List of particles List of particles This is a list of particles in particle physics, including currently known and hypothetical elementary particles, as well as the composite particles that can be built up from them. For a chronological listing of subatomic particles by discovery date, see Timeline of particle discoveries. A new particle is discovered: The first detection of the positron occurred in 1932 in a cloud chamber built by Carl D. Anderson. The track of the positron can be seen, going from top to bottom and curving to the right. Elementary particles An elementary particle is a particle with no measurable internal structure, that is, it is not a composite of other particles. They are the fundamental objects of quantum field theory. Elementary particles can be classified according to their spin, with fermions having half-integer spin and bosons integer spin. Standard Model The Standard Model of particle physics is our current understanding of the physics of elementary particles. All Standard Model particles except the Higgs boson have been observed. Fermions (half-integer spin) Quark structure proton: 2 up quarks and 1 down quark. Fermions have half-integer spin; for all known elementary fermions this is ½.

Each fermion has its own distinct antiparticle. Fermions are the basic building blocks of all matter . They are classified according to whether they interact via the colour force or not. According to the Standard Model, there are 12 flavours of elementary fermions: six quarks and six leptons. Quarks interact via the colour force. Their respective antiparticles are known as antiquarks. Quarks exist in six flavours: Generation >112 See below The Higgs boson (spin-0) is predicted by electroweak theory, and is the only Standard Model particle not yet observed. In the Higgs mechanism of the Standard Model, the massive Higgs boson is created by spontaneous symmetry breaking of the Higgs field. The intrinsic masses of the elementary particles (particularly the massive W± and Z0 bosons) would be explained by their interactions with this field. Many physicists expect the Higgs to be discovered at the Large Hadron Collider (LHC) particle accelerator now under construction at CERN. Hypothetical particles Supersymmetric theories predict the existence of more particles, none of which have been confirmed experimentally as of 2006.

The neutralino (spin-½) is a superposition of the superpartners of several neutral Standard Model particles. It is a leading candidate for dark matter. The partners of charged bosons are called charginos. The photino (spin-½) is the superpartner of the photon . The gravitino (spin-3⁄2) is the superpartner of the graviton boson in supergravity theories. Sleptons and squarks (spin-0) are the supersymmetric partners of the Standard Model fermions. The stop squark (superpartner of the top quark) is thought to have a low mass and is often the subject of experimental searches. Other theories predict the existence of additional bosons. The graviton (spin-2) has been proposed to mediate gravity in theories of quantum gravity. The graviscalar (spin-0) and graviphoton (spin-1). The axion (spin-0) is a pseudoscalar particle introduced in Peccei-Quinn theory to solve the strong-CP problem.

The saxion (spin-0, scalar, R parity=1) and the axino (spin-1/2, R parity = -1) form together with the axion a supermultiplet in supersymmetric extensions of Peccei-Quinn theory. The X boson and the Y boson are predicted by GUT theories to be heavier equivalents of the W and Z. The magnetic photon. Sterile neutrinos are introduced by many extensions to the Standard Model, and may be needed to explain the LSND results. Mirror particles are predicted by theories that restore Parity symmetry. Magnetic monopole is a generic name for particles with non-zero magnetic charge. They are predicted by some GUT theories. Tachyon is a generic name for hypothetical particles that travel faster than the speed of light and have an imaginary rest mass. The preon was a suggested substructure for both quarks and leptons, but modern collider experiments have all but disproven their existence. Composite particles Hadrons are defined as strongly interacting composite particles. Hadrons are either: Fermions, in which case they are called baryons. Bosons, in which case they are called mesons.

Quark models, first proposed in 1964 independently by Murray Gell-Mann and George Zweig (who called quarks "aces"), describe the known hadrons as composed of valence quarks and/or antiquarks, tightly bound by the colour force, which is mediated by gluons. A "sea" of virtual quark-antiquark pairs is also present in each hadron. Baryons (fermions) A combination of three u, d or s-quarks with a total spin of 3/2 form the so-called baryon decuplet. Ordinary baryons ( fermions) contain three valence quarks or three valence antiquarks each. Nucleons are the fermionic constituents of normal atomic nuclei: Hyperons such as the Λ, Σ, Ξ, and Ω particles, which contain one or more strange quarks, are short-lived and heavier than nucleons. Although not normally present in atomic nuclei, they can appear in short-lived hypernuclei. A number of charmed and bottom baryons have also been observed. Some hints at the existence of exotic baryons have been found recently; however, negative results have also been reported.

