[Paper Review] Enhanced signature of vacuum birefringence in a plasma wakefield

Feng Wan, Ting Sun+8 more|arXiv (Cornell University)|Jun 22, 2022

Atomic and Molecular Physics6 citations

TL;DR

This paper proposes a novel method to enhance vacuum birefringence (VB) detection using a plasma wakefield driven by moderately intense lasers (10²⁰–10²¹ W/cm²), enabling large interaction lengths (mm–cm) and strong fields simultaneously. Using polarized γ-photon beams, the method achieves VB signals of ~10⁻⁵ for MeV photons and ~10⁻³–10⁻² for GeV photons, with noise from plasma electrons mitigated via polarization control and plasma density modulation.

ABSTRACT

Vacuum birefringence (VB) is a basic phenomenon predicted in quantum electrodynamics (QED). However, due to the smallness of the signal, conventional magnet-based and extremely intense laser-driven detection methods are still very challenging. This is because in the first case the interaction length is large but the field is limited, and vice versa in the second case. We put forward a method to generate and detect VB in a plasma bubble wakefield, which combines both advantages, providing large fields along large interaction lengths. A polarized $γ$-photon beam is considered to probe the wakefield along a propagation distance of millimeters to centimeters in the plasma bubble. We find via plasma particle-in-cell simulations that the VB signal in terms of Stokes parameters can reach about $ 10^{-5}$ ($10^{-3}$-$10^{-2}$) for tens of MeV (GeV) probe photons with moderately intense lasers ($10^{20}$-$10^{21}~\mathrm{W/cm^2}$). The main source of noise from plasma electrons is mitigated, in particular, by a choice of $γ$-photon polarization and by proper modulation of the plasma density. The proposed method represents an attractive alternative for the experimental observation of VB via laser-plasma interaction.

Motivation & Objective

• To overcome the limitations of conventional VB detection methods that face trade-offs between field strength and interaction length.
• To develop a compact, stable, and experimentally feasible method for detecting vacuum birefringence using laser-plasma interactions.
• To reduce noise from plasma electrons that can obscure the VB signal in high-field plasma environments.
• To demonstrate that moderate laser intensities can produce measurable VB signals over extended interaction lengths in plasma wakefields.
• To explore the potential of this method for probing weakly interacting slim particles (WISPs), such as axion-like particles.

Proposed method

• A plasma bubble wakefield is generated via a high-intensity laser pulse, creating a region of strong, localized electric fields over millimeter to centimeter-scale propagation distances.
• A linearly polarized γ-photon beam is injected along the wakefield axis to probe the vacuum birefringence effect induced by the strong field.
• The VB signal is quantified through changes in Stokes parameters, with the degree of polarization rotation measured as a function of probe photon energy and interaction length.
• Plasma particle-in-cell (PIC) simulations are used to model the interaction, including electron dynamics, photon scattering, and pair production processes.
• Noise mitigation is achieved by selecting optimal γ-photon polarization and applying a smooth, linearly increasing plasma density gradient to suppress electron-driven background effects.
• The method is robust against beam divergence, energy spread, and spatial delay, with optimal performance at a laser-to-probe delay of ~27 μm.

Experimental results

Research questions

• RQ1 Can a plasma wakefield simultaneously provide strong electromagnetic fields and long interaction lengths to enhance vacuum birefringence signals beyond current experimental limits?
• RQ2 How can plasma electron noise be effectively suppressed in γ-photon-based VB detection in laser-driven plasmas?
• RQ3 What laser and plasma parameters are required to achieve a detectable VB signal using moderate laser intensities (10²⁰–10²¹ W/cm²)?
• RQ4 To what extent do beam properties such as angular divergence, energy spread, and transverse radius affect the VB signal fidelity?
• RQ5 Can this method be extended to probe new physics, such as axion-like particles, via vacuum birefringence mediated by weakly interacting slim particles?

Key findings

• The VB signal, measured via Stokes parameter changes, reaches ~10⁻⁵ for tens of MeV γ-photon probes in a plasma wakefield with moderate laser intensities (10²⁰–10²¹ W/cm²).
• For GeV-energy γ-photon probes, the VB signal increases to ~10⁻³–10⁻², significantly enhancing detectability over conventional methods.
• The main noise source from plasma electrons is suppressed by selecting appropriate γ-photon polarization and using a smooth, linearly increasing plasma density profile.
• Scattering of probe photons via Compton scattering (CS) has a negligible probability (~3×10⁻⁸ for 2 mm path length), confirming minimal beam depletion.
• The method remains robust under realistic beam imperfections: angular divergence up to 20 mrad reduces the signal by less than a factor of two, and energy spread has minimal impact on the signal amplitude.
• The optimal laser-to-probe delay is ~27 μm, beyond which the signal decreases due to weaker field exposure in the bubble tail region.