  It is argued that the experimentally observed strong upward-bending of the
logarithm of factorial moments versus that of phase space partition number in
the higher-dimensional phase space of nucleus-nucleus collisions is due to the
superposition of elementary collision processes in these collisions. A direct
implication of this observation is that, at high enough energy and/or density,
when the produced particles from individual elementary processes are melted
into a unique system the above-mentioned superposition effect will disappear
and the factorial moments will not be strongly upward-bending any more. So, the
disappearance of strong upward-bending of higher-dimensional factorial moments
in heavy ion collision may be taken as a signal for the formation of a unique
system, or QGP, in this collision.
