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Ken Bloom | USLHC | USA

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Things fall apart

There are lots of physical phenomena that arise from changes in temperature. High-temperature environments are high-energy environments. That energy goes into the kinetic energy of particles. Perhaps the most common manifestation of this is evaporation — when you set a liquid out on a hot day, the molecules gain thermal energy, and some of them gain enough energy to overcome the attractive forces of the other molecules in the liquid; those molecules then float away into the air.

You can see similar phenomena at the atomic level and below. There, binding energies of particles tend to be bigger, and thus it takes more thermal energy to separate the bound states. For instance, at a temperature of about 158,000 degrees above absolute zero (could someone check my math on that?), electrons in hydrogen atoms will gain enough energy for them to separate from the their nuclei. Under such conditions, atoms don’t really exist anymore; you just have a “plasma” of electrons and protons. And we imagine that the early universe, shortly after the Big Bang, was so hot that protons didn’t exist; the quarks and gluons had enough thermal energy to keep from being bound together into hadrons.

A new result from CMS shows just this kind of phenomenon. The upsilon particle is a bound state of a bottom and anti-bottom quark, much like a hydrogen atom is a bound state of an electron and proton. In the ground state of the upsilon, the two particles are pretty tightly bound and require a lot of energy to separate. But the upsilon, like hydrogen, has a number of higher-energy bound states, in which the quarks have greater kinetic energy, and thus are easier to separate. A bit more thermal energy, a few hundred MeV, and these excited upsilon states should just fall apart.

This is what CMS observes. In proton-proton collisions, the excited upsilon states are clearly visible. But in lead-lead collisions, when there is a lot more ambient energy due to all of the colliding nuclei, the excited states begin to disappear. Actually, all of the upsilon states are suppressed, but the excited states are even more so, by about a factor of three, which indicates that the more energetic states are more sensitive to the increased temperature. It’s a pretty neat trick, and the first time that it’s been observed in bound states of bottom quarks.

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