are extracted from fits to tetraquark candidates among the XYZ states. The main goal of the research in this chapter was to calculate the constituent heavy-light diquark mass using QSR, so as to provide a QCD-based test of the constituent diquark models used in Refs. [84, 10]. Constituent masses of diquarks composed of light quarks only were determined in Refs. [42, 67, 129, 126]. In Ref. [125] heavy-light diquarks with J P = 0 $^{+}$, 1 + were studied using QSR, however, only leading-order perturbative contributions to the OPE were considered. The research presented in this chapter builds upon the work of Ref. [125] by including next-to-leading order perturbative contributions and diquarks with J P = 0 $^{−}$, 1 $^{−}$. 4.2 Results where α , ω are color indices. The heavy-light diquark currents are where the Lorentz structures O = γ$_{5}$ , I , γ$_{µ}$ , γ$_{µ}$γ$_{5}$ respectively probe scalar $^{(}$J P = 0 + $^{)}$, pseu- doscalar (0 $^{−}$), axial vector (1 $^{+}$), and vector (1 $^{−}$) heavy-light diquarks [42, 67]. In Eq. (4.3) C is the charge conjugation operator (A.4), T denotes the transpose, Q is a heavy (charm or bottom) quark field, and q is a light quark field. The axial vector and vector correlation functions are given by where the number of spacetime dimensions d is kept arbitrary because dimensional regular- ization is used. The correlation function in Eq. (4.2) includes a path-ordered exponential, 84 Π (A , $^{V)}$( q ) = 1 d − 1 $^{(}$q $^{µ}$qν q 2 − g µν ) Π (A , V) µν ( q ) , (4.4) J$_{α}$ = ϵ$_{αβγ}$Q T $_{β}$C O q$_{γ}$ , (4.3) Π $^{(}$Q 2 ) = i$^{∫}$ d $^{4}$x e iq · $^{x}$〈 Ω | T [ J$_{α}$ ( x ) S$_{αω}$ [ x , 0] J † ω (0) ] | Ω 〉 , (4.2) The correlation function and currents used to study heavy-light diquarks with J P = 0 $^{±}$, 1 ± are given by