Washington, DC—This week is the Quark Matter 2012 (QM2012) conference—the preeminent meeting for those studying high-energy collisions between heavy ions. I attended a number of talks on Monday, August 13, during which researchers announced the major new results from the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory and the Large Hadron Collider (LHC) at CERN. The conference offered fresh insights on the transition between ordinary matter and the soup of quarks that existed in the early Universe—including a tantalizing hint that might tell us about why the modern cosmos has more matter than antimatter.

We recently ran a detailed review of heavy-ion physics; here's an executive summary. Heavy nuclei (lead at LHC, gold, copper, and uranium at RHIC) are completely stripped of electrons, leaving massive, positively charged ions. These are accelerated to well over 99 percent of the speed of light and smashed into each other. If the energy is sufficiently high, the protons and neutrons in the nuclei "melt" into their constituent quarks and gluons. The result is a substance known as the quark-gluon plasma (QGP), which theory predicts existed during the first 10 microseconds after the Big Bang.

While the hunt for the Higgs boson has dominated press coverage of the LHC, the collider also performs heavy ion experiments using lead (Pb+Pb). In addition to the ATLAS and CMS detectors, which are used both for proton-proton and heavy ion collisions, LHC has a dedicated heavy ion detector named ALICE (A Large Ion Collider Experiment, pronounced "ahLEES"). The two active detectors at RHIC are PHENIX (Pioneering High-Energy Nuclear Interacting Experiment) and STAR (Solenoidal Tracker at RHIC). These study the products of collisions between gold ions (Au+Au); in the most recent experiments, researchers have added gold and copper asymmetric collisions (Au+Cu) and uranium (U+U). The two major colliders are complimentary in many aspects: the LHC has a larger temperature range and can reach lower density, while RHIC is able to explore much higher baryon densities.

To form a more perfect fluid

The detectors at RHIC and LHC measure the particles produced within the QGP, and those formed in the first stages of the collision and pass through the plasma. Many of these particles are hadrons (collections of quarks), but many photons, electrons, and muons are made, some from the decay of exotic species containing strange and charm quarks.

The distribution of the collision products reveals a lot about the interaction between the ions when they collide—including their positions within the region of overlap. Just as Fourier analysis reveals the harmonics contained in a musical note, researchers use it to determine the shape of particles coming out of the QGP. This result in turn reveals a lot about the plasma itself: if it had high viscosity, then the deformations would be damped out, just as a stiff mixture resists carrying waves over long distances. However, because the deformations are carried through, that means the QGP has low viscosity.

The "melting" analogy I used above appears to be appropriate for talking about the transition from stable nuclei to the QGP. While we are often used to thinking of plasmas as gaseous (like they are in stars), the QGP is actually a liquid that flows with nearly zero viscosity. Viscosity is the resistance to flow: water has relatively low viscosity, but the QGP has much less. Theoretical models predict that viscosity can never be zero due to quantum fluctuations, but as Jurgen Schukraft of CERN described it, the QGP's viscosity is very close to the predicted minimum.

Several speakers emphasized how new that result is: three years ago, nobody suspected the deformations could be studied in that much detail, but today they are some of the most powerful data coming from detectors.

Much of the recent work at RHIC and LHC is an attempt to map the phase diagram of the dense hot matter formed in these collisions (called "QCD matter" in reference to quantum chromodynamics, which describes its behavior). The people running the accelerators are trying to map the transitions between the QGP, ordinary matter, and other phases. Ordinary phases of matter include solid, liquid, and gas; physicists add many more, such as various magnetic and superconducting phases.

Water provides an analogous situation to QCD matter: at low temperatures and moderate pressures, it exists as ice, while at higher temperatures it may melt into liquid or boil. However, at higher temperatures and pressure, water reaches a critical point: a place where it is a fluid, but there is no longer a distinction between the liquid and vapor state.

For QCD matter, the relevant quantities are temperature (a proxy for energy) and density of matter. At low temperatures and high density, the result is stable hadrons. At higher temperatures, hadrons melt into the QGP. (At relatively low temperature but extremely high densities, matter forms yet another phase we don't understand very well: the substance that comprises neutron stars.)

Experiments at RHIC have begun probing the phase transition between stable hadrons and the QGP—and looking for a possible critical point. While this critical point hasn't been found yet, there are good theoretical and experimental reasons to think it exists. At QM2012, Steve Vigdor (associate laboratory director for nuclear and particle physics at Brookhaven) pointed out that STAR has seen signs of a transition between QGP and stable hadrons, where the strange quark production seen at higher energies dropped precipitously.

Golden hints about the early Universe

Earlier low-energy RHIC experiments using gold (Au+Au) found electric polarization—a small separation of positive and negative charges—in the overlap region where the ions collided. This effect was potentially worrying, since quantum chromodynamics requires chiral (or mirror) symmetry: there should be no inherent left- or right-handedness resulting from strong force interactions. However, QCD does allow violations of this symmetry at higher temperatures, such as those in the QGP, as long as they average out over all collisions. These violations may have played a role in the early Universe, when more matter was produced than antimatter, according to BNL's Vigdor.

Au+Au collisions result in strong magnetic fields, which complicate analysis. The uranium-uranium collisions have effectively ruled out the possibility that the charge separation is due to the specific shape of the overlap region in the Au+Au interaction. This test was possible because uranium nuclei are highly non-spherical, having nearly the shape of an American football. RHIC researchers collided uranium ions both along their long sides and their narrow ends. The end-to-end collisions resulted in very high energy density, said Stony Brook physicist Barbara Jacak, while the side-to-side collisions produced charge separation without magnetic field effects.

Similarly, the asymmetric gold-copper collisions may hold clues to whether the symmetry violation is a fundamental result, or an artifact of the experimental setup. However, it's too early to draw strong conclusions (Jacak told me the Au+Cu collision results are only a few weeks old), so further analysis will need to happen before we can definitively say if the charge separation has anything important to say about QCD or the QGP.

More exotic quark matter

With much higher energies available at LHC, researchers are using it to examine a high-temperature region of the QGP: the strongly correlated QGP (sQGP). ALICE found that particles passing through the sQGP experienced a great deal of energy loss, suggesting the interactions within it are different from those present at lower energies. The sQGP is still poorly understood from a theoretical point of view, according to CERN theorist Urs Achim Wiedemann.

Analogous systems exist in condensed matter physics (which are very low temperature), but the particle-like excitations that drive so much of the interesting behavior in materials don't seem to exist in the QGP. However, both the theoretical and experimental studies of the sQGP are in very early stages.

To give a sense of how preliminary they are: CERN announced yesterday that the LHC has achieved the highest human-made temperatures yet, but hasn't been able to determine exactly what that temperature is. It's about 38 percent larger than the previous record from RHIC, which was about 4 trillion degrees Celsius. Such high energies should help make clear the structure of the sQGP.

Listing image by Brookhaven National Laboratory