A new concept, rigidity of pseudo-rapidity spectrum, is extended into the study of the multiplicity fluctuations in high energy nucleus-nucleus interactions. In using the novel method to analyze the data of16O-Em at 14.6, 60, and 200 A GeV, and 32S-Em at 200 A GeV, it is found that the rigidity of pseudo-rapidity spectrum in the central domain of pseudo-rapidity might be a mixture of Pois...