Their existence is uncertain. Pentaquarks consist of four valence quarks and one valence antiquark. Mesons (bosons) Mesons of spin 0 form a nonet Ordinary mesons ( bosons) contain a valence quark and a valence antiquark, and include the pion, kaon, the J/ψ, and many other types of mesons. In quantum hadrodynamic models, the strong force between nucleons is mediated by mesons. Exotic mesons may also exist. Positive signatures have been reported for all of these particles at some time, but their existence is still somewhat uncertain. Tetraquarks consist of two valence quarks and two valence antiquarks. Glueballs are bound states of gluons with no valence quarks. Hybrids consist of one or more valence quark-antiquark pairs and one or more real gluons. Atomic nuclei Atomic nuclei consist of protons and neutrons. Each type of nucleus contains a specific number of protons and a specific number of neutrons , and is called a nuclide or isotope. Nuclear reactions can change one nuclide into another. See Isotope table (complete) for a list of isotopes.

Atoms Atoms are the smallest neutral particles into which matter can be divided by chemical reactions. An atom consists of a small, heavy nucleus surrounded by a relatively large, light cloud of electrons. Each type of atom corresponds to a specific chemical element , of which 111 have been officially named. Refer to the periodic table for an overview. Molecules Molecules are the smallest particles into which a non-elemental substance can be divided while maintaining the physical properties of the substance. Each type of molecule corresponds to a specific chemical compound. Molecules are composites of one or more atoms. See list of compounds for a list of molecules. Condensed matter The field equations of condensed matter physics are remarkably similar to those of high energy particle physics. As a result, much of the theory of particle physics applies to condensed matter physics as well; in particular, there are a selection of field excitations, called quasi-particles, that can be created and explored. These include: Phonons are vibrational modes in a crystal lattice. Excitons are bound states of an electron and a hole. Plasmons are coherent excitations of a plasma . Polaritons are mixtures of photons with other quasi-particles.

Polarons are moving, charged (quasi-) particles that are surrounded by ions in a material. Magnons are coherent excitations of electron spins in a material. Other A WIMP (weakly interacting massive particle) is any one of a number of particles that might explain dark matter (such as the neutralino or the axion). The pomeron, used to explain the elastic scattering of hadrons and the location of Regge poles in Regge theory. The skyrmion, a topological soliton of the pion field, used to model the low-energy properties of the nucleon, such as the axial vector current coupling and the mass. A goldstone boson is a massless excitation of a field that has been spontaneously broken. The pions are quasi-Goldstone bosons (quasi- because they are not exactly massless) of the broken chiral isospin symmetry of quantum chromodynamics. A goldstino is a Goldstone fermion produced by the spontaneous breaking of supersymmetry. An instanton is a field configuration which is a local minimum of the Euclidean action. Instantons are used in nonperturbative calculations of tunneling rates.

A dyon is a hypothetical particle with both electric and magnetic charges A geon is an electromagnetic or gravitational wave which is held together in a confined region by the gravitational attraction of its own field energy. An Oh-My-God particle is an ultra-high energy cosmic ray (probably a proton ) falling well beyond the GZK cutoff, the energy limit beyond which virtually no cosmic rays should be detected. A spurion is the name given to a "particle" inserted mathematically into an isospin-violating decay in order to analyze it as though it conserved isospin. Classification by speed Some Particle Properties Some Particle Properties Back A particle, increasing its speed because of some force acting on it, gains energy of motion. An electron (negatively charged) gains one electron volt (eV) of energy in accelerating through a vacuum from the negative end to the positive end of a one-volt battery. The one eV of energy is given up to other particles as the electron crashes into the positive end. A proton (positively charged) traveling from positive to negative pole through the vacuum would also gain one eV of energy and give it up in its collision with particles in the negative end.

