Emission patterns of neutral pions in 40 A MeV Ta+Au reactions

TL;DR

This study measures neutral pion emission in Ta+Au collisions at 40 A MeV, revealing that standard models fail to fully describe the spectra, but high-energy tails fit well with unexpectedly soft thermal distributions.

Contribution

It provides detailed differential cross sections of neutral pions and highlights limitations of existing thermal models in describing the spectra.

Findings

Thermal models do not adequately describe the pion spectra.

High energy tails fit with soft thermal distributions below 10 MeV.

Reabsorption effects are insufficient to explain the spectral features.

Abstract

Differential cross sections of neutral pions emitted in 181Ta + 197Au collisions at a beam energy of 39.5A MeV have been measured with the photon spectrometer TAPS. The kinetic energy and transverse momentum spectra of neutral pions cannot be properly described in the framework of the thermal model, nor when the reabsorption of pions is accounted for in a phenomenological model. However, high energy and high momentum tails of the pion spectra can be well fitted through thermal distributions with unexpectedly soft temperature parameters below 10 MeV.