We report on a measurement of subthreshold $\eta$ and ${\pi }^0$ mesons in the reaction $\mathrm{Ar}+\mathrm{Ca}$ at $180A$ MeV. We find that the ratio of the $\eta$ to ${\pi }^0$ meson-production cross section is more than a factor of 20 smaller than the one expected from threshold-energy scaling of meson production. In addition, the multiplicity of high $m_t$ ${\pi }^0$ increases faster with the centrality of the reaction than the multiplicity of the bulk of ${\pi }^0$ mesons. This behavior is explained by the rescattering of $\pi$ mesons in nuclear matter at the origin of most energetic particles.

• Received 25 June 1998

DOI:https://doi.org/10.1103/PhysRevLett.83.1538