Speaker: Gianluca Gregori (University of Oxford)

Relativistic electron-positron plasmas are ubiquitous in extreme astrophysical environments such as black-hole and neutron-star magnetospheres, where accretion-powered jets and pulsar winds are expected to be enriched with electron-positron pairs.
Their role in the dynamics of such environments is in many cases believed to be fundamental, but their behaviour differs significantly from typical electron-ion plasmas due to the matter-antimatter symmetry of the charged components.
So far, our experimental inability to produce large yields of positrons in quasi-neutral beams has restricted the understanding of electron-positron pair plasmas to simple numerical and analytical studies, which are rather limited.
Here we present the first experimental results confirming the generation of high-density, quasi-neutral, relativistic electron-positron pair beams using the 440 GeV/c beam at CERN’s Super Proton Synchrotron (SPS) accelerator.
We show that the characteristic scales necessary for collective plasma behaviour, such as the Debye length and the collisionless skin depth, are exceeded by the measured size of the produced pair beams.
In the first application of this experimental platform, the stability of the pair beam is studied as it propagates through a meter-length plasma, analogous to TeV γ-ray induced pair cascades in the intergalactic medium. It has been argued that pair beam instabilities disrupt the cascade, thus accounting for the observed lack of reprocessed GeV emission from TeV blazars.
If true this would remove the need for a moderate strength intergalactic magnetic field to explain the observations.
We find that the pair beam instability is suppressed if the beam is not perfectly collimated or monochromatic, hence the lower limit to the intergalactic magnetic field inferred from γ-ray observations of blazars is robust.
We conclude the talk by discussing applications of the technique to other facilities (e.g., the BTF in Frascati), particularly in the study of collective plasma instabilities and coherent emission processes, as those occurring in astrophysical masers.