The compositeness of weakly bound states is discussed using the effective field theory from the viewpoint of the low-energy universality. We introduce a model with coupling of the single-channel scattering to the bare state, and study the compositeness of the bound state by varying the bare state energy. In contrast to the naive expectation that the near-threshold states are dominated by the molecular structure, we demonstrate that a noncomposite state can always be realized even with a small binding energy. At the same time, however, it is shown that a fine tuning is necessary to obtain the noncomposite weakly bound state. In other words, the probability of finding a model with the composite dominant state becomes larger with the decrease of the binding energy, in accordance with the low-energy universality. For the application to exotic hadrons, we then discuss the modification of the compositeness due to decay and coupled-channels effects. We quantitatively show that these contributions suppress the compositeness, because of the increase of the fraction of other components. Finally, as examples of near-threshold exotic hadrons, the structures of $T_{cc}$ and $X\left(3872\right)$ are studied by evaluating the compositeness. We find the importance of the coupled-channels and decay contributions for the structures of $T_{cc}$ and $X\left(3872\right)$, respectively.

中文翻译：

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考虑衰减和耦合通道效应的 TccandX(3872) 的复合性

从低能普适性的角度，利用有效场论讨论了弱束缚态的复合性。我们引入了单通道散射与裸态耦合的模型，并通过改变裸态能量来研究束缚态的复合性。与近阈值态由分子结构主导的天真预期相反，我们证明即使结合能很小，也总是可以实现非复合态。然而，同时表明，需要进行微调才能获得非复合弱束缚态。换句话说，根据低能普适性，随着结合能的降低，找到具有复合主导态的模型的概率变得更大。对于奇异强子的应用，我们然后讨论由于衰变和耦合通道效应而对复合性的修改。我们定量地表明，由于其他成分比例的增加，这些贡献抑制了复合性。最后，作为近阈值奇异强子的例子，${\mathrm{时间}}_{CC}$和$X（3872）$通过评估复合性进行研究。我们发现耦合通道和衰变贡献对结构的重要性${\mathrm{时间}}_{CC}$和$X（3872）$， 分别。