On the identification of the triangle mechanism with pion exchange

The triangle graph with pion exchange is presently widely used for the description of various processes with large momentum transfer. Here it is shown that this mechanism can be reliably identified by observing the picture corresponding to so called moving complex triangle singularities: the position and the width of the peak in the invariant mass distribution of several final particles must distinctly vary with three-momentum transfer from initial fast particle to final one. Furthermore, this procedure gives a simple method to determine the mass of the meson, the exchange of which gives the main contribution to the cross section of the process. These statements are illustrated on an example of the near-threshold η-meson production in the channel pd→pdη.