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By using QCD sum rules, the mass of the hidden charm tetraquark$[cu][\bar{c}\bar{d}]$ state with $I^{G} (J^{P}) = 1^+ (1^{+})$ (HCTV) isestimated, which presumably will turn out to be the newly observedcharmonium-like resonance $Z_c^+(3900)$. In the calculation, contributions upto dimension eight in the operator product expansion(OPE) are taken intoaccount. We find $m_{1^+}^c = (3912^{+306}_{-153}) \, \text{MeV}$, which isconsistent, within the errors, with the experimental observation of$Z_c^+(3900)$. Extending to the b-quark sector, $m_{1^+}^b =(10561^{+395}_{-163}) \,\text{MeV}$ is obtained. The calculational resultstrongly supports the tetraquark picture for the "exotic" states of$Z_c^+(3900)$ and $Z_b^+(10610)$.

Estimating the mass of the hidden charm $1^+(1^{+})$ tetraquark state via QCD sum rules