distorted by the effect of the nearby $\bar{K}Λ$ threshold. We then construct another model which reproduces the $K^{-}Λ$ scattering length by the ALICE collaboration. In this case, the eigenstate pole does not appear in the physically relevant Riemann sheets, and the spectrum shows a cusp structure at the $\bar{K}Λ$ threshold. We finally examine the compatibility of the value of the $\bar{K}Λ$ scattering length and the subthreshold pole of $Ξ(1620)$ including the experimental uncertainties.

hep-ph↗

arXiv2022

Nature of excited $Ξ$ baryons with threshold effects

Spectroscopy of excited baryons with strangeness $S=-2$ is stimulated by recent experimental developments. Here we focus on the $Ξ(1620)$ which locates close to the $\bar{K}Λ$ threshold. To take into account the threshold effects, we construct the coupled-channels meson-baryon scattering amplitude where the $Ξ(1620)$ appears as a resonance. We demonstrate that the $\bar{K}Λ$ threshold effects distort the peak of the $Ξ(1620)$ resonance from the simple Breit-Wigner distribution.

hep-ph↗