Phase-space dependence of particle-ratio fluctuations in Pb+Pb collisions from 20A to 158A GeV beam energy

A novel approach, the identity method, was used for particle identification and the study of fluctuations of particle yield ratios in Pb+Pb collisions at the CERN Super Proton Synchrotron (SPS). This procedure allows to unfold the moments of the unknown multiplicity distributions of protons (p), kaons (K), pions ($pi$) and electrons (e). Using these moments the excitation function of the fluctuation measure $ u_{text{text{dyn}}}$[A,B] was measured, with A and B denoting different particle types. The obtained energy dependence of $ u_{text{dyn}}$ agrees with previously published NA49 results on the related measure $sigma_{text{dyn}}$. Moreover, $ u_{text{dyn}}$ was found to depend on the phase space coverage for [K,p] and [K,$pi$] pairs. This feature most likely explains the reported differences between measurements of NA49 and those of STAR in central Au+Au collisions.