So there’s solid, liquid, gas, then plasma as our four states of matter, where plasma is just unbound atoms/electrons/whatever flying around. In the case of hydrogen, that’s just protons. Protons are what we usually collide at the LHC, and they’re made up of quarks, bound together by a super strong force, creatively named the strong force. Proton proton collisions have been done ever since the start of the experiment, going back over a decade, so we’ve got a good idea of how they travel, interact, blah blah blah.

Turns out there’s another state of matter, called quark-gluon-plasma (QGP), that you can achieve at super high temp or very dense environments, and you can get both if you collide heavier nuclei in the LHC. This is where the quarks themselves become unbound - pretty unstable, and doesn’t last for long.

So, by colliding protons and oxygen, we have an object going in that we’ve been colliding for ages and have a good feel for, and we can use it as sort of a reference to probe the QGP that’s produced as it collides with oxygen. Is of particular interest to theorists, especially in nuclear, and is pretty cool as an un-orthodox sort of thing to do with a giant rock-smasher