The $\gamma^* \gamma^*$ total cross section in next-to-leading order BFKL and LEP2 data

TL;DR

This paper calculates the total cross section for highly-virtual photon collisions at high energies using next-to-leading order BFKL resummation, compares with LEP2 data, and finds the impact of recent impact factor calculations reduces the BFKL contribution significantly.

Contribution

It provides a detailed numerical analysis of the NLO BFKL cross section with recent impact factor calculations, highlighting their effect on the predicted cross section.

Findings

NLO BFKL contribution is significantly reduced by recent impact factor calculations.

The revised BFKL prediction cannot fully explain LEP2 data when combined with QED contributions.

The study compares different representations and optimization methods for the cross section calculation.

Abstract

We study the total cross section for the collision of two highly-virtual photons at large energies, taking into account the BFKL resummation of energy logarithms with full next-to-leading accuracy. A necessary ingredient of the calculation, the next-to-leading order impact factor for the photon to photon transition, has been calculated by Balitsky and Chirilli using an approach based on the operator expansion in Wilson lines. We extracted the result for the photon impact factor in the original BFKL calculation scheme comparing the expression for the photon-photon total cross section obtained in BFKL with the one recently derived by Chirilli and Kovchegov in the Wilson-line operator expansion scheme. We perform a detailed numerical analysis, combining different, but equivalent in next-to-leading accuracy, representations of the cross section with various optimization methods of the perturbative series. We compare our results with previous determinations in the literature and with the LEP2 experimental data. We find that the account of Balitsky and Chirilli expression for the photon impact factor reduces the BFKL contribution to the cross section to very small values, making it impossible to describe LEP2 data as the sum of BFKL and leading-order QED quark box contributions.