A Appendix: The Greek Alphabet lower upper case case English name Qronunciation a A alpha ß B beta BAY-ta 'Y r gamma 5 ~ delta E E epsilon ( Z zeta ZAY-ta 'fl H eta AY-ta e e theta THAY-ta L r iota K, K kappa ,\ A lambda f-L M mu MEW v N nu ~ - xi 0 0 omicron 7r II pi P P rho ROE (J ~ sigma T T tau TAOW 'U Y upsilon cjJ <I> phi X X chi 1jJ W psi SrGH w n omega o-MEG-a B Appendix: Glossary accelerator: A machine used to accelerate particles to high speeds (and thus high energy, as compared to their rest mass-energy). angular moment um: Angular moment um is a conserved quantity, which is used to describe rotational motion much like moment um for linear motion. Rotational motion can be orbital motion of two bodies around one another or the rotation of a rigid body. The intrinsic angular moment um of a particle is called "spin." In quantum mechanics, angular moment um and spin are quantized quantities: They can only have certain discrete values, measured in multiples of Ti, which is Planck's constant h divided by 21f. annihilation: A process in which a particle meets its corresponding antiparticle and both disappear. The energy appears in some other form, perhaps as a different particle and its antiparticle (and their energies), perhaps as many mesons, and perhaps as a single, neutral boson, such as a ZO boson. The produced particles may be any combination allowed by conservation of energy, momentum, electric, and other charge types and by other rules. antifermion: The antiparticle of a fermion. See also antiparticle. antirnatter: Material made from antifermions. We define the 212 Glossary fermions that are common in our universe as matter and their antiparticles as antirnatter . In the particle theory, no apriori distinction exists between matter and antirnatter . The asymmetry of the universe between these two classes of particles is a deep puzzle for which we are not yet completely sure of an explanation. antiparticle: For every fermion type, another fermion type exists that has exact1y the same mass but the opposite value of all other charges (quantum numbers). This is called the antiparticle. For example, the antiparticle of an electron is a particle of positive electric charge called the positron. Bosons also have antiparticles, except for those that have zero value for all charges, for example, a photon or a composite boson made from a quark and its corre­ sponding antiquark. In this case, the particle and the antiparticle have no differences; they are the same object. antiquark: The antiparticle of a quark. An antiquark is denoted by putting a bar over the corresponding quark symbol (d, u, s, etc .. ). astrophysics: The physics of astronomical objects such as stars and galaxies. B Factory: An accelerator designed to maximize the production of B mesons. The properties of the B mesons are then studied with specialized detectors. baryon [BARE-ee-on]: A hadron made from three quarks The proton (uud) and the neutron (udd) are both baryons. They may also contain additional quark-antiquark pairs. baryon-antibaryon asymmetry: The observation that the uni­ verse contains many baryons but few antibaryons; a fact that needs explanation. beam: The particle stream produced by an accelerator usually clustered in bunches. Big Bang theory: The theory of an expanding universe that begins as an infinitely dense and hot medium. The initial instant is called the Big Bang. boson [BOZE-on]: A particle that has integer intrinsic angular momentum (spin) measured in units of 1i (spin = 0,1,2, ... ). All particles are either fermions or bosons. The particles associated B. Glossary 213 with all fundamental interactions (forces) are bosons. Composite particles with even numbers of fermion constituents (quarks) are also bosons. bottom quark (b): The fifth fiavor of quark (in order of increasing mass) , with electric charge -1/3. calorimeter: A device that can measure the energy deposited in it (originally, devices to measure thermal energy deposited, using change of temperaturej particle physicists use the word for any energy-measuring device). CERN: The major European international accelerator laboratory located near Geneva, Switzerland. charge: A quantum number carried by a particle. This quantum number determines whether the particle can participate in an interaction process. A particle with electric charge has electrical interactions, one with strong charge has strong interactions, etc. charge conservation: The observation that electric charge is conserved in any process of transformation of one group of particle into another. charm quark (c): The fourth fiavor of quark (in order of increasing mass) , with electric charge +2/3. collider: An accelerator in which two beams traveling in opposite directions are steered together to provide high-energy collisions between the particles in one beam and those in the other. colliding-beam experiments: Experiments done at colliders. color charge: The quantum number that determines participation in strong interactions; quarks and gluons carry nonzero color charges. color neutral: An object with no net color charge. For composites made of color-charged particles, the rules of neutralization are complex. Three quarks (baryon) or a quark plus an antiquark (meson) can both form color-neutral combinations. confinement: The property of the strong interactions by which quarks or gluons are never found separately but only inside color­ neutral composite objects. 