At the RHIC beamline at Brookhaven National Laboratory, the three-story-tall PHENIX muon spectrometers, designed and built by Los Alamos scientists in collaboration with other scientists from around the world, witness collisions between gold ions (red/yellow, approaching from the right) and deuterons (white, coming in from the left). Two doughnut-shaped magnets (green) steer the ions in these 100-GeV-per-nucleon beams, traveling at 0.9999 the speed of light, into head-on collisions. In such high-energy collisions, the quarks and gluons that make up the protons and neutrons of the colliding particles interact directly. The collision shown here releases pions (purple), which are only partially stable; some of them decay into muons (white). Most of the pions are absorbed inside the hadron absorber (the green magnet). The detector panels (brown) track the muons.
By studying the decay products of these collisions and their behavior, we learn about the fundamental physical laws governing the strong interactions in the universe. These laws and their effects were much more obvious in the high-energy state of the very early universe. The same physical laws hold just as much sway today, but their effects are much more subtle in the low-energy states in which we find most matter now. In order to tease out their effects and patterns, we attempt to recreate those earlier, high-energy and high-density conditions at places like RHIC and study them with detectors like the PHENIX.
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Muon: An elementary particle with negative electric charge and a spin of ½. It has a lifetime of 2.2 µs, longer than any other unstable lepton, meson, or baryon except for the neutron. Because their interactions are very similar to those of the electron, a muon can be thought of as a much heavier version of the electron. Due to their greater mass, muons do not emit as much bremsstrahlung radiation; consequently, they are much more penetrating than electrons.
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