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Describing UPC Data with the Sar$t$re Event Generator

Vaidehi Nattoja, Tobias Toll

TL;DR

The paper addresses vector-meson photoproduction in ultra-peripheral heavy-ion collisions by enhancing the Sar$t$re dipole-model Monte Carlo with a complete photon flux that includes interference and photon $k_T$, as well as an afterburner for neutron-tagged events via $n_0^0n$. It systematically develops the theoretical framework—covering dipole amplitudes with Sat/NonSat, hotspot substructure, Good-Walker incoherent channels, and UPC-specific photon-flux formalism—and demonstrates improved agreement with STAR and LHCb data, particularly at low $p_T^2$ and across neutron classes. The work proves the importance of accounting for interference and $k_T$ in UPC predictions and showcases the practical impact of neutron tagging for event classification. Looking ahead, the authors plan faster generation for multiple vector mesons and to extend the framework to inclusive diffraction for heavy flavors, broadening the applicability of Sar$t$re in high-energy QCD studies.

Abstract

Ultra-peripheral heavy-ion collisions (UPCs) provide a distinct environment for high-energy QCD research, focusing on the production of vector mesons. This proceeding details recent advancements in the Sar$t$re Monte Carlo event generator, a dipole model-based tool, to better describe UPC data. We present the incorporation of the full photon flux, accounting for interference effects and photon transverse momentum ($k_T$), and the integration of an $n_0^0n$ afterburner for neutron-tagged event classification. These extensions significantly improve Sar$t$re's ability to reproduce experimental data from STAR and LHCb, particularly at small $p_T^2$ and for various neutron event classes.

Describing UPC Data with the Sar$t$re Event Generator

TL;DR

The paper addresses vector-meson photoproduction in ultra-peripheral heavy-ion collisions by enhancing the Sarre dipole-model Monte Carlo with a complete photon flux that includes interference and photon , as well as an afterburner for neutron-tagged events via . It systematically develops the theoretical framework—covering dipole amplitudes with Sat/NonSat, hotspot substructure, Good-Walker incoherent channels, and UPC-specific photon-flux formalism—and demonstrates improved agreement with STAR and LHCb data, particularly at low and across neutron classes. The work proves the importance of accounting for interference and in UPC predictions and showcases the practical impact of neutron tagging for event classification. Looking ahead, the authors plan faster generation for multiple vector mesons and to extend the framework to inclusive diffraction for heavy flavors, broadening the applicability of Sarre in high-energy QCD studies.

Abstract

Ultra-peripheral heavy-ion collisions (UPCs) provide a distinct environment for high-energy QCD research, focusing on the production of vector mesons. This proceeding details recent advancements in the Sarre Monte Carlo event generator, a dipole model-based tool, to better describe UPC data. We present the incorporation of the full photon flux, accounting for interference effects and photon transverse momentum (), and the integration of an afterburner for neutron-tagged event classification. These extensions significantly improve Sarre's ability to reproduce experimental data from STAR and LHCb, particularly at small and for various neutron event classes.

Paper Structure

This paper contains 11 sections, 15 equations, 4 figures.

Table of Contents

  1. Introduction
  2. Theoretical Framework
  3. Exclusive Diffraction in the Dipole Model
  4. From ep to eA and the Hotspot Model
  5. Good-Walker Picture and Incoherent Scattering
  6. From eA to UPCs: Photon Flux and Interference
  7. Integrating Sartre with n00n for Neutron Tagging
  8. Experimental Comparisons and Results
  9. STAR Data Comparison
  10. LHCb Data Comparison
  11. Summary and Outlook

Figures (4)

  • Figure 1: Dipole Model
  • Figure 2: Feynman diagrams illustrating vector meson photoproduction in UPCs, including photon flux and interference contributions.
  • Figure 3: Emitted neutron count generated using $n_0^0n$ in Sar$t$re
  • Figure 4: Comparison of Sartre predictions with STAR data, showing coherent, incoherent, and interference contributions to exclusive $J/\psi$ photoproduction