Melting mesons

Since the bottom quarks (b and b) inside the Upsilon 1S (left) are held together more tightly than in the Upsilon 3S (right), the Upsilon 1S is less likely to fall apart when clobbered by quarks and gluons in a hot plasma.

There have been some hot days this summer; not quite hot enough to cook an egg on the sidewalk, but hot enough to bake cookies in a car. But we don’t need a thermometer to quantify temperature. Besides baking in our cars, we can note what melts and what doesn’t. When nuclei collide in the LHC, the temperatures produced are hot enough to melt subatomic particles called mesons. They are a million times hotter than the center of the sun, just as the center of the sun is many thousands of times hotter than summer in Chicago.

The upsilon 1S, 2S and 3S are a family of mesons, all of which are made of a bottom quark and antiquark, bound by a stream of gluons. In an analysis of lead-nucleus collisions, CMS scientists saw far fewer upsilon 2S and 3S mesons relative to upsilon 1S. This is what we expect if the temperature were hot enough to melt them.

For one month toward the end of each year, the LHC collides lead nuclei instead of protons. Lead nuclei are made of more than 200 protons and neutrons; when they collide, there are so many interactions in a small region of space that the debris mixes and becomes fluid, called quark-gluon plasma. If the plasma is hot enough, the whirlwind of quarks and gluons can knock apart the bottom and antibottom quarks of an upsilon meson. This is called melting, as explained by this analogy of melting ice: molecules of hot air can knock apart the bonds holding water molecules together in ice. However, upsilon mesons are much smaller and harder to break up than ice.

The CMS analysis revealed that 60 percent of upsilon 1S mesons and almost 90 percent of upsilon 2S and 3S seem to disappear when immersed in the hot aftermath of lead nucleus collisions. The fact that upsilon 2S and 3S are more suppressed is the most interesting part: scientists believe that it is because the 2S and 3S are more loosely bound than the 1S. Because the bottom quarks of the upsilon 2S and 3S dangle more loosely, less force is needed to knock them apart.

This effect introduces a new way to study quark-gluon plasmas—a new state of matter so hot and so short-lived that subatomic particles are the only thermometer.

Jim Pivarski

The U.S. physicists pictured above played a major role in this analysis.
Upsilon mesons are identified by tracks of the particles they decay into. The above physicists made sure that tracking algorithms work well in collisions of lead nuclei.