  Results on charged pion and kaon production in central Pb+Pb collisions at
20A and 30A GeV are presented and compared to data at lower and higher
energies. A rapid change of the energy dependence is observed around 30A GeV
for the yields of pions and kaons as well as for the shape of the transverse
mass spectra. The change is compatible with the prediction that the threshold
for production of a state of deconfined matter at the early stage of the
collisions is located at low SPS energies.

Nuclear Experiment