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Pole analysis on unitarized $SU(3)\times SU(3)$ one loop $χ$PT amplitudes

We analyze $ππ-K\bar{K}$ and $πη-K\bar{K}$ couple channel [1,1] matrix Padé amplitudes of $SU(3)\times SU(3)$ chiral perturbation theory. By fitting phase shift and inelasticity data, we determine pole positions in different channels ($f_0(980)$, $a_0(980)$,$f_0(600)$, $K_0^*(800)$, $K^*(892)$, $ρ(770)$) and trace their $N_c$ trajectories. We stress that a couple channel Breit--Wigner resonance should exhibit two poles on different Riemann sheets and meet each other on the real axis when $N_c=\infty$. Poles are hence classified using this criteria and we conclude that $K^*(892)$ and $ρ(770)$ are unambiguous Breit--Wigner resonances. For scalars the situation is much less clear. We find that $f_0(980)$ is a molecular state rather than a Breit--Wigner resonance, while $a_0(980)$, though behaves oddly when varying $N_c$, does maintain a twin pole structure.