Energy dependence of kaon-to-proton ratio fluctuations in central Pb+Pb collisions from √[sNN]=6.3 to 17.3 GeV

Kaons and protons carry large parts of two conserved quantities: strangeness and baryon number. It is argued that their correlation and thus also fluctuations are sensitive to conditions prevailing at the anticipated parton-hadron phase boundary. Fluctuations of the (K++K-)/(p+p ) and K+/p ratios have been measured for the first time by NA49 in central Pb+Pb collisions at five CERN Super Proton Synchrotron energies between √sNN=6.3 and √sNN=17.3 GeV. Both ratios exhibit a change of sign in σdyn, a measure of nonstatistical fluctuations, around √sNN=8 GeV. Below this energy, σdyn is positive, indicating higher fluctuation compared to a mixed event background sample, while for higher energies, σdyn is negative, indicating correlated emission of kaons and protons. The results are compared to hadronic transport model calculations which fail to reproduce the energy dependence.