Colliding two lead nuclei at the Large Hadron Collider is like throwing two hefty bags of marbles at each other. Usually, the marbles scatter in all directions, but sometimes one bag passes so close that the marbles don't collide: they just exchange glances. That's how, without a direct hit, the nuclei exchanged light, and in that light, an Upsilon particle was born — a heavy cousin of the proton, made of a charm quark and its antiquark.
Inside a lead nucleus, gluons, the carriers of the strong force that glue quarks together, behave differently than in a free proton. The measurement showed: in lead, each gluon accounts for only 55% of what it would be in a free proton. This is strong suppression, and it hardly depends on how heavy the produced particle is. Previously, the same suppression was seen for the lighter phi particle, and now it's confirmed for the Upsilon — at an energy 100 times higher.
This result suggests that inside a lead nucleus, gluons are not just 'smeared out' but form a dense state that scientists call the color glass condensate. This state is hard to see directly, but by the suppression of particle production, we can infer its properties. In the future, such measurements will help us understand how matter behaves at extreme densities — for example, in the first moments after the Big Bang or inside neutron stars.
The Upsilon meson was discovered in 1977 and was the first evidence for the existence of the fifth quark, the bottom quark.
In Liu Cixin's novel 'The Three-Body Problem', aliens use subatomic particles to create super-dense materials — similar states of matter are studied in heavy-ion experiments.