[Paper Review] Letter of Intent for the LUXE Experiment

H. Abramowicz, M. Altarelli+35 more|arXiv (Cornell University)|Sep 2, 2019

Laser-Plasma Interactions and Diagnostics59 references42 citations

TL;DR

Proposes the LUXE experiment to probe non-perturbative strong-field QED by colliding high-energy XFEL electron beams with intense lasers, exploring nonlinear Compton scattering, nonlinear Breit–Wheeler pair production, and trident processes.

ABSTRACT

This Letter of Intent describes LUXE (Laser Und XFEL Experiment), an experiment that aims to use the high-quality and high-energy electron beam of the European XFEL and a powerful laser. The scientific objective of the experiment is to study quantum electrodynamics processes in the regime of strong fields. High-energy electrons, accelerated by the European XFEL linear accelerator, and high-energy photons, produced via Bremsstrahlung of those beam electrons, colliding with a laser beam shall experience an electric field up to three times larger than the Schwinger critical field (the field at which the vacuum itself is expected to become unstable and spark with spontaneous creation of electron-positron pairs) and access a new regime of quantum physics. The processes to be investigated, which include nonlinear Compton scattering and nonlinear Breit-Wheeler pair production, are relevant to a variety of phenomena in Nature, e.g. in the areas of astrophysics and collider physics and complement recent results in atomic physics. The setup requires in particular the extraction of a minute fraction of the electron bunches from the European XFEL accelerator, the installation of a powerful laser with sophisticated diagnostics, and an array of precision detectors optimised to measure electrons, positrons and photons. Physics sensitivity projections based on simulations are also provided.

Motivation & Objective

• Motivate and outline a path to study quantum electrodynamics in the regime of strong electromagnetic fields using the European XFEL electron beam and a powerful laser.
• Describe experimental configurations (electron–laser and gamma-converted photon–laser modes), detector systems, and diagnostics needed to measure electrons, positrons, and photons.
• Provide sensitivity projections and simulate expected outcomes to establish feasibility and precision for SFQED measurements.

Proposed method

• Use high-energy XFEL electron beams (up to 17.5 GeV) collided with high-power lasers (30–300 TW) to reach xi ~ 0.5–16 and chi_e up to ~3.
• Option 1: direct e–laser collisions to study nonlinear Compton scattering and two-step trident processes in a regime where all orders may contribute.
• Option 2: gamma_B–laser mode where electrons hit a tungsten converter to produce Bremsstrahlung photons that collide with the laser, targeting nonlinear Breit–Wheeler pair production.
• Provide detector arrays (silicon trackers, calorimeters, Cherenkov detectors) with high granularity and linear response, plus beam diagnostics and a DAQ system.
• Present a staged plan: switch between electron–laser and gamma_B–laser modes during annual shutdowns; run ~1 month per configuration to achieve sub-5% statistical precision with similar systematics.

Experimental results

Research questions

• RQ1 Can nonlinear Compton scattering spectra reveal effective electron mass shifts and edge structures as xi varies?
• RQ2 Do nonlinear Breit–Wheeler pair production rates follow perturbative xi^2n scaling at low xi and diverge toward non-perturbative behavior at higher xi and chi_e~1?
• RQ3 Is there evidence of all-order, non-perturbative SFQED effects in e–laser and gamma_B–laser collision modes?
• RQ4 What is the transition point between perturbative and non-perturbative regimes in the measured cross sections and spectra?
• RQ5 How do one-step and two-step trident processes compare in rate and kinematics in the high-intensity regime?

Key findings

• Projected to reach xi up to ~16 and chi_e up to ~3 in electron–laser mode, enabling SFQED exploration beyond perturbative limits.
• Anticipated observation of nonlinear Compton scattering and nonlinear Breit–Wheeler processes with high statistics and controlled systematics.
• Two running modes (e–laser and gamma_B–laser) provide complementary access to a broad SFQED landscape and cross-checks of theoretical predictions.
• Expected to measure transition from perturbative to non-perturbative regimes and to test exponential scaling behaviors predicted in non-perturbative QED regimes.
• Detector concept includes high-granularity tracking and calorimetry to count particles and determine spectra across high dynamic ranges.