Origin and limits of intermediate-mass dilepton thermometry

Abstract

We identify the physical origin and limits of intermediate-mass dilepton thermometry in relativistic heavy-ion collisions. Using a controlled expanding-fireball framework with thermal dilepton rates, we show that the local inverse-slope parameter of the invariant-mass spectrum follows, to percent-level accuracy, the emission-weighted harmonic mean of the temperatures contributing to the spectrum. As the thermal-stage initial temperature increases, the temperatures sampled by the radiation shift upward with the overall thermal scale, causing their harmonic mean to track the initial temperature closely. This provides the physical basis for the strong inverse-slope--initial-temperature correlation, while the resulting mapping remains nonuniversal: changes that only rescale the amount of radiation leave it unchanged, whereas changes in the cooling history or source composition redistribute the radiation among different temperatures and modify the response. The mapping is nevertheless nearly linear over the temperature range studied here. Because different invariant-mass windows weight the emission history differently, source scenarios with the same inverse-slope parameter in $1<M<3~{\rm GeV}$ develop different inverse slopes in harder mass windows. Intermediate-mass dilepton spectra therefore provide a quantitative but nonuniversal probe of the early thermal history, while measurements in multiple mass windows, when confronted with realistic calculations, can provide additional constraints on the thermal evolution and early electromagnetic source content.