This proton collision is similar to the proton beam collision with a target at Fermilab, but at Fermilab the proton energy is much greater. As a particle's speed approaches the speed of light (300,000,000 meters per second), most of the energy gained is not in the form of greater speed but as greater mass. When the particle slows down or collides with another particle the extra mass may be converted back to energy of motion or it may form new particle mass. For example, a proton at its maximum speed in the Fermilab accelerator has more than 800 times the mass it had when not moving (rest mass) and the mass may appear as new particles after a collision. Since energy converts to mass and mass converts to energy ( Einstein's equation E=mc 2 ) both are aspects of the same thing.

Since subatomic masses are so small, it is easiest to express them in the very small electron-volt (eV) unit.* *The approximate conversion factors are: 1 gram=6 x 1026 million electron volts (MeV) 1 electron volt=1.6 x 10-33 grams 1 MeV=1.6 x 10-27 grams Every particle has either no rest mass or a rest mass that is unchanging. For example, an electron's rest mass is .511 MeV, a proton's is 936.2 MeV, and an upsilon's is 9402 MeV. A photon (a particle of light) has no rest mass, only mass when in motion at its normal speed--the speed of light. Every particle is either neutral or has an electrical charge that never changes. The charge of an electron is negative. Its magnitude is often used as a unit of electric charge (e). An alpha particle (2 protons and 2 neutrons) has 2 e positive charges. (The quantity of charge "e" exists in both positive and negative form.) There are other quantifiable properties of particles including spin and magnetism.

The quark, considered to be one of the most fundamental particles, has additional properties given fanciful names such as color and flavor. The Standard Model – The Physics Hypertextbook Discussion introduction The standard model is the name given in the 1970s to a theory of fundamental particles and how they interact. It incorporated all that was known about subatomic particles at the time and predicted the existence of additional particles as well. There are seventeen named particles in the standard model, organized into the chart shown below. The last particles discovered were the W and Z bosons in 1983, the top quark in 1995, the tau neutrino in 2000, and the Higgs boson in 2012. Standard Model Particles (nucleons included for comparison) * The proton and neutron are hadrons (made of quarks), baryons (made of three quarks), and nucleons (found in the nucleus). ** The higgs boson is the only known scalar boson. family 125,900 particle families Fundamental particles are either the building blocks of matter, called fermions, or the mediators of interactions, called bosons.

There are twelve named fermions and five named bosons in the standard model. Fermions obey a statistical rule described by Enrico Fermi (1901–1954) of Italy, Paul Dirac (1902–1984) of England, and Wolfgang Pauli (1900–1958) of Austria called the exclusion principle. Simply stated, fermions cannot occupy the same place at the same time. (More formally, no two fermions may be described by the same quantum numbers.) Leptons and quarks are fermions, but so are things made from them like protons, neutrons, atoms, molecules, people, and walls. This agrees with our macroscopic observations of matter in everyday life. People cannot walk through walls unless the wall gets out of the way. Bosons, in contrast, are have no problem occupying the same place at the same time.

(More formally, two or more bosons may be described by the same quantum numbers.) The statistical rules that bosons obey were first described by Satyendra Bose (1894–1974) of India and Albert Einstein (1879–1955) of Germany. Gluons, photons, and the W, Z and Higgs are all bosons. As the particles that make up light and other forms of electromagnetic radiation, photons are the bosons we have the most direct experience with. In our everyday experience, we never see beams of light crash into one another. Photons are like phantoms. One may pass through the other with no effect. Elementary particles have an intrinsic spin angular momentum S. The adjective intrinsic means innate or essential to the thing itself. Elementary particles don't have spin because someone is spinning them. They just spin — or rather, they just have a measurable quantity with the same units as angular momentum. In current physics, elementary particles are featureless — like a mathematical point. In order for something to be perceived as spinning, the thing spinning would need something like a "front" and a "back". Featureless, point particles don't have anything like that.

