Research done at the LHC occurs in different modes. One mode is when two beams of protons are collided. That’s what happens most of the time and how the Higgs boson was found. A second common mode is when two beams of the nuclei of lead atoms are smashed together. That allows scientists to study matter under very high temperatures. These collisions are horribly complicated, with 416 nucleons (that is, protons and neutrons) simultaneously colliding. A third approach is to collide a beam of protons with a beam of lead nuclei. This approach straddles the two worlds, creating complex environments, but with a simplified probe.
In highly complex environments, scientists tend not to study the trajectory of individual jets of particles, which are the debris of quarks and gluons scattered in the collision. This is because these particles have to batter their way out of the debris of 416 nucleons. And even in lead-proton collisions, there are 209 nucleons in play.
Instead, they look at the particles exiting the collision (which have already penetrated the hot nuclear matter) to see if they are located near one another or farther apart.
The following analogy might make it clearer. Suppose there is a zoo with a huge paddock containing lions, with the lions standing in for the scattered particles. Suppose further that the lions are organized into two prides. Scientists study the location of the lions to see if the location of one lion gives you any information about the location of the others. In this study, scientists excluded lions that were located too near one another, since that particular configuration is sensitive to, for example, mothers and cubs. Instead, they studied lions that were farther apart. In this way, they can study the dynamics of lions within a pride and between lions in different prides without including the specific case of mother-cub connections. Scientists might perhaps discover that the two prides get together to socialize or keep away from each other to avoid conflict. They could also find out how the dynamic changes if there are few lions or lots of lions in the paddock.
In the actual CMS study, scientists studied the separation of particles when there were 1) fewer than 20 particles and 2) between 220 and 260 particles. Previous studies had explored this behavior, but these new studies were being done to discriminate between two models of novel nuclear matter, one closely paralleling hydrodynamics, which is the flow of water, and a second called color glass condensate, in which the gluons in nuclear matter act like a wall on short time scales and more liquid on longer ones.
Neither model reproduced the data, suggesting that the behavior of nuclear matter is more complex than either model predicts. This isn’t surprising, as the complex environment makes it hard to model. More studies will be needed to better understand this difficult state of matter.
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