  The influence of the finite width of $\rho$ meson on the pion momentum
distribution is studied quantitatively in the framework of the S-matrix
approach combined with a blast-wave model to describe particle emissions from
an expanding fireball. We find that the proper treatment of resonances which
accounts for their production dynamics encoded in data for partial wave
scattering amplitudes can substantially modify spectra of daughter particles
originating in their two body decays. In particular, it results in an
enhancement of the low-$p_T$ pions from the decays of $\rho$ mesons which
improves the quantitative description of the pion spectra in heavy ion
collisions obtained by the ALICE collaboration at the LHC energy.
