Measurement of pion enhancement at low transverse momentum and of the Δ resonance abundance in Si-nucleus collisions at AGS energy

Abstract

We present measurements of pion transverse momentum (pt) spectra in central Si-nucleus collisions in the rapidity range 2.0 < y < 5.0 for pt down to and including pt = 0. The data exhibit an enhanced pion yield at low pt compared to what is expected for a purely thermal spectral shape. This enhancement is used to determine the Δ resonance abundance at freeze-out. The results are consistent with a direct measurement of the Δ resonance yield by reconstruction of proton-pion pairs and imply a temperature of the system at freeze-out close to 140 MeV.