Description
The E-320 collaboration is colliding the compressed, 10 GeV FACET-II electron beam with 10 TW-scale NIR laser pulses [1,2]. In the rest frame of the ultra-relativistic electrons the intensity of the laser field is comparable to/exceeds the QED critical value of $10^{29}$ W/cm$^2$. This permits experimental studies of Strong-Field QED, i.e, the regime where the quantum parameter $\chi$ satisfies the relation $\chi≳1$. Here, $\chi$ is defined as the ratio of the laser electric field in the electron rest frame to the QED critical one. In the regime $\chi≳1$ the probability for electron-positron pair production is no longer exponentially suppressed. Already for $\chi≳0.1$ the recoil induced by individually emitted photons is, on average, comparable to the initial electron energy. Therefore, quantum corrections to classical synchrotron radiation become important [3-5].
The character of photon emission and pair production is critically affected by the classical field-strength parameter $a_{0} = eE/(m c\omega)$, which characterizes the formation length of typically emitted, high-energy photons as well as laser-produced electron-positron pairs. For $a_0≲1$ the formation length is of the order of the laser wavelength or larger, implying that the emitting electron experiences an oscillatory field. For $a_0 \gg 1$, the formation region is much smaller than the laser wavelength, implying that the quantum process happens inside a locally static field. Therefore, the transition between both regimes is analogous to the transition between multi-photon and tunneling ionization in atoms. In fact, $1/a_0$ can be considered as a generalized Keldysh parameter for photon emission / pair production in an oscillatory field [6].
In the seminal SLAC experiment 144 nonlinear Compton scattering and pair production were first observed in the multiphoton regime ($a_{0}≲1$), where the laser can be treated perturbatively [7]. In E-320 we are probing the transition from the perturbative ($a_{0}^{2} \ll 1$) to the non-perturbative regime ($a_{0}^{2} \gg 1$), while $\chi{\sim}1$. During this transition, qualitative changes occur in the spectrum of scattered electrons; specifically, the Compton edges are blueshifted due to the ponderomotive shift in the effective electron mass, and the spectrum becomes quasi-continuous.
During the E-320 measurements in May 2025, we obtained a peak value of $a_{0}^{2}≳10$, corresponding to $\chi≳0.5$ at a collision angle of $\sim30^\circ$. According to our preliminary analysis we observed an exponential electron spectrum below the linear Compton edge, with energy losses down to at least 5 GeV. Correspondingly, E-320 was able to observe radiation reaction in the quantum regime. In this talk we will present the current status of the E-320 data analysis.
[1] Yakimenko et al., PRAB 22, 101301 (2019)
[2] Reis and Meuren (for the collaboration), talk at FACET-II meeting (2024)
[3] Fedotov et al., PR 1010, 1 (2023)
[4] Gonoskov et al., RMP 94, 045001 (2022)
[5] Di Piazza et al., RMP 84, 1177 (2012)
[6] Borysov et al., arXiv:2506.04992 (2025)
[7] Bamber et al., PRD 60, 092004 (1999)
