Heavy ion collisions reveal the earliest instants of our Universe
by John Timmer -
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 upgrades 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.
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http://arstechnica.com/science/2012/07/heavy-ion-collisions-reveal-the-earliest-instants-of-our-universe/
