QCD scattering amplitudes in large Nc limit: from trees to loops

This thesis studies the large number of colors expansion of QCD scattering amplitudes in the color flow representation at one-loop. Based on the tree-level formalism, the one-loop color flow amplitude for $n$-gluon scattering is derived, both for gluon loop and quark loop contributions. A remarkable result is that all terms with a decoupled gluon cancel in the tree–loop interference, allowing a simplified amplitude to be used at NLO. Using this, the Born and virtual contributions to the amplitude squared are derived up to the fourth subleading order in color, and the number of subamplitudes needed to construct them is shown to scale as $n^2 2^{n-3}$ for large $n$. Finally, the color flow representation is extended to processes with an external quark-antiquark pair and $n$-gluons, providing the formula for the scattering amplitude.