We present a comprehensive study of the radiative decays of fully heavy tetraquarks ($T_{4c}$, $T_{4b}$) and mixed heavy-flavor tetraquarks ($T_{bc\overline{b}\overline{c}}$) into charmonium/bottomonium states, within the nonrelativistic QCD factorization framework. Numerical result indicates that the decay widths of fully charm tetraquark $T_{4c}$ into $\gamma J/\psi ({\eta }_c)$ are around 1 MeV. Crucially, the $\gamma J/\psi$ signal channel receives less experimental background near the $J/\psi J/\psi$ threshold, providing a compelling way to search for the possible tetraquark states X(6200). For $T_{4b}$, the decay width is highly suppressed by the bottom quark mass, just lying in the tens eV level. We further find that the decay widths of $T_{bc\overline{b}\overline{c}}\to \gamma \mathrm{\Upsilon }$ exceed those to $\gamma J/\psi$ by 3–4 orders of magnitude, indicating preferential $c\overline{c}$-pair annihilation over $b\overline{b}$. These radiative decay modes can be measured in the future experiments, and are helpful to understand the inner structure of the full heavy tetraquark.

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• Bound states
• Effective field theory
• Multiquark bound states
• Particle decays
• Signatures with photons
• Quarkonia
• Lifetimes & widths