  The distribution of the squared momentum difference |P_A|^2 - |P_B|^2 between
the momenta in the laboratory system of two experimentally observed particles A
and B provides a test for whether an observed mass peak indicates a real
resonance rather than nonresonant background or kinematic reflection. The
angular distribution of the relative momenta in the center-of-mass system
exhibits a forward-backward symmetry in the production and decay of any
resonance with a definite parity. This symmetry is not expected in other
nonresonant processes and can be expressed without needing angular
distributions in terms of the easily measured momenta in the laboratory frame
that are already measured and used to calculate the invariant mass of the
system. Our test is especially useful for low statistics experiments where the
full angular distribution cannot be determined. It can be applied to both
fixed-target and collider searches for the Theta^+ and Theta_c pentaquarks.

High Energy Physics - Phenomenology;High Energy Physics - Experiment;Nuclear Experiment