New Evidence for "Hot Spots" from Subthreshold Pions

Abstract

The excitation functions and energy distribution of the reaction ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}+A\to {\pi }^0+X$ for beam energies of several tens of megaelectronvolts per nucleon are analyzed. Statistical-decay theory yields good agreement with the data under the assumption that a hot spot is formed. Its size is determined from the velocity of the emitting source. The cross section for forming a hot spot shows a systematic behavior which fits well with the systematics recently observed in fragmentation reactions.