Tcc in the Diabatic Diquark Model: Effects of D^*D Isospin

Tcc⁺ is an isoscalar 4-quark state with mass lying barely below the D*+ D⁰ threshold, and several times further below the D*0 D⁺ threshold. It allows both di-meson molecular and elementary diquark-antidiquark (cc)( u d) substructures. The diabatic generalization of the adiabatic approximation within the Born-Oppenheimer formalism rigorously incorporates the mixing of such elementary eigenstates with states corresponding to two-particle thresholds. We examine the separate influence of the two D^* \! D isospin channels and find that the influence of D*+ D⁰ is larger than that of D*0 D⁺ but not overwhelmingly so, and that Tcc⁺ contains an $O(10%)$ (cc)( u d) component. We then explore the variation of these results if the isospin breaking between the di-meson thresholds is varied, and also the sensitivity of our results to variation of the mixing-potential parameters.