Particle physics is best described with mathematics. Spin is a convenient label for a measurable quality and not a description of reality. Every elementary particle has associated with it a spin quantum number s (often called the spin number or just the spin), where s is any whole number multiple of a half. Fermions have half integral spin quantum numbers (½, 1½, 2½, etc.) and bosons have integral spin quantum numbers (0, 1, 2, etc.). No spin numbers are possible in between these. Spin is a quantized quantity. The elementary fermions have a spin ½. Particles made from combinations of fermions will have an overall spin that's a combination of the individual spins. A baryon composed three quarks will combine to an overall spin of ½ or 1½, since those are the only possible, non-negative combinations of ½ ± ½ ± ½. That shows that all baryons (like protons and neutrons, for example) are also fermions. Likewise, a meson composed of a quark and an antiquark will combine to an overall spin of 0 or 1 since those are the only possible, non-negative combinations of ½ ± ½.

That shows that all all mesons (like the pion of the residual strong interaction, for example) are also bosons. The force carying bosons of the standard model (gluons, photons, and the W and Z) have spin 1 since they go with vector fields. The Higgs boson corresponds to a scalar field so it has spin 0. If the particle of the gravitational field is ever discovered, it would be called a graviton and would have a spin 2 since it corresponds to a tensor field. A tensor is a mathematical object that's more complex than a vector, which is in turn more complex than a scalar. See the trend? A scalar field with no direction gets a particle with spin 0. A vector field with a direction gets a particle with spin 1. A tensor field that stretches and squeezes space in two directions gets a particle with spin 2. All fundamental and composite particles have a spin quantum number s (lowercase). This is associated with a spin angular momentum S (uppercase). The SI unit of angular momentum is the kilogram meter squared per second [kgm2/s] or, equivalently, the joule second [Js], which is much too large for elementary particles.

Instead ℏ (h bar), also known as the reduced Planck constant (ℏ = h/2π), is used. For reasons that are beyond the scope of this book, the spin quantum number s (which is just a number) and the spin angular momentum S (which is a number with a unit) are not numerically the same. Instead, they are related by a non-obvious equation. S = ℏ Fermions are divided into two groups of six: those that must bind together are called quarks and those that can exist independently are called leptons. The word "quark" originally appeared in a single line of the the novel Finnegans Wake written by the Irish author James Joyce (1882–1941). The protagonist of the book is a publican named Humphrey Chimpden Earwicker who dreams that he is serving beer to a drunken seagull (no joke). Instead of asking for "three quarts for Mister Mark" the inebriated bird says "three quarks for Muster Mark". Since the pre-standard model theory was complete with only three quarks, the name made some sense. The full standard model today needs six quarks.

That hasn't made the word any less fun to say. Quark! The six flavors of quark are up, down, strange, charm, top, and bottom. The names of the flavors are essentially meaningless. Quarks are known to bind into triplets and doublets. The triplets are called baryons, a term derived from the Greek word βαρύς (varys) meaning "heavy". The doublets are called mesons, a term derived from the Greek word μέσος (mesos) meaning "medium". Collectively baryons (the heavy triplets), mesons (the middleweight doublets), and quarks (the fundamental particles) are known as hadrons, from the Greek word αδρός (adros) meaning thick, robust, massive, or large. This name alludes to the ability of the point-like quarks to bind together and form particles that are "thick" in a certain sense. The other six fermions are called leptons, a name derived from the Greek word λεπτός (leptos) meaning thin, delicate, lightweight, or small.

These particles don't need to bind to each other, which keeps them "thin" in a certain sense. Originally leptons were considered the "light" particles and hadrons the "heavy" particles, but the discovery of the tau lepton in 1975 broke that rule. The tau (the heaviest lepton) is almost twice as massive as a proton (the lightest hadron). Baryons found in the nucleus (the proton and neutron) are called nucleons. The Latin word for kernel is nucleus. Nucleons are found in the metaphorical "kernel" of the atom. Baryons that contain at least one strange quark but no charm, bottom, or top quarks are called hyperons. The Greek word for beyond is υπέρ (yper), which morphed into the English prefix hyper- over the years. Hyperons are particles that are "way out" in a certain sense. The neutrinos are an important subgroup within the leptons. They come in three flavors named for their partner leptons. The electron, muon, and tau are matched with the electron neutrino, muon neutrino, and tau neutrino.

