System-size dependence of pion production in nucleus–nucleus collisions

Determine the physical mechanism responsible for the non-monotonic dependence of the negatively charged pion yield per wounded nucleon on collision-system size, particularly the pronounced midrapidity behavior observed in nucleus–nucleus collisions.

Background

The paper compares rapidity spectra of negatively charged pions produced in central Be–Be, Ar–Sc, Xe–La, and Pb–Pb collisions at several collision energies. Unlike the positively charged kaon yield, which shows a monotonic system-size dependence plausibly associated with strangeness enhancement, the pion yield normalized by the mean number of wounded nucleons varies non-monotonically with system size, with the strongest effect near midrapidity. The authors explicitly state that the origin of this behavior is not understood, leaving its physical explanation unresolved.