As I mentioned previously, I spent Monday at the Quark Matter 2012 conference (QM2012) in Washington, DC. Quark Matter is probably the preeminent conference in heavy-ion physics, so I wish to convey my thanks to Karen McNulty Walsh of Brookhaven National Laboratory for my invitation to attend. In my short time at the conference, I heard the latest important results from the Relativistic Heavy Ion Collider (RHIC) at Brookhaven and the heavy-ion experiments performed at the Large Hadron Collider (LHC) at CERN.
Heavy-ion physics straddles the boundary between traditional nuclear physics—which involves bombarding nuclei with particles like protons or neutrons—and particle physics, where individual particles are slammed together at nearly light-speed. In heavy-ion experiments, atoms like gold, lead, and uranium are stripped of all their electrons (ionized), leaving bare nuclei. These are then accelerated to high fractions of the speed of light and formed into beams that travel around rings in opposite directions, as in particle physics experiments. The collisions between the ions probe the structure of the nuclear forces, providing deep insights into the nature of quarks—the constituents of protons and neutrons—and gluons, the carriers of the strong force that binds quarks together.
Under ordinary conditions in the modern Universe, quarks and gluons (collectively known as partons) are tightly bound together in hadrons—the protons and neutrons that comprise atomic nuclei, along with a variety of other particles. In the first 10 microseconds after the Big Bang, however, the temperature and density of matter were sufficient to prevent stable hadrons from forming. Instead, matter existed in the form of quark-gluon plasma (QGP), with free quarks and gluons mingling. The QGP is realized experimentally by going the other direction: starting with stable nuclei and slamming them together with sufficient energy to “melt” them into their constituent partons. Since the theory describing these processes is quantum chromodynamics (QCD, referring to the so-called “color force” between quarks), heavy-ion physics is often referred to as the study of QCD matter.
Because it is difficult to sustain the conditions for QGP, it’s an ephemeral state in colliders like RHIC and LHC. The latest results announced at QM2012 highlight both the difficulty in studying something that only endures a tiny fraction of a second, but also how much researchers have learned within the last two years. I won’t try to cover everything (which would take a lot of space to fill in the requisite background), but two interrelated aspects stood out to me as an outsider to the heavy-ion community: the mapping of the phase diagram of QCD matter, and the effect of the shape of the ions themselves in the collision process.
My semi-complete write-up of the conference can be found at Ars Technica; I’ve got a little more to say about the presentations in a later post (in the spirit of something I wrote for the Phenomenology 2012 conference I attended in May). Stay tuned!