For the first time, an international team of scientists – including researchers from the Institute for Plasmas and Nuclear Fusion (IPFN) / Instituto Superior Técnico – has experimentally generated high-density relativistic electron-positron pair-plasma beams by producing two to three orders of magnitude more pairs than previously reported. The team’s findings appear in Nature Communications.

Black holes and neutron stars are among the densest known objects in the universe. Within and around these extreme astrophysical environments exist plasmas – the fourth fundamental state of matter alongside solids, liquids, and gases. Specifically, the plasmas at these extreme conditions are known as relativistic electron-positron pair plasmas, or plasma “fireballs”, because they comprise a collection of electrons and positrons—all flying around at nearly the speed of light.

“The laboratory generation of plasma ‘fireballs’ composed of matter, antimatter, and photons is a research goal at the forefront of high-energy-density science,” says lead author Charles Arrowsmith, a physicist from the University of Oxford. “But the experimental difficulty of producing electron-positron pairs in sufficiently high numbers has, to this point, limited our understanding to purely theoretical studies”, he adds.

Luís Oliveira e Silva, Professor of Physics at Instituto Superior Técnico and Head of the Group of Lasers and Plasmas/IPFN, together with Pablo Bilbao and Filipe Cruz, PhD students at Técnico, collaborated with Arrowsmith and other scientists to design a novel experiment harnessing the HiRadMat facility at the Super Proton Synchrotron (SPS) accelerator at the European Organization for Nuclear Research (CERN) in Geneva, Switzerland. The IST team was responsible for the numerical simulations and theoretical interpretation supporting the experimental effort.

That experiment generated extremely high yields of quasi-neutral electron-positron pair beams using more than 100 billion protons from the SPS accelerator. Each proton carries a kinetic energy that is 440 times larger than its resting energy. Because of such large momentum, when the proton smashes an atom, it has sufficient energy to release its internal constituents—quarks and gluons—which then immediately recombine to produce a shower of elementary particles. In other words, the beam they generated in the lab had enough particles to start behaving like an astrophysical plasma.

“This opens up an entirely new frontier in laboratory astrophysics by making it possible to experimentally probe the microphysics of gamma-ray bursts or blazar jets,” Arrowsmith says.

The team has also developed techniques to modify the emittance of pair beams, making it possible to perform controlled studies of plasma interactions in scaled analogues of astrophysical systems. “Satellite and ground telescopes are not able to see the smallest details of those distant objects and so far we could only rely on numerical simulations. Our laboratory work will enable us to test those predictions obtained from very sophisticated calculations and validate how cosmic fireballs are affected by the tenuous interstellar plasma,” says coauthor Gianluca Gregori, a professor of physics at the University of Oxford.

In addition to Instituto Superior Técnico, University of Oxford, and CERN, collaborating institutions on this research include the University of Rochester, Science and Technology Facilities Council Rutherford Appleton Laboratory (STFC RAL), the Atomic Weapons Establishment in the UK, the Lawrence Livermore National Laboratory, the Max Planck Institute in Germany, and the University of Iceland.

                                                          Illustration: University of Rochester Laboratory for Laser Energetics / Heather Palmer

HOW IT WORKS: A proton (far left) from the Super Proton Synchrotron (SPS) accelerator at CERN impinges on carbon nuclei (small gray spheres). This produces a shower of various elementary particles, including a large number of neutral pions (orange spheres). As the unstable neutral pions decay, they emit two high-energy gamma rays (yellow squiggly arrows). These gamma rays then interact with the electric field of Tantalum nuclei (large gray spheres), generating electron and positron pairs and resulting in the novel electron-positron fireball plasma. Because of these cascade effects, a single proton can generate many electrons and positrons, making this process of pair plasma production extremely efficient.