Absolute Zero 
The lowest possible temperature in the universe, at which all atomic activity ceases. Equal to -273 degrees Celsius (- 459 degrees Fahrenheit). Used as a benchmark for measuring temperature.

Atom
The smallest particle of an element that can exist either alone or in combination. Composed of an electron cloud and a central nucleus.

Big Bang
The explosion and rapid explansion of matter that occurred at the creation of our universe. In the immediate aftermath of the Big Bang, all matter is thought to have consisted of free quarks and gluons at extremely high temperatures and densities. This plasma then cooled and coalesced into the particles and atoms that now make up all objects in the universe.

Collider
A type physics research machine that brings moving particles into collision with one another for the purpose of studying the outcome. As opposed to a fixed-target machine, which smashes moving particles into a stationary object.

Color
The basic characteristic that differentiates the six varieties of quark.

Electron
A small, indivisible particle that orbits the nucleus of an atom.

Gluon
A particle that holds together quarks via the strong force.

Hadron
Any particle made of quarks.

Heavy ion
The nucleus of a large atom, such as gold, with more or fewer electrons than usual, or no electrons at all.

Ion
Any atom with more or fewer electrons than its normal state, or no electrons at all..

Kelvin temperature scale
A scale of temperature based on Absolute Zero. Zero degrees kelvin is equivalent to - 273 degrees Celsius (- 459 degrees Fahrenheit).

Neutron
A common neutral particle equal in size to a proton, often found in the nucleus of atoms. Composed of two down quarks and one up quark.

Nuclear physics
The study of the atom's nucleus, and the interactions of its parts.

Nucleon
Any particle, such as a proton or neutron, in the nucleus of an atom.

Nucleus (pl. nuclei)
The center of an atom, consisting of one or more protons and associated neutrons.

Phase transition
A process by which a systems changes from one state to another state with different properties, as a result of small changes in temperature or pressure.

Pion
A subatomic particle composed of a quark and an anti-quark. Not seen under everyday conditions, but produced at high temperatures or in reactions between atoms.

Proton
A common subatomic particle found in the nucleus of every atom, often along with neutrons. Made of two up quarks and one down quark, a proton has a positive charge.

Quantum Chromodynamics (QCD)
The widely accepted modern theory of matter, based on the assumption that quarks are distinguished by differences in color and are held together (as in hadrons) by gluons.

Quark
The basic building block of matter, and the foundation for modern theories of matter and the universe. Quarks combine with gluons to make the protons and neutrons that make up every atom in the universe. There are six varieties of quark: up and down, which make up most everyday matter, and the more exotic top, bottom, strange and charm quarks.

Quark-gluon plasma (QGP)
A "soup" of Nature's most basic particles, loosed from their usual confined state within hadrons. Thought to have existed a few millionths of a second after the Big Bang, before matter cooled and organized into hadrons and atoms. Also thought to exist at the centers of neutron stars.

Relativistic
Describes anything travelling at nearly the speed of light, and obeying the special laws of behavior that apply at such speeds.

RHIC
The Relativistic Heavy Ion Collider, a physics research machine at Brookhaven National Laboratory. Will collide two beams of atomic nuclei at 99.95 percent the speed of light.

Speed of light
186,000 miles per second (or 300,000,000 meters per second).The universal "speed limit" that is the fastest that anything can go.

Spin
The characteristic of a particle that causes it to rotate about an axis in a consistent direction.

Strong force
The force that binds quarks and gluons together to form hadrons like protons and neutrons. Hadrons are also bound to one another by the residual strong force between their quarks and gluons.

Subatomic particle
Any particle smaller than an atom.