Influence of impact parameter on thermal description of relativistic heavy ion collisions at(1–2)A GeV

Attention is drawn to the role played by the size of the system in the thermodynamic analysis of particle yields in relativistic heavy ion collisions at SIS energies. This manifests itself in the nonlinear dependence of $K^{+}$ and $K^{-}$ yields in $\mathrm{AA}$ collisions at $(1\mathrm{}–2)A\hspace{0.5em}\mathrm{GeV}$ on the number of participants. It is shown that this dependence can be quantitatively well described in terms of a thermal model with a canonical strangeness conservation. The measured particle multiplicity ratios $({\pi }^{+}/p,{\pi }^{-}/{\pi }^{+},d/p,K^{+}/{\pi }^{+},$ and $K^{+}{/K}^{-}$ but not $\eta /{\pi }^0)$ in central Au-Au and Ni-Ni collisions at $(0.8\mathrm{}–2.0)A\hspace{0.5em}\mathrm{GeV}$ are also explained in the context of a thermal model with a common freeze-out temperature and chemical potential. Including the concept of collective flow a consistent picture of particle energy distributions is derived with the flow velocity being strongly impact-parameter dependent.