Description
Strong-field QED (SFQED) is the regime of physics where effective fields are comparable to Ec = 1.3 x 1018 V/m, the critical field of QED. In this regime several exotic processes can be observed such as non-linear Compton scattering and non-linear Breit-Wheeler pair production. In addition, radiation reaction - the recoil experienced by particles radiating in these fields - takes a different form from classical predictions and exhibits stochasticity. Such fields and physics are relevant to astrophysical objects like magnetars and pulsars, but experimental investigation of this area is limited by the difficulty in generating such large fields.
Currently the highest power laser beams tightly-focused can still only reach fields of order 1014 V/m, but when combined with a counter-propagating relativistic particle beam the fields in the frame of the particles are upshifted to order 1017-1018 V/m, close to Ec. These relativistic beams can be generated by laser-wakefield acceleration, allowing SFQED experiments to be carried out using lasers only as drivers, with the additional advantage that achieving spatial and temporal overlap of the beams is easier. Our collaboration has previously carried out experiments of this type on the Gemini facility [1,2,3], but the ZEUS facility provides higher power beams and therefore access to stronger field interactions.
In this talk we will present an experiment carried out in September 2025 in which electron beams up to 2 GeV were generated by laser-wakefield acceleration and collided with a 200 TW beam focused to intensity ~5 x 1020 W/cm2. The electrons were diagnosed by a magnetic spectrometer, and the gamma rays generated by the collision by a LYSO profiler and CsI stack spectrometer. We will present combined analysis of these, with supporting simulations, to estimate collision parameters and SFQED effects observed.
References:
1. J M Cole et al, Phys Rev X 8, 011020, 2018
2. K Poder et al, Phys Rev X 8, 031004, 2018
3. E E Los et al, Nat Commun 17, 1157, 2026