# QED in extreme regimes

LUXE: a new experiment to study non-perturbative QED in electron-laser and photon-laser collissions

QED is the most precisely tested theory in the Standard Model of constituents of matter and their interactions. Its precise predictions were made possible by calculations with perturbative methods due to the small value of the fine structure constant α and weak electromagnetic (EM) fields.

However, in the regime of strong fields, often called strong-field QED (SFQED), the perturbative methods break down and non-perturbative QED predicts many phenomena that are yet to be confirmed experimentally.

In the QED vacuum, fluctuations occur causing virtual electron-positron pairs to be constantly created and annihilated. When a pair is created, the electron and the positron initially separate, then reunite into annihilation. If during the fluctuation time a high enough external energy source larger than twice the electron mass is supplied, for example by an external strong electric field, the distance between the pair will be larger than the electron Compton wavelength and a real electron-positron pair will be produced out of the vacuum. Virtual photons spontaneously convert into real electron-positron pairs at a critical field, quoted as the Schwinger limit.

The fluctuating vacuum is stimulated by a high electric field to produce real e + e − pairs. The Schwinger limit has not yet been reached experimentally. In fact, its value is orders of magnitude higher than the field of the most powerful lasers available today.

However, in the rest frame of a high-energy probe charge, the EM field strength

is boosted. With a 16.5 GeV electron beam and an incident angle of 17.2 degrees it is possible to reach the Schwinger limit in the electron rest frame with laser power of the order of 40 – 350 TW, as planned for LUXE.

Bibliography:

- «LUXE: A new experiment to study non-perturbative QED in electron-laser and photon-laser collisions» by A. Levy https://arxiv.org/abs/2206.00403