There are few places as complicated as the debris formed from a high-energy collision of two nuclei of atoms, and there is no facility that can muster as much energy as the LHC. For about a month each year, the LHC slams together two beams of nuclei from lead atoms. If the collision is head on, the 416 protons and neutrons involved in the collision melt, freeing the quarks and gluons. The resulting state of affairs is often called quark-gluon plasma (QGP). This mix hasn’t been commonly seen in the universe since about a millionth of a second after the Big Bang.
In order to study the behavior of matter under these conditions, we turn to bottom and charm quarks. These particles decay into muons that do not experience the strong nuclear force, which lets them escape the plasma unscathed. By observing muons, we can understand the behavior of the parent quarks that are affected by the QGP.
In a collision, charm quarks can be produced in a meson called the J/ψ, (consisting of a charm quark and charm antimatter quark) while the bottom quarks can be produced in a meson called the Υ, (which consists of a bottom quark/antiquark pair). In ordinary LHC collisions between pairs of protons, these quark/antiquark pairs see each other and bind together into the mesons. However, in collisions between lead nuclei, the excessive particles shield the quark/antiquark pairs from one another. Because of this shielding, sometimes the pair doesn’t see each other which means that they don’t form a meson. The net result is fewer J/ψ and Υ mesons than predicted from what was observed in proton/proton collisions, which do not create a QGP. That’s exactly what CMS saw. The suppression of meson production was smaller for Υ production because the quark/antiquark pair is held tighter and is more difficult to shield.
CMS also studied the production of J/ψ mesons produced a large distance from the collision point. In this case, the mesons aren’t produced directly in the collision, but are decay products from long-lived particles carrying bottom quarks. There were also fewer mesons observed from this production channel, possibly because it was difficult for the bottom quarks to escape the QGP. In trying to punch through the dense environment, bottom quarks lost energy, making the decay particles harder to see.
Researching the properties of QGP is a challenging endeavor, but studies like these show how we are getting a handle on the behavior of this most exotic form of matter.