Direct Accessibility of the Fundamental Constants Governing Light-by-Light Scattering

Quantum field theory predicts that the vacuum exhibits a nonlinear response to strong electromagnetic fields. This fundamental tenet has remained experimentally challenging and is yet to be tested in the laboratory. We present proof of concept and detailed theoretical analysis of an experimental setup for precision measurements of the quantum vacuum signal generated by the collision of a brilliant x-ray probe with a high-intensity pump laser. The signal features components polarized parallel and perpendicularly to the incident x-ray probe. Our proof-of-concept measurements show that the background can be efficiently suppressed by many orders of magnitude which should not only facilitate a detection of the perpendicularly polarized component of the nonlinear vacuum response, but even make the parallel polarized component experimentally accessible for the first time. Remarkably, the angular separation of the signal from the intense x-ray probe enables precision measurements even in the presence of pump fluctuations and alignment jitter. This provides direct access to the low-energy constants governing light-by-light scattering.

Figure 1

Schematic layout of an experiment to measure the coupling constants. The XFEL is focused to a spot with a wire creating a central shadow in the beam on both sides of the focus while retaining a central intensity peak in the focus. X-ray optics image the wire to a matched aperture plane. The interaction with the pump results in signal photons scattered into the central shadow. The $\perp$, $\parallel$-polarized components are directed to separate detectors using a crystal polarizer.

Figure 2

The top figure shows the simplified geometry used for the shadow contrast measurements. The middle figure displays 0.5 s exposures of the full x-ray beam profile (a) without and (b) with Au wire inserted. The green box indicates the slit position for shadow measurements. (c) The number of photons per 1000 s integrated over the $y$ axis for different configurations. The 0.5 s exposures from (a),(b) are scaled to 1000 s resulting in a lower detection limit of $N_{\mathrm{phot}}=2\times {10}^3$ in (c). For high dynamic range measurements with a 1000 s exposure a slit was used to block the transmitted beam on either side of the wire. With this arrangement three 8 keV photons were observed in the wire shadow consistent with the level predicted due to diffraction and the off-Bragg-peak reflectivity of the silicon crystals.