16.1 Introduction The hard exclusive processes involving light mesons (η, η′, π~0) can be described using the factorization, in the sense of convolution, of the short and long distance dynamics. The short-distance part is process dependent and calculable perturbatively as parton-level subprocess amplitude in which the mesons are replaced by their valance Fock components. The long-distance part is described by process-independent meson distribution amplitudes (DAs). The electromagnetic transition form factors (TFFs) of light pseudo scalar mesons are important ingredients to understand their structure. The aim of theoretical investigations is to explain the experimental data within the framework of QCD using factorization. The leading hard contribution to TFF starts at tree-level itself. For large photon virtualities,the full expression can be expanded as a sum of contributions of increasing twist:{formula} Here μ_F represents the boundary between low and high energy parts. η and η′mesons posses SU (3)_F singlet and octet valance Fock components andalsotwo gluons. There is a DA corresponding to each of these three components. This feature leads to the flavor mixing for η and η′ system, and to mixing of singlet quark and gluon distribution amplitudes under evolution. One of the key objectives of analysis of TFF of mesons is to model the meson DAs and, in the η-η′ case, to extract information about their gluon contents. The gluonic parts of the η-η′ meson DAs, deduced from the ηγ, η′γ data, are important ingredients in studying a wide range of exclusive processes, such as B meson two body non-leptonic exclusive and semi-inclusive decays involving η-η′ mesons. The form factors describing the mesons (η-η′) transition in two (in general virtual) photons are defined as, {formula} {formula} {formula}
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