The partial wave amplitudes of particular five-point scattering are investigated as an extention of four-point amplitudes due to analytic continuation in squared mass of one of their four lines. From this point of view, the appearance of an anomalous complex singularity in this continuation is clue to the circumstance that a new channel is opened as the particle becomes unstable. Thus, the generated complex anomalous cut corresponds to the unitarity cut of the new open channel. In this way, we can immediately find the imaginary part of the contribution from the complex anomalous cut and can rewrite its real part by an integral along the real axis. This procedure is very useful for practical calculations of scattering problem including unstable particles.
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