[Paper Review] Photoproduction at collider energies: from RHIC and HERA to the LHC

A. J. Baltz, G. Baur+27 more|ArXiv.org|Feb 21, 2007

Particle physics theoretical and experimental studies1 references4 citations

TL;DR

This paper summarizes the 2007 ECT* workshop on photoproduction at collider energies, covering theoretical and experimental advances in photon-induced processes at HERA, RHIC, and the LHC. It highlights key physics opportunities in small-x QCD, hard diffraction, and high-energy γγ collisions, with quantitative predictions for ultra-peripheral collision yields in ALICE, including 150,000 J/ψ and 600 Υ events per year.

ABSTRACT

We present the mini-proceedings of the workshop on ``Photoproduction at collider energies: from RHIC and HERA to the LHC'' held at the European Centre for Theoretical Studies in Nuclear Physics and Related Areas (ECT*, Trento) from January 15 to 19, 2007. The workshop gathered both theorists and experimentalists to discuss the current status of investigations of high-energy photon-induced processes at different colliders (HERA, RHIC, and Tevatron) as well as preparations for extension of these studies at the LHC. The main physics topics covered were: (i) small-$x$ QCD in photoproduction studies with protons and in electromagnetic (aka. ultraperipheral) nucleus-nucleus collisions, (ii) hard diffraction physics at hadron colliders, and (iii) photon-photon collisions at very high energies: electroweak and beyond the Standard Model processes. These mini-proceedings consist of an introduction and short summaries of the talks presented at the meeting.

Motivation & Objective

• To review the current status of photoproduction physics at HERA, RHIC, and the Tevatron.
• To assess the potential of ultra-peripheral collisions (UPCs) at the LHC for probing small-x QCD and non-linear effects in nuclear wave functions.
• To explore the feasibility of studying electroweak and beyond-Standard Model physics via high-energy γγ collisions at the LHC.
• To provide a roadmap for experimental and theoretical efforts in photoproduction at the LHC, particularly in the ALICE experiment.
• To facilitate collaboration between theorists and experimentalists on photon-induced processes across collider energies.

Proposed method

• Compilation of short summaries and discussions from the ECT* workshop on photoproduction at collider energies.
• Use of the equivalent photon approximation (EPA) to model quasi-real photon fluxes from relativistic ions.
• Monte Carlo simulations of electron-positron pair production in peripheral heavy-ion collisions, including impact parameter and energy dependence.
• Exact solution of second-order Feynman diagrams for lepton pair production, generalized to all ion charges and energies.
• Comparison of results with Born approximation and Coulomb-corrected methods, identifying limitations in small-momentum approximations.
• Prediction of event rates for coherent vector meson and jet production in UPCs using ALICE detector acceptance and trigger strategies.

Experimental results

Research questions

• RQ1 What are the key theoretical and experimental advances in photoproduction at HERA and RHIC?
• RQ2 How can ultra-peripheral collisions at the LHC probe small-x QCD and gluon saturation in nuclei?
• RQ3 What are the expected yields of J/ψ and Υ mesons via coherent photoproduction in Pb-Pb UPCs at the LHC?
• RQ4 How can γγ collisions in UPCs be used to study QED in strong fields and measure triple gauge boson couplings?
• RQ5 What trigger and analysis strategies are needed to isolate and study low-multiplicity ultra-peripheral events in ALICE?

Key findings

• The ALICE experiment at the LHC is expected to observe approximately 150,000 J/ψ events per year via coherent photoproduction in ultra-peripheral Pb-Pb collisions.
• The yield of Υ mesons in the e⁺e⁻ decay channel is predicted to be around 600 events per year in ALICE.
• High rates for photoproduction of heavy quarks and jets are expected in the ALICE acceptance, with significant signal above background.
• Monte Carlo simulations show that the impact parameter dependence of lepton-pair production cross sections is crucial for understanding nonperturbative QED effects.
• The small-momentum approximation used in Coulomb correction calculations is insufficient for accurate results at small impact parameters.
• Coulomb correction terms in lepton pair production are not exact and require higher-order corrections for precision in strong-field regimes.