Photon structure functions at small x in holographic QCD
Akira Watanabe1*, Hsiang-nan Li1,2,3
1Institute of Physics, Academia Sinica, Taipei, Taiwan
2Department of Physics, National Tsing-Hua University, Hsinchu, Taiwan
3Department of Physics, National Cheng-Kung University, Tainan, Taiwan
* presenting author:Akira Watanabe,
The energetic photons fluctuate into quark-antiquark pairs or vector mesons in high energy scattering processes, which enables us to investigate the internal structure of those photons. In this study, we calculate the photon structure functions at small Bjorken-x in the framework of holographic QCD which is an effective approach to QCD based on the AdS/CFT correspondence. At small x, the nonperturbative hadronic contribution to the cross section becomes dominant. Therefore, since the perturbative approaches are not applicable in this kinematical region, studies based on effective models are needed. It is known that the Pomeron exchange picture can give a reasonable description for various high energy forward scattering, and we apply this to the photon deep inelastic scattering. We evaluate the quasi-real photon structure functions by combining the Brower-Polchinski-Strassler-Tan (BPST) Pomeron exchange kernel and the wave function of the U(1) vector field in the five-dimensional AdS space. In contrast to the cases of the nucleon structure functions, we do not need to introduce approximations nor models to describe the incident and target particles in this study. Hence, the model dependence is less than that of those, and it may be considered to be a cleaner process to test the BPST kernel. Although presently available experimental data for F2γ(x,Q2) at small x are quite limited, our calculations are in good agreement with them. Furthermore, we find that the dependence on the square of the target photon four-momentum in the resulting F2γ(x,Q2) is qualitatively consistent with the QCD predictions. Our results presented in this talk will be tested via the experiment at the future linear colliders, e.g., the International Linear Collider.

Keywords: Photon structure, Deep inelastic scattering, Gauge/string correspondence, Pomeron