We employ the Born-Oppenheimer approximation to the T c s ¯ 0 a ( 2900 ) 0 and T c s ¯ 0 a ( 2900 ) + + states observed by the LHCb Collaboration and study mass spectrum and root-mean-square radius values. For this purpose, we use the dynamical diquark model. We assume that a strange quark is heavy for the usage of the Born-Oppenheimer approximation. Our results strongly indicate that the T c s ¯ 0 a ( 2900 ) states are best described as composed of axial-vector (spin-1) diquark pairs. Furthermore, the calculated root-mean-square radius, ⟨ r 2 ⟩ 1 / 2 ≈ 0.70 – 0.80 fm , which is significantly less than 1 fm, provides compelling evidence that these are compact tetraquarks rather than loosely bound hadronic molecules.
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