Abstract
We report on experimental spectra of photons radiated by 50 GeV positrons crossing silicon single crystals of thicknesses 1.1 mm, 2.0 mm, 4.2 mm, and 6.2 mm at sufficiently small angles to the (110) planes that their motion effectively is governed by the continuum crystal potential. The experiment covers a new regime of interaction where each positron emits several hard photons, whose recoil are not negligible and which are formed on lengths where the variation of the crystal field cannot be ignored. As a result neither the single-photon semiclassical theory of Baier et al. nor the conventional cascade approach to multiple hard photon emissions (quantum radiation reaction) based on the local constant field approximation are able to reproduce the experimental results. After developing a theoretical scheme which incorporates the essential physical features of the experiments, i.e., multiple emissions, photon recoil and background field variation within the radiation formation length, we show that it provides results in convincing agreement with the data.
Highlights
Strong electromagnetic fields such as those produced by intense lasers and by crystals are a unique tool to test QED in the laboratory in unprecedented high-energy regimes, where nonlinear effects in the electromagnetic field energy density dominate the dynamics [1,2,3,4,5,6]
When electrodynamical processes occur in the presence of a sufficiently intense background electromagnetic field, the photon density of the latter is so high that charged particles like positrons interact coherently with several background field photons
By employing a conventional kinetic approach based on the emission probabilities evaluated within the local constant field approximation (LCFA), we show that such an approach is unable to explain the experimental results
Summary
Strong electromagnetic fields such as those produced by intense lasers and by crystals are a unique tool to test QED in the laboratory in unprecedented high-energy regimes, where nonlinear effects in the electromagnetic field energy density dominate the dynamics [1,2,3,4,5,6]. The localization of the emission is a crucial requirement of the method and it corresponds to assuming that the formation length l f of the photon emission process is much smaller than the typical length where the crystal field significantly varies, such that the local value of the probability per unit time, evaluated for a constant field, can be employed [2,3] This local constant field approximation (LCFA) is another remarkable tool in strong-field physics and recent studies have been devoted to investigating its limitations, especially in the realm of SFQED in beamstrahlung [14,15], in intense laser fields [16,17,18,19], and in space-time dependent electric fields [20]. The theoretical spectra obtained with this method result in overall good agreement with the data, which in turn can be interpreted as experimental evidence of quantum radiation reaction beyond the LCFA
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