  Relativistic electron-positron plasmas are ubiquitous in extreme
astrophysical environments such as black holes and neutron star magnetospheres,
where accretion-powered jets and pulsar winds are expected to be enriched with
such pair plasmas. Their behaviour is quite different from typical electron-ion
plasmas due to the matter-antimatter symmetry of the charged components and
their role in the dynamics of such compact objects is believed to be
fundamental. 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. We present 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. The produced pair beams have a volume that fills multiple Debye
spheres and are thus able to sustain collective plasma oscillations. Our work
opens up the possibility of directly probing the microphysics of pair plasmas
beyond quasi-linear evolution into regimes that are challenging to simulate or
measure via astronomical observations.
