The LHC's proton collisions, which have now successfully nailed down the existence of the Higgs boson, get most of the attention, both in the media and at CERN itself. But, for a few weeks each year, the collider is switched over to smashing lead ions. Heavy ion collisions, in fact, are considered to provide such distinct information that the US has kept open the Relativistic Heavy Ion Collider, which is dedicated to smashing heavy ions, even as it shut down the Tevatron, its dedicated proton/antiproton collider.
Right on the heels of the Higgs announcement, Science is running a review of heavy ion collisions, which nicely explains why they tell us something completely different from what's revealed by proton colliders. Plus it provides a nice picture of how the LHC will provide new data, and the upgrades that have taken place at the RHIC to help keep it relevant.
The matter we see around us is comprised mainly of protons and neutrons. These, in turn, are composed of quarks and gluons, which mediate the strong force that binds them together. Because the potency of the strong force increases with distance, breaking up a nucleon (proton or neutron) typically requires high levels of energy that basically blast the nucleon in part. That's precisely the sort of thing that happens during the proton collisions that take place at the LHC.
Heavy ion collisions—lead at the LHC, gold at RHIC—involve huge numbers of nucleons, on the order of 400. That creates a very different environment. The quarks and gluons that spill out of a proton collision tend to have nothing but empty space around them. In a heavy ion collision, the large number of nucleons that are broken apart at once means that, instead of flying into empty space, a given quark or gluon will have the opportunity to interact with those pouring out of nearby nucleons. As a result, for a brief instant, the collisions don't look much like an explosion; instead, it looks more like the boundaries between nucleons melting, leaving behind a sea of quarks and gluons that are interacting.
The resulting material, called a quark-gluon plasma, isn't just interesting on theoretical grounds. In the first moments of the Universe's existence, the energy density was so high that all normal matter was in this state. It took about a second to cool down enough for protons to condense out of the QGP. But, before that second was up, the Universe had gone through its entire inflationary period, sowing the seeds for the large-scale structures we see today.
What actually goes on as the QGP forms? The review divides things up into three stages. In the first portion of things, the newly liberated gluons form a dense mesh of interactions. This sets the stage for phase 2, where the quarks, while under the influence of the fields generated by the gluons, form the actual QGP. This soup of particles quickly "thermalizes," meaning energy becomes evenly distributed among its components. As this happens, the QGP starts to expand.
Despite its extremely high density, the QGP shows a shear viscosity that's tiny, making the QGP one of the closest things we've seen to an ideal quantum liquid. Somewhat surprisingly, its behavior is nicely described by equations used in string theory—to describe a five-dimensional black hole. The correspondence does have limitations, though, and the review suggests that finding ways to extend the comparison might give the experimentalists more things to look for.
In any case, as the QGP expands, its energy density drops, and it eventually reaches the point where it drops below what's necessary to maintain the plasma. At this stage, various particles "freeze out," including nucleons and other more exotic particles.
How do you study this process? One of the simplest ways is to simply track all the particles that freeze out, and trace them back to where they came from. This lets researchers reconstruct the shape of the QGP as particles condense out of it. The collisions also create some highly energetic particles that start outside the plasma, but with a momentum that carries them into it. If these particles are quarks, they can participate in interactions with the QGP. In the case of light quarks, that will slow them down considerably. But heavier quarks will simply radiate off some energy (in the form of gluons) and pass through without losing much momentum, providing a sensitive probe for conditions within the QGP.
This is where the LHC's higher energy could come in handy. The lead ion collisions there are high enough energy that they can produce Z bosons, the carriers of the weak force. These should be able to cross the QGP, and provide a very different probe of the conditions inside it, since it won't interact in the same ways quarks do. So far, the information generated at the LHC has largely extended the findings of RHIC into higher energy domains.
Meanwhile, RHIC isn't standing still. In additions to upgrades to its detectors, the accelerator chain has been modified to provide the ability to collide many different ions. The review's authors, for example, are excited about the prospect of colliding uranium ions. The nucleus of these atoms is asymmetric, so some of the collisions should provide QGPs with distinct shapes, which will vary the transit time that particles take across the plasma. The asymmetric collisions may also provide a glimpse into the earliest steps of forming gluon interaction networks, which remain poorly understood.
In any case, although these collisions aren't going to result in the discovery of new particles, continued work at RHIC and the LHC should provide a clearer picture of the formation and behavior of the quark-gluon plasma and, in the process, give us a better understanding of the Universe's first moments.
Science, 2012. DOI: 10.1126/science.1215901 (About DOIs).
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