Observable Effects of the Leading Landau Singularity of the Box Graph

The effect of the leading Landau singularity of the fourth-order single-loop graph in a two-particle to four-particle amplitude is investigated. The amplitude for the graph is evaluated from a dispersion relation in the mass squared of the two particles interacting in the final state, and its behavior is studied using a simple model. The reactions $\overline{p}p\to \overline{K}K\pi \pi$ and $\overline{p}p\to \overline{\Lambda }\Lambda {\pi }^{+}{\pi }^{-}$ are examined in some detail, and peaks are predicted for the amplitude squared in a certain range of the external variables.