Imprints of dynamic fluidization on dilepton production

TL;DR

This paper tackles how hydrodynamic initialization in hybrid heavy-ion models at low-to-intermediate beam energies influences dilepton production, focusing on the intermediate-mass range (IMR). It implements dynamic fluidization ( DynFlu ) in the SMASH-vHLLE framework, initiating hydrodynamics locally when the energy density exceeds a threshold to produce gradual energy deposition and a natural core-corona structure. Dilepton emission is computed from off-equilibrium SMASH radiation and thermal rates using the Rapp-Wambach-Hees framework, enabling a direct comparison between DynFlu and iso-$\tau$ initializations across beam energies. The extracted IMR temperature $T_{\mathrm{eff}}$, derived from $dN/dm_{ee} \propto m_{ee}^{3/2} \exp(-m_{ee}/T)$, correlates with the maximum hydro temperature via $\langle T_{\mathrm{in}}\rangle = \kappa T_{\mathrm{eff}} + c$, but DynFlu yields a larger slope ($\kappa \approx 1.88$ vs $1.52$ for iso-$\tau$ in the integrated case), signaling initialization-dependent thermometer reliability; employing a high-$p_T$ cut mitigates this and improves the interpretation of early-temperature signals. This work informs how dilepton spectra should be analyzed to extract robust temperature information in heavy-ion collisions.

Abstract

We present a newly developed hybrid hadronic transport + hydrodynamics framework geared towards heavy ion collisions at low to intermediate beam energies, and report on the resulting excitation function of dileptons. In this range of energies, it is unclear how to properly initialize the hydrodynamic evolution. Due to the cumulative electromagnetic radiation throughout the collision, dilepton observables are sensitive to the initial condition. In this work, we study how the dilepton ``thermometer'' is affected by employing dynamical initial conditions, in contrast to the traditional fixed-time approach.

Figures (2)

• Figure 1: Effect of different fluidization conditions for intermediate beam energies. Left: Total energy deposition in fluid core. Right: Invariant mass spectra of dileptons.
• Figure 2: Left: beam energy dependence of the effective dilepton temperature for different fluidization setups. Right: correlation between said temperature with the maximum average temperature reached by the fluid. The bars around each point indicate the fit uncertainty in $T_\mathrm{eff}$ and the $1\sigma$ spatial variation in $\left\langle T_\mathrm{Hydro}\right\rangle$.