Neutrinos have very little mass (even for leptons) and interact so weakly with the rest of the particles that they are exceptionally difficult to detect. The name is a play on words. The Italian word for neutron (neutrone) sounds like the word neutral (neutro) with an augmentative suffix (-one) tacked on the end. That is, it sounds something like "big neutral" to Italian ears. Replace the augmentative suffix -one with the diminutive suffix -ino and you have a "little neutral", which is a good description of what a neutrino is — a diminutive neutral particle. Aaaaaw, so small and precious. Fermions belong to one of three known generations from ordinary (I), to exotic (II), to very exotic (III). (These are the adjectives I selected to describe the generations.) Generation I particles can combine to form hadrons with effectively infinite life spans (stable atoms made of electrons, protons, and neutrons for example). Generation II particles always form unstable hadrons. The longest lived hadron containing a generation II quark is the lambda particle (made of an up, down, and strange quark).

It has a mean lifetime less than a billionth of a second, which is considered long-lived for an unstable hadron. Generation III particles are divided in their behavior. The bottom quark isn't much stranger than a strange quark, but the top quark is so short-lived that it doesn't exist long enough to do anything. It falls apart before the world even knows it exists. Top quarks are only known from their decay products. particle interactions Three of the four fundamental fources of nature are included in the standard model of particle physics — electromagnetism, the strong force, and the weak force. (Gravity is not included in the standard model.) Each force acts between particles because of some property of that particle — charge for electromagnetism, color for the strong force, and flavor for the weak force. The bosons associated with each force are called gauge bosons — the photon for electromagnetism, gluons for the strong force, and the W and Z bosons for the weak force. Charge is the property of matter that gives rise to electric and magnetic phenomena (known collectively as electromagnetism).

Charge is quantized, which means it can only exist in discrete amounts with restricted values — multiples and fractions of the elementary charge (e = 1.6 × 10−19 C). Particles that exist independently (the electron, muon, and tau) carry multiples of the elementary charge (−1e), while quarks carry fractions of the elementary charge (+⅔e or −⅓e). Quarks always bind together in groups whose total charge is an integral multiple of the elementary charge, which is why no one has ever directly measured a fractional charge. In addition, since opposite charges attract, electrons tend to bind to protons to form atoms that are neutral overall. We don't normally notice the electrical nature of matter because of this. Charged particles interact by the exchange of photons — the carrier of the electromagnetic force. Whenever an electron repels another electron or an electron orbits a nucleus, a photon is responsible. Photons are massless, uncharged, and have an unlimited range. The mathematical model used to describe the interaction of charged particles through the exchange of photons is known as quantum electrodynamics (QED). Quarks stick to other quarks because they possess a characteristic known as color (or color charge).

Quarks come in one of three colors: red, green, and blue. Don't let the words mislead you. Quarks are much too small to to be visible and thus could never have a perceptual property like color. The names were chosen because of a convenient analogy. The colors of quarks in the standard model combine like the colors of light in human vision. Red light plus green light plus blue light appears to us humans as "colorless" white light. A baryon is a triplet of one red, one green, and one blue quark. Put them together and you get a color neutral particle. A color plus its opposite color also gives white light. Red light plus cyan light looks the same to humans as white light, for example. A meson is a doublet of one colored quark and one anticolored antiquark. Put them together and you get another color neutral particle. There's something about color that makes it want to hide itself from anything bigger than a nucleus. Quarks can't stand being apart from one another. They just have to join up and always do so in a way that hides their color from the outside world. One color is never favored over another when quarks get together.

