Azimuthal Modulations in Photon-Induced Processes

TL;DR

This paper reviews recent advances in understanding azimuthal modulations in photon-induced processes, highlighting their role in probing photon polarization, interference effects, and applications in nuclear imaging and fundamental physics tests.

Contribution

It provides a comprehensive overview of theoretical and experimental progress in azimuthal asymmetries in photon-induced reactions across different collider environments.

Findings

Excellent agreement between theory and experiment in UPC dilepton and vector meson production.

Identification of double-slit interference effects in nucleus-nucleus collisions.

Azimuthal asymmetries enable direct phase extraction in gamma-gamma to pion-pair processes.

Abstract

We review recent theoretical developments and experimental measurements of azimuthal modulations in photon-induced processes, covering both ultra-peripheral heavy-ion collisions (UPCs) and $e^+e^-$ colliders. The azimuthal asymmetries $\cos(n\phi)$ ($n=1,2,3,4$) serve as precision diagnostics that probe the linear polarization of coherent photons, final-state soft radiation effects, and quantum interference phenomena at the femtometer scale. In UPCs, we discuss dilepton production, diffractive dijet and vector meson production, where theoretical predictions show excellent agreement with STAR, ALICE, and CMS measurements. The unique double-slit interference effect in nucleus-nucleus collisions plays a crucial role in describing experimental observations. At $e^+e^-$ colliders, $\gamma\gamma\to\pi\pi$ azimuthal asymmetries enable the direct extraction of helicity amplitude phases, with important implications for hadronic light-by-light scattering and the muon anomalous magnetic moment. Azimuthal modulations establish a powerful tool for multi-dimensional nuclear imaging and precision QED/QCD tests.