Three colours in a Venetian Mosaic

This post was written during the 27th installment of the Quark Matter conference held in Venice in May, 2018.

Today, in Venice, the sun does not shine, it roars. Yesterday, the city was completely soaked as a storm paraded through it, giving thunderous signals of its arrival. But today golden hues flood the air, contrasting with the shadows of the trees near the Palazzo del Casinó. The wind blows calmly and the smell of sea salt fills the air. Outside, the sea hums, the boats sail, and the tourists roam the streets of the islands in search of a taste of the past. I am sitting outside of the venue of the conference, drinking a coffee, admiring the day, and admiring the excruciatingly white buildings in front of me.

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Today in the morning, close to Lido.

These are the buildings that hold the Venice’s international Film Festival every year. In a couple of months, this place will be brimming with celebrities, producers, paparazzi and curious tourists. Today, however, it is not full with them. Today there are no film stars, directors, or screen writers around. There are no films getting presented, and most definitely no red carpet.

Today, it is full with scientists.

But why are we here, you may ask? We are discussing the existence and the properties of deconfined nuclear matter. This may sound a bit weird and technical now, but you will see that it is not that complicated. It is, in reality, very intuitive once you get the hang of the names and concepts that make up this area of research.

Remember that any material is made out of atoms. The atom itself is made out of a nucleus, which is surrounded by ever-vibrating and ever-moving electrons. The electrons are very interesting in their own right, as their properties and behaviour is what makes atomic physics. This means that they determine how atoms interact with each other and their environment (and yeah, that includes atom bonding, which is the physics part of chemistry.).

The nucleus is positive, electrons are negative, and opposite charges attract each other.

-You, at some point when you were in school.

The funny thing is that electrons are extremely tiny. In fact, they are believed to be fundamental particles, which means that they are not divisible (in a literal atomists’ way) into smaller pieces. Nevertheless, as small as they can be, their radius of action around the positive nucleus is “quite big”. How big? Around a tenth of a nanometer – a nanometer being a millionth of a millimeter! -. A nice analogy is the planets orbiting the Sun. They are very small, but the size of their orbits is quite big compared to them. The electrons can be thought of very much the same way…

So why did I say the orbit of the electron was”quite big”? Well, a tenth of a nanometer may sound tiny, however, compared to the size of the nucleus, this is huge! The nucleus, which holds basically all the mass in the atom, is one hundred thousand times smaller. We want to understand nuclear properties, which means that now, we may have to take a closer look.

A nucleus is made of protons and neutrons. Now think about the following question. How does the nucleus stay together? I mean, it is made from particles of the same charge – hence repelling each other -, right? How does it manage to not blow apart? It took physicists a long time to develop an answer to that question. But during the sixties, our understanding of the nucleus grew little by little until the full formulation of the theory of the strong (or nuclear) force. This mysterious force can only be understood when you look deeper into the nucleus. Ever deeper also means ever smaller.

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The different scales that we will go through as we zoom into the nuclear size. We start with our size (1-2 meters), and go through the size of small insects (some millimeters), bacteria (some micrometers) . Then we arrive to the size of atoms (around an ångstrom) and finally we zoom in enough to see the proton and the nucleus, at a staggeringly small size of around 1-6 femtometers.

And we are going deep into the proton.

Turns out the protons and the neutrons are not fundamental, and they are made from little particles called quarks and gluons. Gluons  are the force carriers, they are the way quarks talk to each other, how they interact. They bind the quarks into the proton, neutron, pions, kaons amongst many others… They are the glue that binds them. Now you are probably slapping your face in disappointment at such a terrible name, right? Well, it is a very weird historical accident that everything which surrounds the studies of quarks and gluons is funnily named. The word quark comes from a poem from Joyce, and it is a whimsical as some of the properties they have, although quark also is a special kind of yoghurt in Germany. So, quarks come in 6 different flavours  (which is the actual scientific word, believe it or not) : up, down, charm, strange, bottom and top. This gives rise to an infinite amount of weird and dumb puns. Phrases like charm physics, top physics or strange suppression can be heard today at today’s conference…

Additionally, each of these flavours can be found in one of three different… smells. Ok, no, I am just kidding. We are not that goofy, they come in colours. These are of course not real colours, but labels we use to understand different states these particles exhibit. Anyway, these labels are why the Theory of Strong interactions is also called Quantum Chromodynamics (chromo from colour in greek). Quarks can take labels such that they can be red, green or blue. The reason why those colours were chosen is that if you mix red, green and blue light (RGB in a projector/beamer), you get white. And in nature, there are no coloured particles, hence all the measurable ones are white. Finally, the burning question: If we don’t measure those numbers, why do we need them? It is impossible to explain why protons, neutrons and similar particles behave without the existence of quarks and gluons… (for more about this see next post)

We have investigated deeply the properties of quarks and gluons throughout the history of particle physics. We have a very good idea of what they say to each other individually, how the behave in isolation. You could say we understand the “few-body physics” of the particles that are inside the nucleus quite well. But what happens when you have not three or six but six hundred quarks and gluons or even more? Does it behave individually, or as a gas, or a liquid, or something even more exotic? If it becomes a fluid (a medium), does it exhibit thermal equilibrium, and if yes, what temperature does it have? What happens when this collection of particles is stressed and put to be under extreme conditions? We are interested in all of this. In our work, we always keep in mind their fundamental (individual) properties but our goal is to better understand how they behave collectively, or if I may, socially.

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How I imagine the collision. A burst of color, expanding in space and time, overflowing of new things yet to be made.

But how do you do this?

To answer this, we smash protons with protons, protons with nuclei, or nuclei with nuclei to better understand what is happening. We do this at very high energies, which means that we are colliding them almost at the speed of light! When these systems collide, the neutrons/protons  that interact with each other, are destroyed to reveal their inner workings. Think of two piñatas exploding when they suddenly touch each other, and revealing all those sweet candies you were looking forward to see.

For us it is very important to do the three kinds of experiments listed before to see how different number of initial protons/neutrons in the system affect the outcome of the physics we see in the experiment. That means that we want to see how the initial number of protons/neutrons affect how much of collective behaviour it exhibits. But, why this barbaric treatment, and why not use a microscope, or shine light on it? That is the topic of the next post, but let me give you a small spoiler, the collision is our microscope…

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Flowing colours in a Murano glass. When I saw it, it reminded me that as the two ions collide in the experiment, they explode into a colorful spectrum of particles, each carrying the signatures of the collision, plus a little more. So it is this glass, the sum of its constituents, plus a lot more. Photo by Heike Frömke.

So we gather here in Venice this year to discuss our knowledge and ideas. Venice, which has been known for centuries for so many things. It was a naval superpower some many centuries ago, and it was there where Galileo first presented the telescope. And it is also very well-known for its sturdy and beautiful glass, the famous Vetro di Murano. Watching the glass-master making all these glasses and vases and sculptures with absolute calm and focus is something outstanding. It occurred to me that we are not so different. He heats the glass to make it malleable and fluid, we are smashing nuclei together to observe collective behaviour, to see its collective, “molten“, properties. The glass master seems to fully understand intuitively his material, how the temperature, flow, or the rotation that he gives affect the final result. We want to understand also those same factors and more in our material, and we are slowly and steadily on our way there.

This article is just a little introduction to my area of work. Next week I will try to expand and explain why nuclear matter is so interesting. Stay tuned!

2 thoughts on “Three colours in a Venetian Mosaic

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