Things are about to get messy.
Give me a moment of your day, and let me put a picture in your mind. Imagine you and a friend each have a soup in a plate, and each soup has two carrot pieces, one potato and not so much broth in it. You are bored, it is a slow afternoon, so you decide to perform a little experiment.
You decide to probe the properties of the soup, and the best way to do this, as sometimes it tends to be, is to run and pitch the soups against each other, as in a culinary mirrored baseball game. The reason to do this, you say to yourself because you want to justify this madness, is that if you see how the pieces bounce, and the broth splashes around, you will better understand the kinetic and dynamic properties of the soup.
So you take on this monumental task of soup-scattering and at first pitch not so fast, and nothing really fascinating happens. Mostly, the little veggies fly away and smash the walls. But you want to know more, and continue to waste good soup until you find out that, actually, this creative exercise is every kid’s nightmare, because when carrots and potatoes hit each other, they can spontaneously produce more veggies (in this imaginary universe it happens, ok?). More in quantity and also with more diversity. That is cool, right?
But you and your friend are not really satisfied, and start thinking of new things you can experiment about. Bigger plates, more carrots, more potatoes. What about you heat up the soup? This is when you start noticing that the broth actually gets thicker, and denser as you turn up the temperature… So you totally want to smash them together and see the results. How can this new variable affect the outcome?
And then you perform the experiment, and are blown away by the results. You smash them together, and as normally, you see a lot of carrots and potatoes just flying away. Some of them actually collide to produce a jet of broccoli, which will stain that wall forever, and some others just bounce and produce some peas, and more carrots. But the broth! The broth encounters the opposing broth and while it should have flown away into the walls, it seems now that there is a force that is preventing the broth to fly away! It is not that it does not expand and try to go, it is that it doesn’t do it how a normal fluid would. This bubble continues to briefly expand, and at some point, it seems to freeze and breaks into small bubbles.
These bubbles are very interesting. You know they are made from broth and little veggie pieces, but from the outside, they don’t really look like that. They look solid, or something similar to solid. Furthermore, it seems that they also have some kind of interaction, as they don’t immediately fly away, but it is definitely smaller than before. And very soon their new bubble gang breaks up and flies away, only to smash against the walls.
You and your friend are just there, jaw open wide, surprised about these previously unknown properties.
This ridiculous parable I just made you read is not that far away from the reasons why we want to microscope into nuclei. You see, as you may recall from earlier posts (if not, they are here and here), nuclei are made from nucleons – protons and neutrons, namely. These in turn are made of little particles called quarks, which are bound together by other particles which are called gluons.
The rules for making particles out of them are very clear, and easy to follow. Quarks come in three different types, or colours, which physicists named red, green and blue. It is important to always remember that colours are just states that quarks can have. A quark can be one of these, or superpositions of them (quantum quarks, yeah!). These states will get shuffled if a gluon bounces on them, so we say gluons are charge exchangers.
The interesting part is that for a particle to be real and measurable, they should have no net colour. Quarks and gluons hide themselves from measurement by this mechanism, which is called confinement. By the way, as a fun fact, the colour name comes from the phenomenon, where in optics, combining red, blue and green light gives you white light (you can try this using a slide show projector!). And so, neutrons, protons, pions, deltas, rhos, J/Psi’s (which are all made from quarks, as opposed to electrons and neutrinos) have to be colour neutral – “white”.
Nuclei are themselves made from neutrons and protons, and it may come with little to no surprise that they -as we so often do- behave different socially than individually. The soup story is a very far-fetched analogy of what it is currently researched. Our experiment consists of colliding nuclei (soups) close to the speed of light. At very high energies, the number of gluons explode in comparison to the number of quarks, which makes the soup thicker, or denser, in a way.
Why do they rise in number? Because gluons can split, endlessly. The higher the energy they carry, the more probable their splitting is. This splitting should come to an end at some energy scale, since the nucleus/proton/neutron don’t really get larger with the energy, and the gluons literally have no more space to fill at some point. This predicted effect is called Saturation. In a way, part of all this research endeavor is to understand and see saturation phenomena.
“With great energy comes great splitting capability”
-Me, hoping I don’t get sued by Sony.
After the collisions of the two soups, there comes the very violent part of the collision called pre-thermal state. It is so extreme that a temperature can’t even be defined! This epoch is given by the struggle from quarks and gluons to share the energy in a more equilibrated manner. For this, they collide with each other, and create turbulence at the quantum level. Particles with very low speeds are created in huge quantities, and a bit of existing ones are accelerated to higher and higher speeds. This may seem counter-intuitive, but this is how in the Universe equilibrium is reached. The system needs to stretch itself so that all energy scales are occupied.
In this way, the quark-gluon plasma is thermalised. Now, it will behave as a very special fluid. One with a huge viscosity, but that flows similar to an ideal gas. Also, a temperature can now be assigned to the system, and it is deemed to be a huge one: it is around 1000000000000 ºC ! With such an insane temperature, this new, hot soup finds itself inevitably expanding through space-time. By doing so, it also looses temperature. One may say that the quark-gluon plasma cools itself in its lifetime by expanding into the vacuum.
At the critical temperature, gluons freeze. They don’t like to be all alone when it gets cold, and they help quarks to form particles like baryons (from the greek word that means heavy, like protons and neutrons) and mesons (from the greek word that means light, like pions, kaons, and other quarkonia). This process is called hadronisation, and it is in some way similar to when one cools down water and it becomes a bunch of ice pieces. However, in this case, we have actually found that the process is smoother, less abrupt. Instead of being a phase transition (as in the phases, phase diagram of water), we call it a cross-over, a smooth conversion from the quark-gluon plasma to the hadronic gas phase. If you are curious about it, nuclei, and therefore us, live in the -normal, everyday life– hadronic liquid phase.
By the way, hadron is just a name we give to colour-neutral -or colourless- particles.
From then on, hadrons can interact by bouncing on each other, but only to some little extent. After a short time, they stop speaking to each other, and fly away to the detector, to never see each other again.
This is what a Heavy Ion Collision, or Heavy Ion Experiment, consists of. It is our own violent way to microscope the smallest bits of nature that we have come across until now. This makes it a perfect way of testing nuclear matter in extreme conditions. There are other ways, of course, which would give insight to different aspects of nuclear stuff. For example, in compact stars, like neutron stars, which came close to be Black Holes (bold added for dramatism) but didn’t, nuclear matter is packed into very dense, very heavy form. In these settings, it has been theorised that really crazy stuff happens, like coloured quarks behaving as superconductors, transmitting color currents without any resistance. But unfortunately it is very difficult, you can imagine, to access these laboratories… So, we will keep on doing our own little bangs.
“Quarks, huh, yeah! What is it good for? ”
-Also me, and also hoping I don’t get sued by Edwin Starr
So, yeah, what is all of this good for? Is it absolutely nothing, like that song I just paraphrased? Honestly, we don’t know. Are we going to ever be able to use this for energy, transport, leisure, or anything else? I don’t know, perhaps. So, is it really worth it to spend all that money and time in something which may never be useful? Yes.
Because, hey, the same questions were asked before about electricity.