Researchers have studied the spacelike-collinear limit of gauge-theory scattering amplitudes using the Method of Regions. They found that the resulting splitting amplitudes violate strict collinear factorization, showing a dependence on non-collinear partons. While the associated color dependence has long been known, starting at two loops, the splitting amplitude also acquires a dependence on the kinematics of these partons. This finding is crucial for understanding the complexity of particle interactions at high energies.

The research reveals that this kinematic dependence originates from a unique hidden region, present in the asymptotic expansion of the five-point amplitude in the spacelike-collinear limit, but absent in the timelike limit. The authors propose that these hidden regions provide the mechanism by which crossing-related asymptotic limits cease to be analytically connected. They developed an algorithm for systematically identifying these hidden regions, applying it to the five-point amplitude in super Yang-Mills theory.

By applying this algorithm, the scientists computed the hidden-region contributions to the complete set of basis integrals and recovered the exact kinematically dependent, factorization-violating splitting amplitude. In momentum space, the hidden region is characterized by soft and Glauber loop momenta. This discovery explains why the Wilson-line calculation captures the complete kinematic dependence, thereby accounting for the observed universality across gauge theories.