Dynamical production of dileptons within a hybrid approach

The work examined several questions related to dilepton production using hadronic transport and hybrid approaches that couple transport to hydrodynamics. The first topic concerned the in-medium modifications of resonances, particularly the $\rho$ meson, which dominates the low-mass dilepton spectrum. Many-body calculations predict a strong broadening of resonance spectral functions in hot and dense matter. Off-shell transport models account for this phenomenologically through density-dependent widths, while on-shell approaches such as SMASH propagate hadrons with their vacuum properties. Nevertheless, inelastic scatterings shorten resonance lifetimes and effectively increase their widths, a mechanism known as collisional broadening. By following the interaction history of resonances, the collisional width was quantified in both a thermalized hadron gas and in heavy-ion collisions at HADES energies. Comparisons to experimental dilepton data and many-body results showed that collisional broadening alone cannot reproduce the full extent of in-medium modifications, which also require effects such as chiral mixing. Still, it produces qualitatively similar trends, with universal broadening as a function of hadron density. Interestingly, when comparing equilibrated systems to approximately thermal regions of actual collisions, significant differences emerged due to chemical imbalance. This indicates that thermal spectral functions commonly employed for dilepton rates may miss important off-equilibrium effects present in real reactions. The next study investigated dielectron elliptic flow at low beam energies, where HADES data indicate nearly vanishing values in the resonance mass region. Since elliptic flow is normally interpreted as a signal of collectivity, this result raised questions about the sensitivity of dileptons to the medium’s expansion. Simulations with SMASH confirmed that the total dilepton flow is small but also revealed that individual sources develop substantial elliptic flow. Their contributions largely cancel when averaged, explaining the experimental observation. Thus, the small net signal does not imply a lack of collectivity but rather the interplay of competing channels. To address this, a new analysis method was proposed: the tagged scalar product technique, which correlates dilepton emission with specific hadron species. This method allows the flow of individual sources to be isolated without requiring additional data. In particular, proton-tagged dileptons reproduce closely the flow associated with $\Delta$ baryons, demonstrating that source-resolved information can be extracted from inclusive measurements. At higher collision energies, the system formed in the overlap region thermalizes rapidly and is well described by hydrodynamics. Standard hybrid models implement this by switching from transport to fluid dynamics at a fixed proper time. At lower energies, however, the finite crossing time of the nuclei and strong stopping effects invalidate such assumptions. To overcome this, a new scheme called dynamic fluidization was developed. Here, hadrons are converted into fluid cells gradually and locally whenever the surrounding energy density exceeds a threshold. This prescription correlates naturally with physical criteria such as mean free path and Knudsen number and provides an automatic separation between dense core and dilute corona. In implementing dynamic fluidization, the hydrodynamic code was adapted to Cartesian coordinates, replacing the usual hyperbolic smearing. While the results depend on technical parameters such as the smearing width, a constant choice across beam energies already produced reasonable outcomes between $\sqrt{s_{NN}}=3\to9.1\ \mathrm{GeV}$. The first application examined dilepton radiation in the intermediate-mass region, where thermal emission dominates. The analysis showed that the effective temperature extracted from the spectra depends strongly on the initialization scenario, highlighting the sensitivity of electromagnetic probes not only to medium properties but also to the modeling of the early stages. Finally, the thesis addressed the role of the hadronic afterburner for heavy flavor. Heavy quarks are produced in the earliest collisions and are regarded as clean probes of the quark–gluon plasma. Once hadronized into open-charm mesons or charmonia, however, they continue to traverse the hadronic medium, where further interactions can occur. These are often neglected in phenomenological studies. To assess their importance, a simplified afterburner model was constructed. The results showed that $D$ mesons experience significant rescattering and tend toward partial thermalization, while charmonia undergo dissociation with nontrivial momentum dependence. The dilepton yield from correlated charm decays was also found to be substantially modified, especially in the intermediate-mass region. These findings demonstrate that late-stage hadronic effects cannot be ignored in heavy-flavor analyses, particularly as the field enters an era of precision measurements. Taken together, these studies reinforce a central message: electromagnetic observables and heavy flavor carry the imprints of both the partonic and hadronic stages of the collision. Collisional broadening illustrates how off-equilibrium conditions affect resonance properties; elliptic flow demonstrates the cancellation of collective signals across different sources; dynamic fluidization emphasizes the role of initialization in shaping thermal radiation; and heavy-flavor rescattering reveals the impact of the hadronic afterburner on open and hidden charm. In each case, the hadronic medium emerges not as a passive background but as an active agent that modifies observables in measurable ways. In summary, the thesis has shown that the interpretation of electromagnetic and heavy-flavor observables requires a careful treatment of hadronic dynamics. By systematically exploring these effects with transport and hybrid models, the work demonstrates how dileptons and charm can serve as windows into the full evolution of heavy-ion collisions, from the earliest interactions to the final freeze-out stage.