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

Tomáš Ježo, Michael Klasen, Alexander Puck Neuwirth

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.

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.

Paper Structure

This paper contains 12 sections, 23 equations, 3 figures.

Table of Contents

  1. Introduction
  2. Dielectron pairs from photons
  3. Kroll-Wada
  4. Collinear QED splittings
  5. Parton shower
  6. Numeric results
  7. PHENIX
  8. ALICE
  9. Conclusion
  10. Open Access.
  11. Pythia and Vincia code
  12. POWHEG directphoton input

Figures (3)

  • Figure 1: Comparison of Kroll-Wada and parton-shower based predictions in pp collisions at $\sqrt{S_{pp}}=5020GeV$. The fits are restricted to $M_{ee} < 1$ GeV (full line) and extrapolated (dashed line). All uncertainties are of statistical origin. The fit results are obtained using the smpl framework alexander_puck_neuwirth_2025_15624550.
  • Figure 2: All data except the Simple and Vincia curves are taken from PHENIX:2009gyd. The uncertainties from the Pythia runs are statistical only.
  • Figure 3: The Kroll-Wada function for $pp$ at $\sqrt {S_{pp}} = 7TeV$ used in ALICE:2018fvj is shown in comparison to our PS results. The analysis cuts applied are $3 < p_{T,ee} < 4GeV$, $|\eta_e| < 0.8$ and $p_{T,e} > 0.2GeV$. The Pythia and Vincia results are given with statistical uncertainties as vertical bars, while the uncertainty band of POWHEG+ Vincia are the typical seven point scale variation.