  The final stage of a relativistic heavy-ion collision is a hadron gas.
Final-state interactions therein distort the $p_T$ spectrum of particles coming
from the phase transition upon cooling the quark-gluon plasma. Using recent
state-of-the-art parametrizations of pion interactions we provide theoretical
computations of the pionic depth of the gas: how likely is it that a given pion
rescatters in it (we find a high probability around $p_T=0.5$ GeV at
midrapidity, corresponding to the formation of the $\rho$ resonance), a
comparison of the collision and Bjorken expansion rates, and how many pions
make it through without interacting as a function of $p_T$. This is in the
range 10-24$\%$ and shown in this plot, the main result of the contribution.
