The Subatomic World Particles Prior to Accelerators By the mid 1930s, the understanding of the fundamental structure of matter seemed almost complete. Decades before, Rutherford had shown that atoms have relatively tiny but massive nuclei. The quantum theory had made sense of atomic spectra and electron orbitals. The discovery of the neutron had explained nuclear isotopes. So protons, neutrons, and electrons provided the building blocks of all matter. Some puzzles remained, however: What holds the protons and neutrons together to form the nucleus? What are the forces involved in the radioactive decays of nuclei that make alpha, beta, and gamma rays? Enter the Accelerator To study the nucleus and the interactions of neutrons and protons that form it, physicists needed a tool that could probe within the tiny nucleus, as earlier scattering experiments had probed within the atom. The accelerator is a tool that allows physicists to resolve very small structures by producing particles with very high momentum and thus short wavelength. The wavelength () of the associated wave is inversely proportional to the momentum (p) of the particle (= h/p), where h = Planck's constant. Particle experiments study collisions of high energy particles produced at accelerators. In modern experiments, large multi-layered detectors surround the collision point. Each layer of the detector serves a separate function in tracking and identifying each of the many particles that may be produced in a single collision. The Particle Explosion To the surprise of the physicists, accelerator experiments revealed that the world of particles was very rich; many more particle types similar to protons and neutrons (called baryons) - and a whole new family of particles called mesons - were discovered. By the early 1960s a hundred or so types of particles had been identified, and physicists still had no complete understanding of the fundamental forces. The Quark Proposal In 1964, two physicists - Murray Gell-Mann and George Zweig - independently hit upon the idea that neutrons and protons and all those new particles could be explained by a few types of yet smaller objects; Gell-Mann called them quarks. They could explain all the observed baryons and mesons with just three types of quarks (now called up, down, and strange) and their antiquarks. The revolutionary part of their idea was that they had to assign the quarks electric charges of 2/3 and -1/3 in units of the proton charge; such charges had never been observed! Antiquarks are the antimatter partners of quarks; they have the same masses as, but the opposite charge from, the corresponding quarks. When a quark meets an antiquark, they may annihilate, disappearing to give some other form of energy. The Standard Model Nearly thirty years and many experiments later, the quark idea has been confirmed. It is now part of the Standard Model of Fundamental Particles and Interactions. New discoveries have shown that there are six types of quarks (given the odd names of up, down, strange, charm, bottom, and top, in order of increasing mass). Also, there are six types of particles including the electron, called leptons. The Standard Model accounts for the strong, weak, and electromagnetic interactions of the quarks and leptons, and thus explains the patterns of nuclear binding and decays. The Particles Made from Quarks The reason that fractional electric charges like those of quarks have not been seen is that the quarks are never found separately, but only inside composite particles called hadrons. There are two classes of hadrons: baryons, which contain three quarks, and mesons, which contain one quark and one antiquark. The sample hadron tables on the Standard Model chart give a few examples of the many known particles. Particles made from the first five quark types have been produced and studied at accelerators. The top quark is so massive it took many years and very high-energy accelerators to produce it. The top quark was finally discovered in April 1995 at Fermilab. The Leptons In contrast to the quarks, any of the six leptons may by found by itself. The electron is the best known lepton. Two other charged leptons, the muon, (discovered in 1936) and the tau (discovered in 1975) differ from the electron only in that they are more massive than it. The other three leptons are very elusive particles called neutrinos, which have no electric charge and very little, if any, mass. There is one type of neutrino corresponding to each type of electrically charged lepton. For each of the six leptons there is an antilepton with equal mass and opposite charge. Forces and Interactions Now we know the building blocks of matter, but we must also ask: What holds it together? All forces are due to the underlying interactions of the particles. Interactions come in four types: gravitational, electromagnetic, strong, and weak. Gravity is perhaps the most familiar force to us, but it is not included in the Standard Model because its effects are tiny in particle processes and, furthermore, physicists have not yet figured out how to include it. Electromagnetic forces are also familiar; they are responsible for binding the electrons to the nucleus to form electrically-neutral atoms. Atoms combine to form molecules or crystals because of electromagnetic effects due to their charged substructure. Most everyday forces, such as the support of the floor or friction, are due to the electromagnetic forces in matter that resist displacement of atoms or electrons from their equilibrium positions in the material. In particle processes the forces are described as due to the exchange of particles; for each type of force there is an associated carrier particle. The carrier particle of the electromagnetic force is the photon; gamma ray is the name given a photon from a nuclear transition. For distances much larger than the size of an atomic nucleus, the remaining two forces have only tiny effects -- so we never notice them in everyday life. But we depend on them for the existence of all the stuff from which the world is made, and for the decay processes that make some types of matter unstable. The strong force holds quarks together to form hadrons; its carrier particles are whimsically called gluons because they so successfully "glue" the quarks together. The binding of protons and neutrons to form nuclei is a residual strong interaction effect due to their strongly-interacting quark and gluon constituents. Leptons have no strong interactions. Weak interactions are the only processes in which a quark can change to another type of quark, or a lepton to another lepton. They are responsible for the fact that all the more massive quarks and leptons decay to produce lighter quarks and leptons. That is why stable matter around us contains only electrons and the lightest two quark types (up and down). The carrier particles of weak interactions are the W and Z bosons. Beta decay of nuclei was the first observed weak process: in a nucleus where there is sufficient energy a neutron becomes a proton and gives off an electron and an antielectron neutrino. This decay changes the atomic number of the nucleus. Beta ray is the name given to the emerging electron. So now we have explained beta and gamma rays; what about the alpha? The alpha particle is a helium nucleus - one of the products of a nuclear fission. Fission is the breakup of a massive nucleus into smaller nuclei; this occurs when the sum of the masses of the smaller nuclei is less than the mass of the parent nucleus. This is a residual strong interaction effect. What Questions Remain? The Standard Model answers many of the questions of the structure and stability of matter with its six types of quarks, six of leptons, and the four force types. But the Standard Model leaves many other questions unanswered: Why are there three types of quarks and leptons of each charge? Is there some pattern to their masses? Are there more types of particles and forces to be discovered at yet higher-energy accelerators? Are the quarks and leptons really fundamental, or do they, too, have substructure? How can the gravitational interactions be included? What particles form the dark matter in the universe? Questions such as these drive particle physicists to build and operate new accelerators, so that higher-energy collisions can provide clues to their answers.