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In the past two decades, one of the most puzzling phenomena discovered inhadron physics is that a nominal hadronic state can actually correspond to twopoles on the complex energy plane. This phenomenon was first noticed for the$\Lambda(1405)$, then for $K_1(1270)$, and to a lesser extent for$D_0^*(2300)$. In this Letter, we show explicitly how the two-pole structuresemerge from the underlying universal chiral dynamics describing thecoupled-channel interactions between heavy matter particles and pseudoNambu-Goldstone bosons. In particular, the fact that two poles appear inbetween the two dominant coupled channels can be attributed to the particularform of the leading order chiral potentials of the Weinberg-Tomozawa form.Their lineshapes overlap with each other because the degeneracy of the twocoupled channels is only broken by explicit chiral symmetry breaking of higherorder. We predict that for light-quark~(pion) masses heavier than theirphysical values (e.g., about 200 MeV in the $\Lambda(1405)$ case studied), thelower pole becomes a virtual state, which can be easily verified by futurelattice QCD simulations. Furthermore, we anticipate similar two-pole structuresin other systems, such as the isopin $1/2$ $\bar{K}\Sigma_c-\pi\Xi'_c$ coupledchannel, which await for experimental discoveries.

Two-pole structures as a universal phenomenon dictated by coupled-channel chiral dynamics