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

TL;DR

This paper compares the traditional Kroll-Wada method with modern parton shower event generators for modeling dilepton production from photon conversion in high-energy heavy-ion collisions, highlighting improved accuracy of the latter.

Contribution

It demonstrates that parton shower event generators can effectively model dilepton production, incorporating higher-order effects and realistic experimental conditions, surpassing the traditional Kroll-Wada approach.

Findings

Parton shower models improve dilepton spectrum accuracy at higher invariant masses.

Parton showers naturally preserve normalization and include higher-order corrections.

Comparison shows better phase-space coverage with 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.