A Paradigm for the Coupled-Channel Origin of Resonances: the Exotic $T_{c\bar{s}}$ in $D_{s1}(2460/2536)\to D_s\pi\pi$

Abstract

The $T_{c\bar{s}}$ state observed in the decay $D_{s1}(2460)^+ \to D_s^+\pi^+\pi^-$ provides direct evidence for an isovector open-charm tetraquark state with strangeness--a discovery that demands a systematic framework connecting its origin to the nature of the parent $D_{s1}$. We successfully achieve this connection by two mechanisms, triangle loops and the coupled channel of $DK$-$D_s\pi$ with pure off-diagonal potentials. We first point out the behavior of propagator of $D_s\pi$ will influence the effective potential of $DK\to DK$, then we can successfully obtain the pole of $T_{c\bar{s}}$ on the second Reimann Sheet. By combing with the $\pi\pi$-$KK$ rescattering, not only the two-peak structure in $D_{s1}(2460)$ decay is well reproduced, but also a single-peak structure is predicted in $D_{s1}(2536)$ decay. The marked difference, testable at LHCb and Belle II, is driven by the $S$-wave versus $D$-wave nature of their $D^*K$ couplings, revealing the underlying structural distinction between the two $D_{s1}$ states. By directly linking hadronic structure to decay patterns, this work provides a template for deciphering the nature of such exotic states. More broadly, by revealing how non-perturbative coupled-channel effects manifest in exotic hadrons, our analysis connects to a universal mechanism shared by systems ranging from halo nuclei to atomic Feshbach resonances, offering a unified perspective across these fields.