Matter is color neutral down to the very small scale. Colored particles are bound together by the appropriately named gluons. Gluons are also colored, but in a more complicated way than the quarks are. Six of the eight gluons have two colors, one has four, and another has six. Gluons glue quarks together, but they also stick to themselves. One consequence of this is that they they can't reach out and do much beyond the nucleus. The mathematical model used to describe the interaction of colored particles through the exchange of gluons is known as quantum chromodynamics (QCD). The whole sticky mess is called the strong force or the strong interaction since it results in forces in the nucleus that are stronger than the electromagnetic force. Without the strong force, every nucleus would blow itself to smithereens. There are twelve named elementary fermions. The difference between them is one of flavor. The word "flavor" is used here to mean "type" and it applies only to fermions. Don't let the word mislead you. Subatomic particles are much too small to have any characteristics that could be directly observed by human senses.

Flavored particles interact weakly through the exchange of W or Z bosons — the carriers of the weak force (also known as intermediate vector bosons). When a neutron decays into a proton, a W− boson is responsible. The mathematical model used to describe the interaction of flavored particles through the exchange of W and Z bosons is sometimes known as quantum flavordynamics (QFD), but this is a term that is not used by working particle physicists. At higher energies, the weak and electromagnetic forces begin to look more and more alike. The mathematical model that describes these interactions together is known as electroweak theory (EWT). This is the conventional name for the theory of the weak force. mass and gravity All fermions are thought to have a nonzero rest mass. Particles in generation I are less massive than those in generation II, which are less massive than those in generation III. Within the generations, quarks are more massive than leptons and neutrinos are less massive than the other leptons. Bosons are divided when it comes to mass. Gluons and photons are massless. The W, Z, and Higgs bosons are massive. Mass is energy.

A moving particle is more massive than a stationary particle because it has kinetic energy. Logically then, a stationary particle should have no mass. If we could stop a photon (which we can't) it would weigh nothing. Our logic appears to be working. If we could stop an electron (which we can) it would weigh something. Our logic is broken. Why do some particles weigh something at rest and others weigh nothing? Mass is energy and energy comes in two types: kinetic energy (the energy of motion) and potential energy (the energy of arrangement). The kinetic energy contribution to mass is minor. Most of the mass around us comes from some sort of potential energy. For example, a proton is made of two up quarks and a down quark. The masses of these three quarks do not add up to the mass of a proton. mp ≠ 9.4 MeV/c2 The masses of the parts are only 1% of the mass of the whole. The remaining 99% comes from the potential energy of the strong force holding the proton together. The particles that mediate the strong force are gluons. The interaction energy of these massleess particles is what gives the proton most of its mass.

So why do quarks have mass but the gluons don't? Or as the question was historically stated, why do the W and Z bosons have mass but the photon doesn't? Maybe there's another kind of potential energy. Maybe there's another interaction out there — an interaction that some particles feel and others don't. If there's an interaction, there's a particle — a particle that gives mass to other particles when they're just sitting around doing nothing. The interaction that gives mass to elementary particles was proposed in 1964 by scientists in three independent locations. Gerald Guralnik , Carl Hagen , and Tom Kibble at Imperial College, London It should be called the Englert–Brout–Higgs–Guralnik–Hagen–Kibble mechanism, but it isn't. For whatever reason, the interaction that gives mass to elementary particles is called the Higgs mechanism and the particle that mediates the interaction is called the Higgs boson, the Higgs particle, or (rarely) the higgson. All of space is assumed to be filled with a Higgs field — a background sea of virtual Higgs bosons that pop in and out of existence.

The quarks, leptons, and W and Z bosons moving around through space interact with this field, which is why these particles have mass. The photons and gluons do not interact with the Higgs field, which is why these particles do not have mass. Even the Higgs boson itself interacts with the Higgs field. It gives itself mass! The Higgs boson is different from the other bosons (gluons, photons, W and Z bosons) in that the Higgs mechanism doesn't result in anything resembling a force (like the strong, electromagnetic, and weak forces). The Higgs field is a scalar field and the Higgs boson is a particle with spin zero. Gravity is the force between objects due to their mass. The mathematical model that would describe gravity on the particle level is sometimes called quantum geometrodynamics (QGD), but is more usually referred to as quantum gravitation. The standard model of particle physics does not include gravity (nor could it ever) and there currently is no quantum theory of gravitation. If there was, it would have to include a force carying particle. The proposed name for this particle is the graviton. General relativity predict the existence.

