On Sunday, the Large Hadron Collider officially made the switch from protons to lead ions. At 82 protons a piece, lead ions are much heavier and thus collide with a lot more energy—574 Tera electron volts (TeV), as opposed to the proton collisions which were at just 7 TeV.

When they hit, the lead nuclei are reaching temperatures of up to 100,000 times the temperature of the Sun, and that causes something unusual to happen. Normally quarks are bound into tightly in protons and neutrons by the strong nuclear force. The strong force is a tough nut to crack at everyday energies, but paradoxically, it gets weaker as the energy rises. By the time you get to the energies inside the LHC, it is weak enough to allow the little quarks to bounce around almost freely.

This quark-gluon soup is what physicists are interested in studying. It tells them more about the strong force and possibly about the Higgs mechanism, which endows protons and neutrons (and ultimately everything else) with mass.

Given that lead collisions are higher energy and really interesting, you might wonder why the LHC doesn’t do it all the time? The photos on the right from the ALICE detector hold the answer: All those protons and neutrons colliding at once create a big mess. Protons alone provide a much cleaner signal, and that means it will take both kinds of collisions to probe fundamental physics.

Credit: ALICE collaboration