Black holes and neutron stars are among the densest known objects in the universe. These extreme environments host plasmas, the fourth fundamental state of matter alongside solids, liquids, and gases. Particularly notable are relativistic electron-positron pair plasmas, comprising electrons and positrons moving at nearly the speed of light.

While such plasmas are common in deep space, replicating them in a laboratory has been challenging. Now, an international team of scientists, including researchers from the University of Rochester's Laboratory for Laser Energetics (LLE), has successfully generated high-density relativistic electron-positron pair-plasma beams in the lab. Their findings, published in Nature Communications, report producing two to three orders of magnitude more pairs than previously achieved.

This breakthrough paves the way for experiments that could uncover fundamental insights into the universe's workings.

"The laboratory generation of plasma 'fireballs' composed of matter, antimatter, and photons is a key research goal in high-energy-density science," says lead author Charles Arrowsmith, a physicist from the University of Oxford joining LLE in the fall. "The difficulty of producing enough electron-positron pairs has, until now, restricted our understanding to theoretical studies."

Rochester researchers Dustin Froula and Daniel Haberberger collaborated with Arrowsmith and other scientists to design a novel experiment at CERN's HiRadMat facility. This experiment utilized the Super Proton Synchrotron (SPS) accelerator to generate high yields of quasi-neutral electron-positron pair beams using over 100 billion protons, each with kinetic energy 440 times its rest energy. The high momentum of these protons enabled them to smash atoms and release quarks and gluons, which recombine to produce showers that decay into electrons and positrons.

The resulting beam behaved like a true astrophysical plasma. "This opens a new frontier in laboratory astrophysics by allowing us 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, enabling controlled studies of plasma interactions in analogues of astrophysical systems. "Satellite and ground telescopes can't see the smallest details of distant objects. Our lab work will enable us to test predictions from sophisticated calculations and understand how cosmic fireballs are affected by the tenuous interstellar plasma," says coauthor Gianluca Gregori, a professor of physics at the University of Oxford.

Gregori also emphasizes the importance of global collaboration in achieving such breakthroughs. In addition to LLE, University of Oxford, and CERN, the research involved institutions like the Science and Technology Facilities Council Rutherford Appleton Laboratory (STFC RAL), University of Strathclyde, Atomic Weapons Establishment in the UK, Lawrence Livermore National Laboratory, Max Planck Institute for Nuclear Physics, University of Iceland, and Instituto Superior Técnico in Portugal.

The team's findings come amid ongoing efforts to advance plasma science through ultrahigh-intensity laser collisions, a research direction to be further explored using the NSF OPAL Facility.