What is jet quenching? See CMS.
This event in the first lead-ion run of the LHC has two nearly back-to-back jets of particles from a single event. Their momenta should have about the same magnitude (conservation laws!) but the jet at the top right falls well short of the jet at the bottom left. The jet on the right seems to have interacted with the quark-gluon plasma and transfered some of its initial momentum to the particles which make up the plasma, resulting in a lower momentum measured in the calorimeter. This is "jet-quenching." The plasma is like that which existed in the very beginning of our universe: could there have been similar effects as particles emerged from that hot primordial soup?
Credit: Copyright CERN on behalf of the CMS Collaboration.