Quark-gluon plasma at the LHC

In 2012 CERN's Large Hadron Collider smashed lead nuclei together and briefly created a quark-gluon plasma, the soup of particles that normally hides inside protons and neutrons. The reading was about 5.5 trillion degrees, hundreds of thousands of times hotter than the core of the Sun. The plasma lasts only a tiny fraction of a second.

The temperature is worked out from the energies of the particles that come out, not read from a thermometer. At this scale kelvin and Celsius are effectively the same number.

Context

The Large Hadron Collider runs lead-ion collisions for a few weeks each year in addition to its main proton program. In those collisions, protons and neutrons melt into their component quarks and gluons, recreating conditions similar to those a few millionths of a second after the Big Bang.

Physicists expect the change from ordinary matter to quark-gluon plasma to occur at around 2 trillion kelvin, according to lattice calculations of the strong force. Reported peak values higher than that, such as the 5.5 trillion in this entry, come from interpreting measurements of the debris, and estimates depend on the method and model used.

At these temperatures the unit hardly matters, since the offset between kelvin and Celsius is only 273 degrees. The plasma behaves like an almost frictionless liquid and exists for a tiny fraction of a second before cooling into ordinary particles. The Sun's core, by comparison, is about 15 million degrees.