Multistep production of η and hard π0 mesons in subthreshold Au-Au collisions

The neutral pi(o) and eta mesons are studied in Au-197-Au-197 collisions at an incident energy of 800A MeV, substantially below the threshold for eta production in N-N collisions. While the gross pi(o) multiplicity increases almost linearly with the number of participant nucleons, the multiplicities of eta and hard pi(o) mesons show a stronger than linear dependence. The nonlinearity is governed by the average transverse-mass excess [m(t)] - (root s - 2m(N)) of the mesons and is insensitive to their final-state interaction in the nuclear medium.