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Is the breakup time a free parameter when describing heavy ion collisions?

Thermodynamic descriptions of heavy ion collisions generally include the so-called breakup time (when to switch from continuum picture to a collisionless gas). While an appropriate choice of this time can help to reproduce the detected spectra, the same freedom may prevent a unique reconstruction of the compressed hot stages from the observables, unless the calculated final spectra are insensitive on the breakup time at least in a range. Here we show such an example in a model system. This example may give further arguments for the thermodynamic description.