It was theorized that quark-gluon plasma - a state of matter where quarks and the strong force-carrying gluons have yet to condense into separate families of particles - would likely act like a gas. So, in the early 2000′s, physicists were surprised to find that quark-gluon plasma actually had a collective behavior. This discovery of tiny plasma blobs after proton-lead collisions supports the finding that primordial plasmas behave like liquids.
Interestingly, the researchers had no clue that proton-lead collisions would be powerful enough to create a quark-gluon plasma - such collisions, theoretically, shouldn't generate enough energy.
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"The proton-lead collisions are something like shooting a bullet through an apple while lead-lead collisions are more like smashing two apples together: A lot more energy is released in the latter," said Velkovska.
The data that Velkovska's team analyzed came from a calibration run in September 2012, but they found, by accident, that five percent of the more violent proton-lead collisions in that run exhibited this collective behavior. The only way to explain this behavior was that tiny droplets of quark-gluon plasma had formed around the "hole" made by the speeding protons (bullets) blasting through the more massive lead particles (apples). The quark-gluon liquid droplets formed by proton-lead collisions were 1/10th the size of those produced by lead-lead collisions, making them the smallest drops of quark-gluon debris to be measured from any particle collision event.