On the LHC sensitivity for non-thermalised hidden sectors

TL;DR

The paper demonstrates that hidden sectors never in thermal equilibrium with the early Universe are also inaccessible at the LHC, implying that observable particles must have been in thermal contact or produced via decays from such sectors.

Contribution

It introduces a general parametrisation and linear programming approach to show the LHC cannot detect non-thermalised hidden sectors under broad assumptions.

Findings

LHC cannot produce more than a few events from non-thermalised hidden sectors.

Particles detectable at LHC must have been in thermal equilibrium or from decays of such sectors.

Non-thermalised hidden sectors are effectively invisible to LHC experiments.

Abstract

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.