Phase-space dependence of particle-ratio fluctuations in Pb + Pb collisions from 20 A to 158 A GeV beam energy phase-space dependence of particle-ratio ... T. Anticic et al.

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 one to measure the moments of the multiplicity distributions of protons ($p$), kaons ($K$), pions ($\ensuremath{\pi}$), and electrons ($e$) in case of incomplete particle identification. Using these moments the excitation function of the fluctuation measure ${\ensuremath{\nu}}_{\mathrm{dyn}}[A,B]$ was measured, with $A$ and $B$ denoting different particle types. The obtained energy dependence of ${\ensuremath{\nu}}_{\mathrm{dyn}}$ agrees with previously published NA49 results on the related measure ${\ensuremath{\sigma}}_{\mathrm{dyn}}$. However, ${\ensuremath{\nu}}_{\mathrm{dyn}}$[$K$,$p$] and ${\ensuremath{\nu}}_{\mathrm{dyn}}$[$K$,$\ensuremath{\pi}$] were found to depend on the phase-space coverage. This feature most likely explains the reported differences between measurements of NA49 and those of STAR in central Au + Au collisions.