PHENIX Muon Spectrometers

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.

Glossary

Deuteron: Deuterium is a stable isotope of hydrogen with a natural abundance in the oceans. The nucleus of deuterium, called a deuteron, contains one proton and one neutron, whereas the far more common hydrogen nucleus consists only of a proton and no neutrons.

Gluon: Particle that keeps protons bound together in the nucleus of an atom. Gluons mediate the color force (quantum chromodynamics) that binds the quarks together.

Hadron: A subatomic particle which experiences the strong interaction (strong nuclear force). These are not fundamental particles but are composed of fermions (quarks and antiquarks) and bosons (gluons). All hadrons are single-particle excitations of the basic theory of strong interactions called quantum chromodynamics.

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.

Nucleon: A collective name for two baryons: the proton and neutron.

PHENIX: Pioneering High-Energy Nuclear Interaction Experiment

Pion: Collective name for three subatomic particles: π⁰, π⁺ and π^-. Pions are the lightest mesons and play an important role in explaining low-energy properties of the strong interaction.

Quark: Quarks are one of the two basic constituents of matter (leptons are the other). Quarks are the only fundamental particles that interact through all four of the fundamental forces. Quarks have charge equal to +1/3, -1/3, and ±2/3 a multiple of electron charge. Isolated quarks cannot be found in nature. Three quarks make up protons (2 up, 1 down) and neutrons (1 up, 2 down); quark-antiquark pairs make up particles called mesons. The quarks within protons, neutrons, and mesons are held together by the strong interaction. In order of decreasing mass, there are six quarks: top, bottom, charm, strange, up, and down.

RHIC: Relativistic Heavy-Ion Collider

Strong Interaction: Force that keeps protons and neutrons bound together in the nucleus, carried by gluons (also called the strong nuclear force).

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