Experiments on quark matter

Far more Upsilon-2S and 3S particles disappear in the droplet of quark matter than Upsilon-1S, as can be seen in this plot from the CMS experiment.

Last week’s Physics in a Nutshell described how lead nuclei melt when they collide in the LHC. The molten nuclei form a new kind of fluid, one made of the random motions of quarks and gluons rather than atoms or molecules. But this quark matter exists for only a few trillionths of a trillionth of a second—how can CMS scientists learn anything about it?

The droplet is far too small and short-lived to poke it with any instruments, so scientists must rely on probes that are created along with it. Fortunately, the high-energy collision of two nuclei creates a lot of particles that are understood from previous studies. Upsilon mesons, for instance, are particles made of two unusually heavy quarks, discovered at Fermilab 35 years ago. The swarm of quarks and gluons in the fluid hit the Upsilons, sometimes enough to break them apart and absorb their quarks into the mix. The exact rate of this “meson melting” yields key insights into the temperature and nature of the droplet.

Not all Upsilons are alike: Upsilon-1S mesons are the smallest and least massive, while Upsilon-2S and -3S are more massive and larger. The Upsilon-3S is the biggest target and is therefore the easiest to hit and break apart. In a recent experiment, CMS scientists clearly observed fewer surviving Upsilon-2S and -3S relative to Upsilon-1S, a phenomenon known as sequential melting.

The plot above shows what this looks like: The particles observed in the aftermath of the collision can be identified by their masses (the horizontal axis). Collisions without quark matter (orange peaks) make about half as many Upsilon-2S as Upsilon-1S, but collisions with quark matter (pink peaks) result in far fewer Upsilon-2S and almost no Upsilon-3S. Since the only difference between the control and the experiment is the presence of a quark-gluon fluid, the heavy Upsilons must have been gobbled up by the fluid.

This effect was first seen by CMS scientists in an early run, but subsequent data have made it much more clear. The data sets are now large enough to see how the effect depends on the exact way that lead nuclei collide and to see it with particles other than just Upsilons. This experiment is one example of how familiar particles can be used as tools to explore a new state of matter.

—Jim Pivarski

The U.S. physicists pictured above helped to measure the temperature of quark matter by studying sequential Upsilon melting.
After a successful term as co-head of the Fermilab LHC Physics Center, Ian Shipsey is stepping down. Meenakshi Narain is joining Rick Cavanaugh to helm the LPC. LPC heads serve for overlapping two-year terms.