STARlight: A Monte Carlo simulation program for ultra-peripheral collisions of relativistic ions

TL;DR

STARlight provides a two-phase Monte Carlo framework for ultra-peripheral collisions, combining $\gamma$-flux calculations in impact-parameter space with Glauber-based coherence to compute cross-sections and then generate large samples of events for two-photon and photonuclear final states. It builds 2D look-up tables in $W$ and $Y$ (and, for some cases, $p_T$) to enable fast event generation, including decays with spin-parity information; interfacing with DPMJET and PYTHIA extends coverage to general photonuclear final states. The tool covers coherent and incoherent vector-meson production, two-photon lepton and meson production, and photonuclear breakup with neutron emission, validated against RHIC and LHC data and used to calculate detector efficiencies. STARlight thus offers a practical, validated platform for UPC phenomenology, cross-section calculations, and rapid-event generation for experimental analyses.

Abstract

Ultra-peripheral collisions (UPCs) have been a significant source of study at RHIC and the LHC. In these collisions, the two colliding nuclei interact electromagnetically, via two-photon or photonuclear interactions, but not hadronically; they effectively miss each other. Photonuclear interactions produce vector meson states or more general photonuclear final states, while two-photon interactions can produce lepton or meson pairs, or single mesons. In these interactions, the collision geometry plays a major role. We present a program, STARlight, that calculates the cross-sections for a variety of UPC final states and also creates, via Monte Carlo simulation, events for use in determining detector efficiency.

Figures (5)

• Figure 1: Rapidity of one track vs. rapidity of the other for low-mass electron-positron pairs (pair mass between 0.1 and 0.3 GeV/c$^{2}$) produced in $Xn-Xn$ Au+Au collisions at RHIC energies. Pair rapidity is restricted to $|y_{pair}| < 0.1$.
• Figure 2: Transverse momentum distribution for muon pairs with pair mass greater than 1 GeV/c$^{2}$ produced in Pb+Pb collisions at LHC energies.
• Figure 3: A fit to the available data for the cross-section for Upsilon production as a function of energy in $\gamma$-p interactions is shown in the upper left ZeusUpsilonH1UpsilonAaij:2015kea. STARlight uses this fit as the basis for calculating the photonuclear cross-section. Rapidity distributions for Upsilon mesons produced at LHC energies are shown for $pp$ collisions (upper right), coherent Pb+Pb collisions (lower left) and incoherent Pb+Pb collisions (lower right).
• Figure 4: Transverse momentum distribution for J/$\psi$ mesons produced at LHC energies.
• Figure 5: Parent mass distribution for 4-pion final state produced in Au+Au collisions at RHIC energies. The 3 GeV/c$^{2}$ cutoff is due to the choice of $W_{\rm MAX}.$