ensembles GCE, CE, and MCE. For the sake of argument we will assume that we only want to measure $P(N_{\Omega}^-)$, i.e. the probability distribution of negatively charged pions in a limited segment $\Omega$ of momentum space. Hence $\pi^-$ with momenta inside $\Omega$ are observed, while $\pi^-$ inside the complementary segment $\bar \Omega$ are not observed. $\pi^+$ and $\pi^0$ are never detected. In GCE and CE the presence of $\pi^0$ as a degree of freedom is of no relevance, while in MCE it constitutes a heat bath for the remaining system. For consistency we use the same system throughout this discussion.

In order to keep the model simple, we assume a static homogenous fireball. Our considerations therefore exclude collective motion, i.e. flow, and resulting momentum spectra are purely thermal. We also omit resonance decay contributions in this work. The spectra presented in Fig. \[spectra\] are normalized to the total $\pi^-$ yield in GCE and CE. Thus they are the same in both ensembles. In MCE one expects in the large volume limit only small deviations from Boltzmann spectra. None of the forthcoming arguments are affected by this.

In the following we will use the transverse momentum and rapidity spectra presented in 