Heavy Ions in CMS

Probing the Plasma

Lead nuclei consist of large numbers of protons and neutrons, both made up of quarks. When the nuclei collide, a range of particles will be produced, some of which are expected to behave differently if a QGP is produced and such behaviour will tell us something about the plasma.

The QGP will for instance affect the frequency with which we see special kinds of mesons made of a pair of “heavy” quarks. When no QGP is formed, quarks remain locked within their particles and a host of these particles fly into the detector, usually seen through their decays to muon pairs. However, when a QGP has been produced, the more loosely-bound of these mesons are no longer seen because they dissolve into the scalding quark soup.

When the plasma gets denser and hotter, the more strongly-bound of these particles also “melt”. By studying the number and types of the surviving particles we can then deduce something about the state of the matter and estimate the temperature of the QGP.

By looking at the energies of the detected jets (narrow sprays of particles produced by quarks or gluons) we can also deduce how dense the plasma is.

Jets are produced in pairs in collisions and fire out in opposite directions. Jets produced in the centre of the dense plasma are “quenched”, losing energy like bullets in water, and most never emerge. Those produced at the edge however might escape, but only in one direction, because the opposite jet will be lost to the plasma mass. If the plasma is dense enough we therefore expect to see single jets rather than jet pairs.

The barely emerging jet will have lost much of its energy. Knowing how much energy loss the plasma caused tells us how dense it is, but to calculate this loss we need to know the jet’s original energy. To do this we look at jets where on the opposite side of the collision a photon or Z boson was produced instead of the second jet. Because these particles do not interact via the strong force they can sail through the QGP unaffected, and tell us the original energy of the interaction.

Z bosons decay into two muons; therefore the final particles that CMS must detect are photons, muon pairs and jets. With very precise hadron and electromagnetic calorimeters and a high performance muon detector system, CMS is ideally equipped to probe the existence of the quark-gluon plasma.