Experimental Signatures of the Quantum Nature of Radiation Reaction in the Field of an Ultraintense Laser

Abstract

The description of the dynamics of an electron in an external electromagnetic field of arbitrary intensity is one of the most fundamental outstanding problems in electrodynamics. Remarkably, to date, there is no unanimously accepted theoretical solution for ultrahigh intensities and little or no experimental data. The basic challenge is the inclusion of the self-interaction of the electron with the field emitted by the electron itself-the so-called radiation reaction force. We report here on the experimental evidence of strong radiation reaction, in an all-optical experiment, during the propagation of highly relativistic electrons (maximum energy exceeding 2 GeV) through the field of an ultraintense laser (peak intensity of 4 × 10 20 W=cm 2 ). In their own rest frame, the highest-energy electrons experience an electric field as high as one quarter of the critical field of quantum electrodynamics and are seen to lose up to 30% of their kinetic energy during the propagation through the laser field. The experimental data show signatures of quantum effects in the electron dynamics in the external laser field, potentially showing departures from the constant cross field approximation.

Spin-dependent radiative deflection in the quantum radiation-reaction regime

“…The bunch get polarized along y because the electrons with positive s y tend to be scattered upwards while those with negative s y downwards. Experimental measurement of this anti-symmetric phenomenon provides a new degree of freedom to study the quantum RR effect in addition to spin-free phenomena [7,10,[31][32][33]. Effects of the electron beam energy spread and angular divergence are illustrated in figure 4(c).…”

Realising single-shot measurements of quantum radiation reaction in high-intensity lasers

“…A major barrier to the measurement of quantum effects on RR in a collider setup (as shown in figure 1) is the shot-to-shot fluctuation of the electron energy spectrum. This prevents comparison with the currently accepted quantum model, based on the locally constant field approximation, with the required finesse to determine its accuracy [33,34]. One solution to this would be to use a conventional particle accelerator to provide the electron beam.…”

“…Previous experiments have used different approaches to overcome the problem of shot-to-shot variation of the electron beam. Use of a gas-cell target [34], for example, improves the stability of the electron beam, but it is unlikely that fluctuations in the electron energy can be completely eliminated. There is also a question about the timing of the collision of the electron bunch with the laser pulse, leading to uncertainty in the value of a 0 at collision.…”

“…The geometry required is similar to experiments previously used to probe Thomson and Compton scattering in the nonlinear regime [11,[30][31][32], see figure 1. Recent experiments have shown that detection of RR is achievable on current facilities [33,34]; however due to the shotto-shot variation of both the electron bunch and the colliding laser pulse, it was not possible to clearly distinguish between classical and quantum (stochastic) effects on the electron motion. Here we propose a solution to this problem by incorporating a pre-interaction drift which causes the electron's transverse momentum to become correlated to their transverse position in the bunch.…”

Reaching supercritical field strengths with intense lasers

“…The largest possible quantum parameter is χ 0 =χ e (θ=0). Experiments at a 0 ;0.4, χ e ;0.3 have demonstrated nonlinear QED effects including pair creation [6,7], and recently evidence of radiation reaction has been reported at a 0 ;10, χ e ;0.1 [27,28]. At present, the highest field strengths are equivalent to a 0 ;50 [29,30], or χ 0 ;1 at γ 0 m;1 GeV; a 0 >100 is expected in the next generation of laser facilities [31][32][33].…”