Ion motion in plasma wakes driven by collider-density beams
In plain words
Collider bunches are so dense that they pull the plasma's ions inward while passing, which spoils the smooth focusing the wake normally gives. How much this damages beam quality at collider parameters is not settled.
Precise statement
For witness bunches with density high enough that background ions move within the bunch duration (ion phase advance of order 1), the focusing force becomes nonlinear and varies along the bunch. Determine the equilibrium or saturated emittance growth for TeV-scale collider parameters and whether matching or ion-species choice limits it below 10 percent. An answer is a quantitative growth estimate validated by 3D simulation.
What would settle it
3D particle-in-cell simulations of collider-parameter bunches over full stage lengths with converged emittance results.