Conversion of photons to dileptons in the Kroll-Wada and parton shower approaches

Abstract

The study of dileptons in high-energy heavy-ion collisions provides critical insights into the properties of the quark-gluon plasma and the thermal radiation emitted throughout its evolution. In the low-mass region, dileptons originate from both direct photon conversion and hadronic decays, with the Kroll-Wada equation traditionally used to relate direct real and direct virtual photon production. In this work, we explore the possibility of using parton shower event generators to model this conversion process, leveraging their unitary treatment of internal photon conversions that naturally preserves normalisation, as well as their ability to incorporate higher-order corrections, recoil kinematics, and realistic experimental selection criteria. We compare the Kroll-Wada approach to simulations using the Pythia8 simple shower, the Vincia sector shower, and the POWHEG shower matched NLO event generator. Our results reveal that the parton shower approach offers improved accuracy in describing the dilepton spectrum, particularly towards larger invariant masses where phase-space suppression effects become relevant.