Self-organized collimation mechanism of linear Breit-Wheeler positrons in single-pulse laser-plasma system

The linear Breit-Wheeler (LBW) process (γγ→e⁺e^-) is a fundamental prediction of the theory of quantum electrodynamics, but yet to be observed in experiments using real photons. Previous research [New J. Phys. 23, 115005 (2021), Phys. Rev. Lett. 131, 065102 (2023)] have shown that a pronounced amount of LBW pairs can be produced by irradiating an ultra-intense laser pulse into a foam target. Our numerical simulations show that the majority of the produced positrons in such a system can be trapped and pinched by the in-situ plasma fields, forming a backward-moving collimated positron beam reaching 10⁶ str^-1MeV^-1 with an opening angle of around 30° in the near hundred MeV regime. Such trapping is due to the strong azimuthal plasma magnetic fields created by the laser-driven electron current, leading to the backward drifting of the LBW positrons. Consequently, a self-organized pinching mechanism is enabled by the gradual decrease of such magnetic fields experienced by the LBW positrons. Our results suggest a potentially feasible experimental regime for the first-ever detection of the LBW process under laboratory conditions using real photons, almost a century after this process was theoretically proposed.