Electromagnetic probes as signatures for a first-order QCD phase transition

TL;DR

This work investigates electromagnetic signatures of a first-order QCD phase transition using a non-equilibrium chiral fluid dynamics framework. By coupling a Langevin-driven chiral order parameter to Bjorken-like hydrodynamics and computing dimuon rates in both QGP and hadronic phases, the study separates the roles of entropy production, reheating, and extended lifetime on the invariant-mass spectra. The main finding is a substantial, beam-energy dependent enhancement of dilepton yields in non-equilibrium scenarios, strongest at the lowest energies (around $\sqrt{s_{NN}}=2.2$ GeV) due to reheating and longer lifetimes, with the enhancement persisting after normalizing to pion multiplicities. These results underscore the potential of dileptons as a diagnostic of non-equilibrium FOPTs in QCD matter and motivate more realistic EoS and hadronic-rate treatments in future work.

Abstract

We investigate dimuon production in the context of a first-order phase transition in QCD matter using a chiral fluid dynamics model. This approach incorporates non-equilibrium effects such as entropy production and reheating, which emerge during the dynamical evolution through a first-order phase transition. By comparing equilibrium and non-equilibrium scenarios across a range of beam energies ($\sqrt{s_{NN}}=2.2-6.2$~GeV), we analyze the resulting invariant mass spectra. Our results reveal a substantial enhancement of dilepton yields in the non-equilibrium scenario, particularly pronounced at lower beam energies, where reheating leads to a prolonged lifetime of the fireball and increased emission. The enhancement persists even after normalizing to pion multiplicities, indicating sensitivity beyond effects of entropy production.

Figures (4)

• Figure 1: Temperature as function proper time for non-equilibrium (solid lines) and equilibrium (dashed lines) evolutions. The two lowest center-of-mass energies show clear reheating and lifetime extension.
• Figure 2: Comparison of QGP, hadronic, and mixed production rate of dileptons for $\sqrt{s}=2.2$ (left) and $4.51$ GeV (right).
• Figure 3: (left) Dilepton production rate for fixed transverse momenta of $p_T=0.3$, and $1.0$ GeV for the non-equilibrium calculation. (right) Comparison of Dilepton production rates for non-equilibrium (solid lines) and equilibrium (dashed lines) scenarios with a FOPT, relative enhancement of the non-equilibrium to equilibrium scenario.
• Figure 4: Enhancement of dileptons relative to enhancement of pion production for the full evolution (left) and the scenario with equal lifetimes (right). The right hand side of each plot shows the enhancement at fixed values of $M$ as function of the energy $\sqrt{s_{\rm NN}}$.