The triangle graph with pion exchange is presently widely used for the
description of various processes with large momentum transfer. Here i
t is shown that this mechanism can be reliably identified by observing
the picture corresponding to so called moving complex triangle singul
arities: the position and the width of the peak in the invariant mass
distribution of several final particles must distinctly vary with thre
e-momentum transfer from initial fast particle to final one. Furthermo
re, 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 eta-meson production in the channel pd --> pd e
ta. (C) 1998 Elsevier Science B.V. All rights reserved.