  We show under rather general assumptions that hidden sectors that never reach
thermal equilibrium in the early Universe are also inaccessible for the LHC. In
other words, any particle that can be produced at the LHC must either have been
in thermal equilibrium with the Standard Model at some point or must be
produced via the decays of another hidden sector particle that has been in
thermal equilibrium. To reach this conclusion, we parametrise the cross section
connecting the Standard Model to the hidden sector in a very general way and
use methods from linear programming to calculate the largest possible number of
LHC events compatible with the requirement of non-thermalisation. We find that
even the HL-LHC cannot possibly produce more than a few events with energy
above 10 GeV involving states from a non-thermalised hidden sector.

High Energy Physics - Phenomenology