of gravitational waves that would behave like tensors — that is, they would distort space-time in two perpendicular directions as they propagate outward. This two dimensional behavior leads theoretical physicists to believe that the graviton would have spin two. It is hoped that gravity will be taken care of in a theory beyond the standard model. In an extreme case of overconfidence, some theorists propose that such a theory would be a theory of everything. Given the history of science (and of life in general), anything that claims to be the ultimate representation of reality (scientific, economic, cultural, or religious) is certainly doomed to be superseded by something bigger and better — or at the very least, something newer. names, names, names The theme of this topic seems to be "names, names, names". Particle groups named after physicists * Classical particles (the molecules of an ideal gas, for example) are not a part of the Standard Model, but are included for comparison.

group 3rd order special unitary group the set of all 3 × 3 unitary matrices with unit determinant SU(2) 2nd order special unitary group the set of all 2 × 2 unitary matrices with unit determinant isomorphic to the group of quaternions of absolute value 1, {x ∈ ℍ: |x| =1} diffeomorphic to a hypersphere (3-sphere) homomorphic to the rotation group SO(3), the set of all rotations about the origin in ordinary three dimensional euclidean space U(1) 1st order unitary group the set of all 1 × 1 unitary matrices isomorphic to the circle group, the multiplicative group of complex numbers with absolute value 1, T = {x ∈ ℂ: |x| =1} isomorphic to SO(2), the second order special orthogonal group What is this? The Standard Model Lagrangian. ℒ = −¼FμνFμν + iψ̄D̸ψ + ψiyijψjφ + h.c.

+ |Dμφ|2 − V(φ) Molecular Expressions Microscopy Primer: Physics of Light and Color - Light: Particle or a Wave? Movie Gallery Light: Particle or a Wave? The exact nature of visible light is a mystery that has puzzled man for centuries. Greek scientists from the ancient Pythagorean discipline postulated that every visible object emits a steady stream of particles, while Aristotle concluded that light travels in a manner similar to waves in the ocean. Even though these ideas have undergone numerous modifications and a significant degree of evolution over the past 20 centuries, the essence of the dispute established by the Greek philosophers remains to this day. One point of view envisions light as wave-like in nature, producing energy that traverses through space in a manner similar to the ripples spreading across the surface of a still pond after being disturbed by a dropped rock. The opposing view holds that light is composed of a steady stream of particles, much like tiny droplets of water sprayed from a garden hose nozzle. During the past few centuries, the consensus of opinion has wavered with one view prevailing for a period of time, only to be overturned by evidence for the other.

Only during the first decades of the twentieth century was enough compelling evidence collected to provide a comprehensive answer, and to everyone's surprise, both theories turned out to be correct, at least in part. In the early eighteenth century, the argument about the nature of light had turned the scientific community into divided camps that fought vigorously over the validity of their favorite theories. One group of scientists, who subscribed to the wave theory, centered their arguments on the discoveries of Dutchman Christiaan Huygens. The opposing camp cited Sir Isaac Newton's prism experiments as proof that light traveled as a shower of particles, each proceeding in a straight line until it was refracted, absorbed, reflected, diffracted or disturbed in some other manner. Although Newton, himself, appeared to have some doubt about his corpuscular theory on the nature of light, his prestige in the scientific community held so much weight that his advocates ignored all other evidence during their ferocious battles. Huygens' theory of light refraction, based on the concept of the wave-like nature of light, held that the velocity of light in any substance was inversely proportion to its refractive index.

In other words, Huygens postulated that the more light was "bent" or refracted by a substance, the slower it would move while traversing across that substance. His followers concluded that if light were composed of a stream of particles, then the opposite effect would occur because light entering a denser medium would be attracted by molecules in the medium and experience an increase, rather than a decrease, in speed. Although the perfect solution to this argument would be to measure the speed of light in different substances, air and glass for example, the devices of the period were not up to the task. Light appeared to move at the same speed regardless of the material through which it passed. Over 150 years passed before the speed of light could be measured with a high enough accuracy to prove that the Huygens theory was correct. Despite the highly regarded reputation of Sir Isaac Newton, a number of prominent scientists in the early 1700s did not agree with his corpuscular theory. Some argued that if light consisted of particles, then when two beams are crossed, some of the particles would collide with each other to produce a deviation in the light beams.

