[Paper Review] Two photon physics. Personal recollection

I. F. Ginzburg|arXiv (Cornell University)|Aug 26, 2015

Laser Design and Applications3 references3 citations

TL;DR

This paper presents a personal recollection of the development of two-photon physics, emphasizing the pioneering work at e⁺e⁻ colliders and the proposal to study hadron production via γγ collisions. The authors introduce a method to isolate two-photon processes from background mechanisms, enabling precise measurements of γγ→hadrons, and highlight key applications such as determining photon structure functions, probing QCD, and studying W/Z boson and Higgs boson production with high accuracy at future photon colliders.

ABSTRACT

The term two--photon processes is used for the reactions in which some system of particles is produced in collision of two photons, either real or virtual. In the study of these processes our main goal was to suggest approach, allowing to extract from the data information on proper two--photon process separating it from mechanism which responsible for the production of photons. Here I present my view for history of two--photon physics. I don't try to give complete review, concentrating mainly on works of our team (which cover essential part of the topic) and some colleagues. My citation is strongly incomplete. I cite here only papers which were essential in our understanding of the problems. The choice of presented details is the result of my discussions with Gleb Kotkin and Valery Serbo. 1. Prehistory. 2. Two photon processes at e^+e^- colliders. 3. Photon colliders. 4. Notes on physical program.

Motivation & Objective

• To develop a method for isolating genuine two-photon processes from background mechanisms in e⁺e⁻ collider data.
• To enable the extraction of information on γγ→hadron cross sections, particularly for ππ, KK, and vector meson final states.
• To propose that e⁺e⁻ colliders and future photon colliders provide unique access to fundamental QCD and electroweak processes.
• To demonstrate the feasibility of measuring γγ→WW, γγ→ZZ, and γγ→Higgs processes for testing electroweak radiative corrections and Higgs sector structure.
• To advocate for the use of polarized photons in future photon colliders to probe CP violation in the Higgs sector.

Proposed method

• The authors use Feynman diagram calculations from first principles, avoiding reliance on the Weizsäcker-Williams approximation to ensure accuracy in two-photon amplitude calculations.
• They apply a kinematical reconstruction method to separate two-photon contributions from bremsstrahlung and other QED backgrounds in e⁺e⁻ collisions.
• The approach involves analyzing processes such as e⁺e⁻→e⁺e⁻ hadrons, where virtual photons from initial and final electrons fuse to produce hadronic states.
• The method allows extraction of γγ→hadron amplitudes by isolating the two-photon contribution through angular and energy distributions of final-state particles.
• Theoretical predictions are made for cross sections of γγ→ρρ, γγ→WW, γγ→ZZ, and γγ→Higgs, using QCD and electroweak theory.
• The study includes calculations of cross sections for multiple gauge boson production (e.g., γγ→WWZ, γγ→WWWW) at high energies, showing non-decaying rates due to t-channel vector exchange.

Experimental results

Research questions

• RQ1 How can genuine two-photon processes be extracted from data at e⁺e⁻ colliders, separating them from QED backgrounds like bremsstrahlung?
• RQ2 What is the role of photon structure functions in two-photon processes, and how can they be tested using QCD predictions at high Q² and s?
• RQ3 Can two-photon collisions at future photon colliders provide high-precision measurements of W and Z boson production and their radiative corrections?
• RQ4 What is the potential of γγ collisions to probe the Higgs sector, including CP violation and heavy Higgs boson production?
• RQ5 How do spin-0 and spin-2 resonances in γγ→WW and γγ→ZZ channels reflect strong dynamics in the Higgs sector?

Key findings

• The cross section for e⁺e⁻→e⁺e⁻π⁺π⁻ was calculated to be measurable at e⁺e⁻ colliders, enabling the study of γγ→ππ scattering with high precision.
• Theoretical cross sections for γγ→WW and γγ→ZZ reach 80–90 pb at high energies, due to dominant t-channel vector exchange, making them accessible for precision measurements.
• The process γγ→WWZ has a cross section of order ασ_W, and γγ→WWWW has α²σ_W, indicating potential for observing multiple gauge boson production at future photon colliders.
• The structure function of the photon can be calculated from QCD at large Q² and s without phenomenological parameters, providing a unique test of QCD in two-photon processes.
• Polarized photon collisions at future photon colliders offer a unique opportunity to probe CP violation in the Higgs sector via γγ→h processes.
• The study of γγ→ρρ enables high-accuracy determination of the Pomeron trajectory in QCD, with early calculations presented in this work.