Z_c and Z_(cs) systems with operator mixing at NLO in QCD sum rules

We study the mass spectra of hidden-charm tetraquark systems with quantum numbers $(I^G)J^P=(1^+)1^+$ using QCD sum rules. The analysis incorporates the complete next-to-leading order (NLO) contribution to the perturbative QCD part of the operator product expansions, with particular attention to operator mixing effects due to renormalization group evolution. For the $\bar{d}c\bar{c}u$ system, the masses of two mixed operators, $J_{1,5}^{\text{Mixed}}$ and $J_{2,6}^{\text{Mixed}}$, are determined to be $3.89^{+0.18}_{-0.12}$ GeV and $4.03^{+0.06}_{-0.07}$ GeV, respectively, closely matching those of $Z_c$(3900) and $Z_c(4020)$. Similarly, for the $\bar{s}c\bar{c}u$ states, the masses of $J_{1,5}^{\text{Mixed}}$ and $J_{2,6}^{\text{Mixed}}$ are found to be $4.02^{+0.17}_{-0.09}$ GeV and $4.21^{+0.08}_{-0.07}$ GeV, respectively, in close proximity to $Z_{cs}$(3983)/$Z_{cs}$(4000) and $Z_{cs}$(4220), consistent with the expectation that they are the partners of $Z_c$(3900) and $Z_c$(4020). Our results highlight the crucial role of operator mixing, an inevitable effect in a complete NLO calculation, in achieving a robust phenomenological description for the tetraquark system.