In theoretical physics, crossing symmetry is a relation between the S-matrix that describe processes that can be obtained from each other by replacing incoming particles with outgoing antiparticles after taking the analytic continuation. For example, the annihilation of an electron with a positron into two photons is related to an elastic scattering of an electron with a photon by crossing symmetry. This relation allows to calculate the scattering amplitude of one process from the amplitude for the other process if negative values of energy of some particles are substituted.
Crossing symmetry exists because of the Wick rotation.
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