Heavy-light diquark masses from QCD sum rules and constituent diquark models of tetraquarks

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Diquarks with Jᵖ=0±, 1± containing a heavy (charm or bottom) quark and a light quark are investigated using QCD Laplace sum rules. Masses are determined using appropriately constructed gauge invariant correlation functions, including for the first time next-to-leading order perturbative contributions. The Jᵖ=0+ and 1+ charm-light diquark masses are respectively found to be 1.86±0.05 GeV and 1.87±0.10 GeV, while those of the 0+ and 1+ bottom-light diquarks are both determined to be 5.08±0.04 GeV. The sum rules derived for heavy-light diquarks with negative parity are poorly behaved and do not permit unambiguous mass predictions, in agreement with previous results for negative parity light diquarks. The scalar and axial vector heavy-light diquark masses are degenerate within uncertainty, as expected by heavy quark symmetry considerations. Furthermore, these mass predictions are in good agreement with masses extracted in constituent diquark models of the tetraquark candidates X(3872) and Yb(10890). Thus these results provide QCD support for the interpretation of the X(3872) and Yb(10890) as JPC=1⁺⁺ tetraquark states composed of diquark clusters. Further implications for tetraquarks among the heavy quarkonium-like XYZ states are discussed.