Collimated and neutral electron-positron plasmas in the laboratory

An electron-positron pair plasma (EPP) is a unique state of matter that presents an exact symmetry between its negatively charged (electrons) and positively charged (positrons) constituents. Despite being virtually absent on Earth, it is believed to be a rather common occurrence in the Universe and it is thought to represent the main constituent of the atmosphere of immense astrophysical objects such as black holes, quasars, and pulsars. There is general consensus in the astrophysical community that EPPs are likely candidates for being the main ingredient of some of the most powerful emissions of matter from these objects, in the form of collimated jets, which are ultimately associated with the emission of ultra-bright and short-lived gamma-ray bursts. These astrophysical objects are exceptional natural laboratories to test physics at its limits and, due to their immense distance from Earth (some exceeding a billion light years), they provide a unique window on the primordial stages of our Universe.

Despite their fascinating role in a wide range of fundamental physical scenarios, EPPs are poorly understood due to the extreme difficulty in generating them in the laboratory. All our knowledge of this peculiar state of matter relies exclusively on astrophysical observations and their theoretical interpretation. However, astrophysical measurements are intrinsically limited and it is virtually impossible to access the microphysics of these objects due to their immense distance from Earth. Laboratory experiments would greatly advance our knowledge of this state of matter and only recently the Principal Investigator and his collaborators have succeeded in producing a neutral electron-positron plasma in the laboratory.

We propose here to continue this line of exciting experimental research by performing a series of experiments using the HERCULES laser hosted by the Centre for Ultrafast Optical Sciences (CUOS) at the University of Michigan, US. In particular, we plan to improve the quality of laser-generated EPPs by minimising their divergence and therefore significantly extend their propagation distance in controlled background electron-ion plasmas. The successful generation of a collimated and neutral EPP will allow, for the first time, for the study of EPP dynamics in background electron-ion plasmas in conditions of relevance to the propagation of astrophysical jets in the intergalactic medium. This preliminary work will not only be of interest in its own right but it will also serve as the basis for obtaining more extended grant funding on this newly developing area of experimental research.

The results of the proposed research will be greatly beneficial for the plasma physics community as well as the astrophysical community. In both cases, it is envisaged that the generation of collimated and neutral electron-positron beams will attract great interest from both the more experimentally and theoretically minded academics. For the former, it would represent a precious tool for the detailed study of pair plasmas in the laboratory, whereas for the latter it would represent an unmatched opportunity for benchmarking numerical calculations and validating theoretical models routinely used in the interpretation of astrophysical observations.

When successful, the proposed research will pave the way for a new branch of experimental physics, namely the laboratory study of electron-positron plasmas and the laboratory small-scale reproduction of astrophysical jets.

Moreover, we envisage that the achievement of the proposal's objectives is likely to fascinate the layman as well. Great interest has already been generated around the preliminary results obtained by the Principal Investigator and his collaborators, as demonstrated by dedicated media coverage in popular science magazines (such as the New Scientist) and websites (such as Physics World, The Conversation UK, and IFL Science). A dedicated article on the work of the Principal Investigator in this area has already been read by more than 120,000 people, placing it to be the 6th most read article in The Conversation of all times. The generation of a small-scale astrophysical jet in the laboratory is likely to attract similar popular interest, boosting the perception of science in the UK.