214 Glossary conservation: When a quantity (e.g., electric charge, energy, or moment um) is conserved, it is the same after areaction between particles as it was before. cosmology: The study of the history of the universe. dark matter: Matter that is in space but is not visible to us because it emits no radiation by which we can observe it. The motion of stars around the centers of their galaxies implies that about 90% of the matter in a typical galaxy is dark. Physicists speculate that dark matter also exists between the galaxies, but this is harder to verify. decay: A process in which a particle disappears and in its place two or more different particles appear. The sum of the masses of the produced particles is always less than the mass of the original particle. (The mass-energy is conserved, however.) detector: Any device used to sense the passage of a particle. Also, the word detector is used for a collection of such devices designed so that each serves a particular purpose in allowing physicists to reconstruct particle events. down quark (d): The second flavor of quark (in order of increasing mass), with electric charge -1/3. electric charge: The quantum number that determines participa­ tion in electromagnetic interactions. electromagnetic interaction: The interaction due to electric charge; this includes magnetic effects, which have to do with moving electric charges. electron [e-LEC-tron] (e): The least-massive electrically charged particle, hence, absolutely stable. It is the most common lepton, with electric charge -1. electroweak interaction: In the Standard Model, electromagnetic and weak interactions are related (unified); physicists use the term electroweak to encompass both of them. event: What occurs when two particles collide or a single particle decays. Particle theories predict the probabilities of various possible events occurring when many similar collisions or decays are studied. They cannot predict the outcome for any single event. B. Glossary 215 exclusion principle: See fermion. Fermilab: Fermi National Accelerator Laboratory in Batavia, Illinois (near Chicago). N amed for particle physics pioneer Enrico Fermi. fermion [FARE-mee-on]: Any particle that has odd, half-integer (1/2, 3/2, ... ) intrinsic angular momentum (spin), measured in units of n. As a consequence of this peculiar angular momentum, fermions obey a rule called the Pauli Exclusion Principle, which states that no two fermions can exist in the same state at the same place and time. Many of the properties of ordinary matter arise because of this rule. Electrons, protons, and neutrons are all fermions, as are all fundamental matter particles, quarks and leptons. fixed-target experiment: An experiment in which the beam of particles from an accelerator is directed at a stationary (or nearly stationary) target. The target may be asolid, a tank containing liquid or gas, or a gas jet. flavor: The name used for the different quark types (up, down, strange, charm, bottom, top) and for the different lepton types (elec­ tron, muon, tau). For each charged lepton flavor, a corresponding neutrino flavor exists. In other words, flavor is the quantum number that distinguishes the different quark/lepton types. Each flavor of quark and charged lepton has a different mass. For neutrinos, we do not yet know if they have a mass or what the masses are. freeze out: As the universe expands and cools, the probability of any collision-driven process decreases, because the rate of the necessary collisions decreases. A process can be ignored when the average time between collisions is long compared to the age of the universe at that time. Such a process is then said to have frozen out. fundamental interaction: In the Standard Model, the funda­ mental interactions are the strong, electromagnetic, weak, and gravitational interactions. At least one more fundamental inter­ action (Riggs) is in the theory; it is responsible for fundamental particle masses. Five interaction types are all that are needed to explain all observed physical phenomena. fundamental particle: A particle with no internal substructure. 216 Glossary In the Standard Model, the quarks, leptons, photons, gluons, W± bosons, and ZO bosons are fundamental. All other objects are made from these. galaxy: A collection of stars held together by gravitational forces. general relativity: The theory of gravitation formulated by Einstein. generation: A set of one of each charge type of quark and lepton, grouped by mass.