Abstract
We study vacuum birefringence and x-ray photon scattering in the head-on collision of x-ray free electron and high-intensity laser pulses. Resorting to analytical approximations for the numbers of attainable signal photons, we analyze the behavior of the phenomenon under the variation of various experimental key-parameters and provide new analytical scalings. Our optimized approximations allow for quantitatively accurate results on the one-percent level. We in particular demonstrate that an appropriate choice of the x-ray focus and pulse duration can significantly improve the signal for given laser parameters, using the experimental parameters to be available at the Helmholtz International Beamline for Extreme Fields at the European XFEL as example. Our results are essential for the identification of the optimal choice of parameters in a discovery experiment of vacuum birefringence at the high-intensity frontier.
Highlights
Quantum field theory predicts the quantum vacuum to be characterized by the omnipresence of vacuum fluctuations
We in particular demonstrate that an appropriate choice of the x-ray focus and pulse duration can significantly improve the signal for given laser parameters, using the experimental parameters to be available at the Helmholtz International Beamline for Extreme Fields at the European x-ray free electron laser (XFEL) as example
We have constructed an improved analytic approximation for the differential numbers of signal photons encoding the experimental signature of vacuum birefringence and diffraction in the head-on collision of an XFEL probe with a high-intensity laser pump
Summary
Quantum field theory predicts the quantum vacuum to be characterized by the omnipresence of vacuum fluctuations. One of the most prominent signatures of quantum vacuum nonlinearity in macroscopic electromagnetic fields is vacuum birefringence [11]: as a consequence of the effective interaction of electromagnetic fields originally linearly polarized light traversing a strong-field region can pick up a small ellipticity, attributing a birefringence property to the quantum vacuum. This quasi-elastic scattering phenomenon is, not limited to the vacuum birefringence signal, but rather amounts to a generic property exhibited by arbitrarily polarized probe photons traversing a spatiotemporally localized strong field region, and provides an additional experimental signature of quantum vacuum nonlinearity. Our paper is organized as follows: in Sec. II we construct an improved analytic approximation for the differential number of signal photons encoding the signatures of vacuum birefringence and x-ray photon scattering in the head-on collision of an XFEL probe with a high-intensity laser pump.
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