[Paper Review] Ultra-peripheral Collisions at RHIC: An Experimental Overview

S. R. Klein|arXiv (Cornell University)|Feb 24, 2015

High-Energy Particle Collisions Research3 references6 citations

TL;DR

This paper reviews ultra-peripheral collisions (UPCs) at RHIC, focusing on photonuclear and two-photon interactions to probe nuclear structure and quantum electrodynamics in strong fields. Key results include evidence for nuclear suppression in coherent ρ⁰ photoproduction—challenging simple γp cross-section extrapolations—highlighting the importance of higher-order effects and paving the way for electron-ion collider physics.

ABSTRACT

Ultra-peripheral collisions (UPCs) of ions allow us to study photonuclear and two-photon interactions at energies above those available at fixed target accelerators. For heavy ions, the couplings are large enough so that multi-photon interactions are possible, and higher order corrections are expected to be significant. In this writeup, I present some recent UPC results from the Relativistic Heavy Ion Collider (RHIC), and discuss some future prospects. I also draw parallels between UPC data and that expected at an electron-ion collider (EIC), and show how UPCs are a useful lead-in to EIC physics. This writeup is based on a talk at "Initial State 2014," (IS2014), with a focus on the newest results. One important result is that comparison of the RHIC (and LHC) results on coherent $ρ^0$ photoproduction show evidence for nuclear suppression, compared to a calculating based on $γp$ cross-sections.

Motivation & Objective

• To summarize recent experimental results from ultra-peripheral collisions (UPCs) at the Relativistic Heavy Ion Collider (RHIC), focusing on photonuclear and two-photon interactions.
• To investigate nuclear effects such as shadowing and suppression in vector meson photoproduction, particularly ρ⁰ and J/ψ production.
• To explore the role of multi-photon interactions in enhancing signal detection and probing hard photon spectra via mutual Coulomb excitation.
• To draw parallels between RHIC UPC data and future electron-ion collider (EIC) physics, especially in studying nuclear gluon distributions and generalized parton distributions (GPDs).
• To demonstrate how UPCs serve as a critical testbed for EIC physics, including spin-dependent observables and higher-twist effects in nuclear matter.

Proposed method

• Utilizes the photon flux from electromagnetic fields of relativistic heavy ions in large-impact-parameter collisions, enabling γγ and γA interactions without strong-force hadronic interactions.
• Applies Glauber-type calculations to model coherent vector meson photoproduction, incorporating multiple scattering and nuclear size effects via σ ∝ A² for small dipoles.
• Employs impact-parameter integration over P₁(b)P₂(b)...P_Nohad(b) to compute cross-sections, with unitarization applied to account for high-probability Coulomb excitation.
• Analyzes exclusive final states via proton detection in Roman pots, enabling clean selection of central diffraction events with rapidity gaps.
• Uses invariant mass distributions and pT balance to identify exclusive processes such as f₂(1270) production and suppress backgrounds.
• Compares experimental data on e⁺e⁻ and ρ⁰ photoproduction with lowest-order and all-order QED calculations, including Coulomb corrections.

Experimental results

Research questions

• RQ1 To what extent do RHIC and LHC data on coherent ρ⁰ photoproduction exhibit nuclear suppression beyond predictions based on γp cross-sections?
• RQ2 How do multi-photon interactions in UPCs modify the photon spectrum and enhance correlations in final states compared to single-photon processes?
• RQ3 What is the role of higher-order QED corrections in explaining observed e⁺e⁻ pair production rates in UPCs, particularly in the context of bound-free pair production?
• RQ4 Can exclusive photoproduction in pp UPCs—such as f₂(1270) or J/ψ—be used to probe double-Pomeron exchange and test QCD dynamics?
• RQ5 How do UPC measurements at RHIC inform the study of generalized parton distributions (GPDs), particularly the GPD-E helicity-flip distribution, in preparation for EIC experiments?

Key findings

• RHIC and LHC data on coherent ρ⁰ photoproduction show significant nuclear suppression compared to predictions based on γp cross-sections, indicating non-trivial nuclear effects.
• The cross-section for e⁺e⁻ pair production in UPCs with gold beams reaches 100,000 barns, with a substantial fraction of low-pT leptons contributing to background in vertex detectors.
• Bound-free pair production was observed via single-electron ion showers detected by PIN diodes downstream of the interaction region, consistent with expected cross-sections.
• Exclusive pp UPC events with central state production and rapidity gaps were cleanly selected using pT balance and proton detection in Roman pots, showing a hint of f₂(1270) production.
• The ρ⁰ρ⁰ pair production amplitude exhibits an enhancement at small transverse momentum difference, consistent with coherent dipole interactions and A² scaling in the coherent regime.
• Future measurements of J/ψ photoproduction on polarized proton targets in pA UPCs offer a unique pathway to access the GPD-E helicity-flip parton distribution, a key observable for nucleon spin structure.