Grandma, the quark-gluon plasma, and all that

Among many other things, my lovely grandma Janet, 88, thinks I work in a nuclear plant. I told her that, rather, I was doing particle physics and that it is something which has nothing to do with working in a plant (God only knows what I might be doing there!). Let me be clear: it is of course not infamous to work in a nuclear plant, but simply, that's absolutely not my job. Anyway, I am pretty sure she knows it: I guess she keeps telling that to her (many!) friends more to tease me than from a genuine misunderstanding. Still, I realize that it may not be clear to everyone (including myself sometimes) what I'm doing for a living. This was confirmed to me the other day when my friend Eléonore asked me the question explicitely: WHAT are you doing, EXACTLY?

In what follows, I'll try to explain simply and in a few words what my research field is all about (btw, if some of my colleagues happen to read my blog, please correct/add anything you like). So... once upon a time... no, too sweety. Father atom and mother... childish. That's why I love divulgating science: it's really tough.

Let me start with an atom. That's already pretty small: typically 0.000 000 000 1 meter (in other words, 1 billion atoms next to each other would be 10cm long). We know from Rutherford that is composed of a tiny nucleus, which size is approx. 100 000 times smaller than that of the atom. What makes the atom so BIG, comparatively, comes from the electrons moving around that very nucleus (the so-called electronic cloud). Now forget about the electrons and let's focus on the nucleus itself. It is made of at least one proton plus some neutrons. Quite often the number of neutrons is roughly the same as the number of protons, but that's not always true. The simplest atom is a nucleus composed of one proton (but zero neutron) with an electron moving around: ladies and gentlemen let us welcome the Hydrogen atom.

The protons and the neutrons are quite similar, except for their electric charge (the neutron being... neutral). To know what the atom is made of, say its structure, Rutherford measured how an electron beam scatters on a gas of hydrogen atoms. Much more recently but basically in the very same way, it has been discovered in the 70's that protons and neutrons are themselves composed of more elementary particles: the quarks and the gluons. In the "standard" model of particle physics, quarks and gluons are "elementary" particles, meaning that, unlike the atoms or the protons and neutrons, they are not themselves made of more elementary particles. As a matter of fact, no quark or gluon structure has yet ever been seen experimentally.

Surprisingly, and now it comes slightly more technical but also hopefully more exciting, those quarks and gluons always bind together to form particles like the proton or the neutron (called "hadrons" in the jargon). No one has ever seen a single, unbound, quark or gluon. We say that they are confined. This confinement mechanism is pretty mysterious and its origin is still pretty much unknown (despite many people working on it for years). However, there's a general consensus that when the Universe was extremely hot (roughly 1000 billion degrees), some 0.000 001 second after the Big Bang, the quarks and gluons were able to move freely, without binding together. It is only later -as the Universe kept cooling because of its expansion- that confinement showed up and gave birth to the hadron particles and, among them, the proton and the neutron that we've already met.

As you can see from the length of the post, we're getting close to what I'm doing, but not quite there yet. In short, we would like to observe experimentally the soup (let's call it the plasma, it sounds better) of freely moving quarks and gluons. A soup is always better eaten hot. To heat up the matter up to those extremely high temperatures physicists collide heavy nuclei, such as gold or lead (any element in the bottom lines of the Mendeleev periodic table, for those who may recall what this is). When the collision takes place, at CERN in Geneva for instance, many quarks and gluons are produced and start to interact with each other. Eventually, a gas (some say a liquid but I don't want to go in too much detail here) of quarks and gluons is formed, characterized by a temperature. Just like the Universe, this gas expands and cools and you can guess what happens then: below a "critical" temperature, confinement strikes back and quarks/gluons merge again into hadrons which are much, much later detected experimentally (how to detect a particle would deserve a post in its own). The tricky thing is that everything happens at very short space and time scales: the soup, which we call quark-gluon plasma (cool, isn't it?), is roughly 10 times larger than a proton -pretty small indeed- and its lifetime (before hadrons show up) is as short as 10 millionth of a billionth of a billionth of a second. Poor guys, not much time to enjoy freedom. You understand of course that it is both way too small and too short to be observed. Rather, we have to focus on some properties of the quark-gluon plasma which could survive well after its extinction (think dinosaurs!). Those indirect signals (we call them signatures) could then give evidence to the formation of the quark-gluon plasma in the very early stage of the collision, before the damned confinement process takes over.

That's basically what my research field is about. What am I doing then precisely? Well, the post is pretty long already and I don't want to confuse my rare and precious readers so I'd better stop here. Let's say I (try to) study some of those signatures, elaborate models and compare with the measurements done in the experiments to be able to tell whether or not, the quark-gluon plasma, sometimes refered to as the "Little Bang", has actually been produced.

You see grandma, that's my job. Now go and impress your friends :-) I hope this was instructive and not too confusing. Please feel free to ask any question. Now let me be back to more prosaic things: find something to eat and check whether some trains circulate this afternoon from Paris to Geneva.