Subatomic hydrodynamics

 The same basic equations that govern the flow of water are important for describing the collisions of lead nuclei. In today’s article, we’ll get a glimpse of how this works.

It’s hard for most people to imagine what it’s like at the heart of a particle collision. Two particles speed toward one another from opposite directions and their force fields intertwine, causing some of the particles’ constituents to be ejected. Or possibly the energy embodied in the interaction might be high enough to actually create matter and antimatter. It’s no wonder the whole process seems confusing.

Things get a little easier to imagine when the particles are the nuclei of atoms (note that I said easier, not easy). For collisions between two nuclei of lead, one can imagine two small spheres, each containing 208 protons and neutrons, coming together to collide. Depending on the violence of the collision, some or many of the protons and neutrons might figuratively melt, releasing their constituent quarks so they can scurry around willy-nilly. Physicists call this form of matter a quark-gluon plasma, and it acts much like a liquid.

Part of this liquid-like behavior is due to the fact that so many particles are involved. An LHC collision between two lead nuclei might involve thousands or tens of thousands of particles. Because these particles are quarks and gluons, they experience the strong nuclear force. So as long as they are close enough to each other, the particles interact strongly enough that they clump a bit together. The net outcome is that the flow of particles from collision between lead nuclei looks vaguely like splashes of water. In these cases, the equations of hydrodynamics apply. Mathematical descriptions like these have been used to make sense of other features we see in LHC collisions between lead nuclei.

However, there is more to understand. We can imagine collisions between the collective 416 protons and neutrons of lead nuclei as splashes of water, but when a pair of protons collide, the collision doesn’t yield enough particles to exhibit hydrodynamic behavior. So as the number of particles involved goes down, the “splash” behavior must slowly go away. In addition, in the first studies of lead nuclei collisions, only the grossest features of the collision were studied. This is because it is impossible to identify individual quarks and gluons.

There are ways to dig into these sorts of questions. One way is to look at collisions in which one beam is a proton and the other is a lead nucleus. This is a halfway point between the usual LHC proton-proton collisions and the lead-lead ones. In addition, we can turn our attention to quarks that we can unambiguously identify, such as bottom, charm and strange quarks, to better understand the hydrodynamic behavior.

In this study, physicists looked at particles containing strange quarks. Since strange quarks don’t exist in the beam protons, studying them gives a unique window into the dynamics of lead-lead collisions. By combining studies of particles with strange quarks in lead-lead and lead-proton collisions, scientists hope to better understand the complicated and liquid-like behavior that is just beginning to reveal its secrets.

Don Lincoln

 These scientists from Rice University contributed to this analysis.
 These Fermilab professionals are a crucial part of the support team for the CMS Phase 1 Upgrade.