Fundamental physics at an X-ray free electron laser

TL;DR

The paper surveys the potential of X-ray free electron lasers (XFELs) to probe fundamental physics beyond traditional XFEL applications, focusing on nonperturbative QED and beyond-Standard-Model phenomena such as Schwinger vacuum pair production, the Unruh effect, and axion production. It describes the XFEL operating principle (SASE in undulators) and current design capabilities, then analyzes the field strengths, focusing limits, and experimental schemes required to realize these fundamental tests. Key conclusions are that, with substantial technological upgrades—notably TW-scale energy extraction and diffraction-limited focusing to ~0.1 nm, and feasible photon-regeneration setups—the XFEL regime could enable observable signatures of these effects, albeit in a developing research area. The work emphasizes that pursuing these goals could unlock unique tests of quantum field theory under extreme conditions and searches for light new particles in the X-ray domain, despite significant challenges remaining.

Abstract

X-ray free electron lasers (FELs) have been proposed to be constructed both at SLAC in the form of the so-called Linac Coherent Light Source as well as at DESY, where the so-called XFEL laboratory is part of the design of the electron-positron linear collider TESLA. In addition to the immediate applications in condensed matter physics, chemistry, material science, and structural biology, X-ray FELs may be employed also to study some physics issues of fundamental nature. In this context, one may mention the boiling of the vacuum (Schwinger pair creation in an external field), horizon physics (Unruh effect), and axion production. We review these X-ray FEL opportunities of fundamental physics and discuss the necessary technological improvements in order to achieve these goals.

Figures (6)

• Figure 1: Principle of a single-pass X-ray free electron laser in the self amplified spontaneous emission mode Materlik:2001qr.
• Figure 2: Left: Spectral peak brilliance of X-ray FELs and undulators for spontaneous radiation at TESLA, together with that of third generation synchrotron radiation sources Materlik:2001qr. For comparison, the spontaneous spectrum of an X-ray FEL undulator is shown. Right: Schematic view of the TESLA XFEL electron beam transport (top) and the XFEL laboratory (bottom) Materlik:2001qr.
• Figure 3: Time evolution of the number density of produced $e^+e^-$ pairs at the focus of an X-ray laser Alkofer:2001ik.
• Figure 4: Schematic diagram of an experiment to detect the Unruh effect at an X-ray free electron laser Chen:priv.
• Figure 5: Exclusion region in mass $m_A$ vs. axion-photon coupling $g_{A\gamma}$ for various current experiments (adapted from Ref. Groom:2000in, where also the corresponding references can be found). Also shown in this figure, and labeled with "SASE-5", is the projected sensitivity Ringwald:inprep of a photon regeneration experiment using the SASE-5 XFEL (cf. Table \ref{['tab:xfel_par']}), as well as the one of a hypothetical XFEL with average power $\langle P\rangle = 10$ GW ("10 GW XFEL").