The impact factor for the virtual photon to light vector meson transition

TL;DR

This work computes the forward impact factor for the transition $\gamma^*\to V$ with longitudinal polarization in the BFKL framework at next-to-leading order. It demonstrates that, in the hard regime, the factorization into a perturbative hard-scattering amplitude $T_H$ and a twist-2 vector-meson distribution amplitude $\phi_\parallel$ persists at NLA, after absorbing ultraviolet and collinear divergences into renormalized quantities. The calculation includes both $q\bar q$ and $q\bar q g$ intermediate states, and yields a closed analytical expression for the NLA impact factor, with explicit $z$-dependent corrections $\tau_i(z)$. This result allows, in combination with the BFKL Green's function, fully perturbative predictions for processes like $\gamma^*\gamma^*\to V V$ and vector-meson electroproduction, and informs energy-scale choices in high-energy QCD analyses.

Abstract

We evaluate in the next-to-leading approximation the forward impact factor for the virtual photon to light vector meson transition in the case of longitudinal polarization. We find that in the hard kinematic domain, both in the leading and in the next-to-leading approximation, the expression for the impact factor factorizes, up to power suppressed corrections, into the convolution of a perturbatively calculable hard-scattering amplitude and a meson twist-2 distribution amplitude.

Figures (9)

• Figure 1: The kinematics of the virtual photon to vector meson impact factor.
• Figure 2: Feynman diagrams at the lowest order for the $\gamma^*R\to VR^\prime$ transition amplitude.
• Figure 3: The contributions to the impact factor from two-particle (a) and three-particle (b) intermediate states.
• Figure 4: The lowest order Feynman diagrams describing Reggeon-photon (a) and Reggeon-meson (b) vertices.
• Figure 5: The contribution of the quark-antiquark intermediate state at the next-to-leading order (NLO).