$Δ$ resonances in Ca+Ca, Ni+Ni and Au+Au reactions from 1 AGeV to 2 AGeV: Consistency between yields, mass shifts and decoupling temperatures

Tom Reichert, Paula Hillmann, Marcus Bleicher

Published 2020-04-21Version 1

The Ultra-relativistic Quantum Molecular Dynamics (UrQMD) transport approach is used to calculate $\Delta$(1232) yields in Ca+Ca, Ni+Ni and Au+Au collisions between 1 AGeV and 2 AGeV. We compare and validate two different methods to extract the yields of $\Delta$(1232) resonances in such low energy nuclear collisions: Firstly, the $\pi^-$ spectra at low $p_\mathrm{T}$ are used to infer the $\Delta$(1232) yield in A+A collisions, a method employed by the GSI/FOPI experiment. Secondly, we employ the invariant mass method used by the HADES collaboration, which has recently reported data in the $\Delta^{++}\rightarrow\pi^++p$ channel. We show that both methods are compatible with each other and with the theoretical calculations, indicating that the new HADES results are compatible with the previous FOPI measurements. Then we use the $\Delta/nucleon$ ratio to extract the kinetic decoupling temperatures of the $\Delta$(1232) resonances. We find that the extracted temperatures are consistent with the predicted mass shift of the $\Delta$ resonance and the freeze-out parameters estimated from complementary studies (blast wave fits, coarse graining).