Well, this week is a big week, in some ways. The Large Hadron Collider has gone into a new phase! For a while, the experiment has turned aside from the task of searching for the origin of mass (the Higgs Particle, or whatever it is that mediates the generation of the masses of elementary particles – see earlier posts, and features like this, etc) and is turning to heavy ion collisions. Rather than studying processes in which only a few particles at a time are interacting at super-duper-uber-high energies, the experiment will instead collide together the nuclei of lead atoms, so that you get lots of particles colliding together and creating a messy “soup” of high energy stuff all together. The goal is to understand the constituent nuclear particles (quarks and gluons) working collectively at high temperature (and low to moderate density), instead of focusing on issues concerning individual fundamental particles. Today (starting late yesterday, actually) is an exciting day because it marks the first step on the journey to probe deeper into this physics. The ALICE experiment has started looking at these collisions. See top right for some screen-shots of the mess of particle tracks that are left after the soup flies apart. The trick is to analyze all these tracks from millions of such collisions to work out the properties of the soup.
As you perhaps know from reading this blog, while of course I’m interested in the behaviour of fundamental particles and the origin of mass, and so on and so forth, I’m very interested in this nuclear issue too. Some of the most interesting work that is going on in a subset of my field right now concerns this, it turns out, since some of the work we are doing in string theory has turned out to be very directly connected to some of the experimental work that has been going on in this area, using the RHIC experiment at Brookhaven. I’ve spent a lot of time here talking about that, and so refer you to earlier posts. Just search on RHIC, for example. (Recall also the special issue of Physics Today for which I wrote an article with Brookhaven’s Peter Steinberg on this matter, and the AAAS symposium I told you about here, a report on a conference at Cambridge where nuclear and string people came together to exchange ideas, and so on and so forth).
I’ve been thinking of applications of string theory to experimental heavy ion collisions for about a decade now, and so a new experiment is of obvious interest. There were huge surprises uncovered about the “quark-gluon plasma” during the RHIC phase in the middle of the last decade, and one of the things on my mind is whether or not the surprise persists, and what more we can learn. The LHC – ALICE experiment will ultimately take the temperature higher, and so one of the questions is whether or not the remarkable strongly coupled fluid behaviour of the plasma (which was, I remind you, expected to be a gas) persists, and how much it persists. What other properties will it have, etc, etc? The rather nice thing was that techniques from string theory allow for the study of strongly coupled fluids with similar curious properties to those seen in the experiment, and this led to a lot of useful dialogue, since such things are hard to study using “traditional” techniques. (You can also ponder the issue of whether it is all a convenience, a trick, a mistake, a fortuitous tool we can use and so who cares where it came from, or whether it’s all “real” and an actual experimental test of string theory’s take on quantum gravity, black holes and extra dimensions (all relevant in these computations) in some sense. Take your pick. See here.)
From my perspective, an interesting and central question is “Will string theory computations remain relevant to these systems?”. (This relates directly to the issue of how long the fluid behaviour of the soup will persist. At some temperature high enough, presumably something happens and it turns into a gas… what then? And how does that cross-over happen? Lots of exciting physics here to think about, and I don’t know how much will be accessible with ALICE.)
Time will tell. A report on the first collisions, done today, can be found here.