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The light-front quark model analysis of the meson-photon transition formfactor $F_{P\gamma} (Q^2)$ amenable both for the spacelike region ($Q^2 >0$)and the timelike region ($Q^2 <0$) provides a systematic twist expansion of$Q^2 F_{P\gamma} (Q^2)$ for the high $|Q^2|$ region. Investigating $F_{P\gamma}(Q^2) (P = \eta_c,\eta_b)$ for the entire kinematic regions of $Q^2$, weexamine the twist-2 and twist-3 distribution amplitudes of $(\eta_c,\eta_b)$mesons in the light-front quark model and quantify their contributions to $Q^2F_{(\eta_c,\eta_b)\gamma}(Q^2)$. Our numerical results for the normalizedtransition form factor$F_{(\eta_c,\eta_b)\gamma}(Q^2)/F_{(\eta_c,\eta_b)\gamma}(0)$ and the decaywidth $\Gamma_{(\eta_c,\eta_b)\to\gamma\gamma}$ are compared with the availabledata checking the sensitivity of our model to the variation of the constituentquark masses.

Systematic twist expansion of $(\eta_c,\eta_b)\to\gamma^*\gamma$ transition form factors in light-front quark model