Systematic twist expansion of (ηc,ηb) →γ∗γ transition form factors in the light-front quark model

The light-front quark model analysis of the meson-photon transition form factor FPγ(Q2) amenable both to the spacelike region (Q2>0) and the timelike region (Q2<0) provides a systematic twist expansion of Q2FPγ(Q2) for the high |Q2| region. Investigating FPγ(Q2)(P=ηc,ηb) for the entire kinematic regions of Q2, we examine the twist-2 and twist-3 distribution amplitudes of (ηc,ηb) mesons in the light-front quark model and quantify their contributions to Q2F(ηc,ηb)γ(Q2). Our numerical results for the normalized transition form factor F(ηc,ηb)γ(Q2)/F(ηc,ηb)γ(0) and the decay width Γ(ηc,ηb)→γγ are compared with the available data, checking the sensitivity of our model to the variation of the constituent quark masses.