QCD Scattering in the Regge Limit
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Fixed-order computations of QCD amplitudes in general kinematics are limited to either one, two or three loops, depending on the number of particles produced. This strongly motivates our theoretical research programme aimed at understanding the behaviour of quark and gluon scattering amplitudes in special kinematic limits, in which new factorization and exponentiation properties arise. A particularly interesting limit is the Regge limit, where major simplifications take place. A remarkable and well-known property of this limit is the exponentiation of energy logarithms, a phenomenon known as gluon Reggeization, leading to power-like dependence on the energy (a Regge pole). This phenomenon and its breaking can be investigated using non-linear rapidity evolution equations. In the planar limit the evolution is consistent with a Regge pole to any logarithmic accuracy. However in full non-planar QCD multi-Reggeon interactions give rise to Regge cuts, in addition to the pole. Over the past decade an effective theory of multi-Reggeon interactions was developed, leading to a clear interpretation of state-of-the-art 2 → 2 and 2 → 3 QCD amplitude computations. Specifically, we are now able to fix all parameters associated with the Regge pole to the next-to-next-to-leading order (NNLO): the 3-loop trajectory, the 2-loop impact factors, and since recently, the two-loop Reggeon-gluon-Reggeon vertex. These, along with the effective multi-Reggeon theory, can be used to determine or resum higher-loop corrections in 2 → n scattering in the multi-Regge limit, and push BFKL theory to NNLO.
