QCD compositeness as revealed in exclusive vector boson reactions through double-photon annihilation: e + e ? → γγ ? → γV 0 and e + e ? → γ ? γ ? → V 0 V 0

摘要

We study the exclusive double-photon annihilation processes, e + e ? → γ γ ? → γ V 0 and e + e ? → γ ? γ ? → V a 0 V b 0 , where the V i 0 is a neutral vector meson produced in the forward kinematical region: s ? ? t and ? t ? Λ QCD 2 . We show how the differential cross sections d σ d t , as predicted by QCD, have additional falloff in the momentum transfer squared t due to the QCD compositeness of the hadrons, consistent with the leading-twist fixed- θ CM scaling laws, both in terms of conventional Feynman diagrams and by using the AdS/QCD holographic model to obtain the results more transparently. However, even though they are exclusive channels and not associated with the conventional electron–positron annihilation process e + e ? → γ ? → q q ˉ , these total cross sections σ ( e + e ? → γ V 0 ) and σ ( e + e ? → V a 0 V b 0 ) , integrated over the dominant forward- and backward- θ CM angular domains, scale as 1 / s , and thus contribute to the leading-twist scaling behavior of the ratio R e + e ? . We generalize these results to exclusive double-electroweak vector-boson annihilation processes accompanied by the forward production of hadrons, such as e + e ? → Z 0 V 0 and e + e ? → W ? ρ + . These results can also be applied to the exclusive production of exotic hadrons such as tetraquarks, where the cross-section scaling behavior can reveal their multiquark nature.