A new study for the first time investigates the effect of thermal broadening of light vector-meson spectral functions on hadron production in heavy-ion collisions. Measured hadron yields in these collisions are well described by thermal particle production at temperatures close to the QCD pseudo-critical transition. However, if particles are produced in thermal equilibrium, in-medium effects, such as the broadening of their spectral functions, must be taken into account.

Researchers generalized the efficient resonance decay framework FastReso to include resonances with finite-width spectral functions. They then computed the pion, kaon, and proton spectra from the decays of hadron resonances for both in-medium and vacuum spectral functions. A simultaneous blast-wave fit with decay feed-down was performed to measured particle spectra in central lead-lead (Pb-Pb) and xenon-xenon (Xe-Xe) collisions at the Large Hadron Collider (LHC).

The findings indicate an increase in pion spectra at low momenta when including thermally broadened ρ and ω mesons. This additional pion yield, stemming from finite-resonance widths, reduces the commonly observed discrepancy between models and experimental data. While it does not completely resolve the "soft pion puzzle" in heavy-ion collisions, this work represents a significant step forward in understanding particle production mechanisms in the quark-gluon plasma.