Evidence of suppressed beam-plasma instabilities in a laboratory analogue of blazar-induced pair jets*

Here we report on an experimental platform at the HiRadMat facility, within CERN’s accelerator complex aimed at recreating a laboratory analogue of ultra-relativistic blazar-induced pair jets propagating into the intergalactic tenuous plasma. More than 10¹³ electron-positron pairs are produced by irradiating a target with 440 GeV protons from the Super Proton Synchrotron. The pair yield and plasma extent are orders of magnitude larger than currently achievable at laser facilities, producing for the first time pair plasma conditions necessary for the study of relativistic kinetic plasma instabilities. In our experiment, the pair beams are remarkably stable as they propagate through 1-m of plasma. Linear theory predicts that the growth of kinetic instabilities is strongly suppressed when non-idealized beam conditions are assumed, such as the inclusion of a small transverse temperature, and particle-in-cell simulations suggest that beam divergences of a few percent are enough to significantly suppress the instability. An experimentally inferred growth rate, when scaled to blazar's jets, is comparable to the inverse-Compton cooling time of the pairs on the cosmic microwave background. Given that a cascade of GeV inverse-Compton scattered photons is not observed from blazar's jets, our results imply that such an absence must be the related to the presence of intervening magnetic fields in the intergalactic plasma of primordial origin.