Laboratory pair plasma with measurable collective effects
In plain words
For the pairs from a cascade to act as a true plasma, they must be dense enough to respond together to fields. The question is whether existing lasers and electron beams can produce such a pair plasma and show its collective behavior.
Precise statement
Pairs created by beam-laser or laser-laser cascades act collectively only if the relativistic pair plasma frequency exceeds the inverse interaction time; Qu, Meuren and Fisch (2021) predict a measurable blue shift of the laser frequency as a signature. Determine whether the frequency upshift, or another collective signature such as field screening or plasma-wave emission, can be measured with existing multi-PW lasers and 10 GeV-class electron beams, and give the expected magnitude and required detection resolution.
What would settle it
An experiment measuring laser spectral upshift or another collective signature that scales with pair density as predicted.