Obviously, this is not the case, so they concluded that light must not be composed of individual particles. Interactive Tutorial Particle and Wave Refraction When a beam of light travels between two media having differing refractive indices, the beam undergoes refraction, and changes direction when it passes from the first medium into the second. This interactive tutorial explores how particles and waves behave when refracted through a transparent surface. Huygens, for all his intuition, had suggested in his 1690 treatise Trait� de la Lumi�re that light waves traveled through space mediated by the ether, a mystical weightless substance, which exists as an invisible entity throughout air and space. The search for ether consumed a significant amount of resources during the nineteenth century before finally being laid to rest. The ether theory lasted at least until the late 1800s, as evidenced by Charles Wheatstone's proposed model demonstrating that ether carried light waves by vibrating at an angle perpendicular to the direction of light propagation, and James Clerk Maxwell's detailed models describing the construction of the invisible substance. Huygens believed that ether vibrated in the same direction as light, and formed a wave itself as it carried the light waves.

In a later volume, Huygens' Principle, he ingeniously described how each point on a wave could produce its own wavelets, which then add together to form a wavefront. Huygens employed this idea to produce a detailed theory for the refraction phenomenon, and also to explain why light rays do not crash into each other when they cross paths. When a beam of light travels between two media having different refractive indices, the beam undergoes refraction, and changes direction when it passes from the first medium into the second. To determine whether the light beam is composed of waves or particles, a model for each can be devised to explain the phenomenon (Figure 3). According to Huygens' wave theory, a small portion of each angled wavefront should impact the second medium before the rest of the front reaches the interface. This portion will start to move through the second medium while the rest of the wave is still traveling in the first medium, but will move more slowly due to the higher refractive index of the second medium. Because the wavefront is now traveling at two different speeds, it will bend into the second medium, thus changing the angle of propagation.

In contrast, particle theory has a rather difficult time explaining why particles of light should change direction when they pass from one medium into another. Proponents of the theory suggest that a special force, directed perpendicular to the interface, acts to change the speed of the particles as they enter the second medium. The exact nature of this force was left to speculation, and no evidence has ever been collected to prove the theory. Another excellent comparison of the two theories involves the differences that occur when light is reflected from a smooth, specular surface, such as a mirror. Wave theory speculates that a light source emits light waves that spread in all directions. Upon impacting a mirror, the waves are reflected according to the arrival angles, but with each wave turned back to front to produce a reversed image (Figure 4). The shape of arriving waves is strongly dependent upon how far the light source is from the mirror. Light originating from a close source still maintains a spherical, highly curved wavefront, while light emitted from a distance source will spread more and impact the mirror with wavefronts that are almost planar. The case for a particle nature for light is far stronger with regards to the reflection phenomenon than it is for refraction.

Light emitted by a source, whether near or far, arrives at the mirror surface as a stream of particles, which bounce away or are reflected from the smooth surface. Because the particles are very tiny, a huge number are involved in a propagating light beam, where they travel side by side very close together. Upon impacting the mirror, the particles bounce from different points, so their order in the light beam is reversed upon reflection to produce a reversed image, as demonstrated in Figure 4. Both the particle and wave theories adequately explain reflection from a smooth surface. However, the particle theory also suggests that if the surface is very rough, the particles bounce away at a variety of angles, scattering the light. This theory fits very closely to experimental observation. Interactive Tutorial Particle and Wave Reflection An excellent comparison of the wave and particle theories involves the differences that occur when light is reflected from a smooth, specular surface, such as a mirror. This interactive tutorial explores how particles and waves behave when reflected from a smooth surface. Particles and waves should also behave differently when they encounter the edge of an object and form a shadow (Figure 5).

Newton was quick to point out in his 1704 book Opticks, that "Light is never known to follow crooked passages nor to bend into the shadow". This concept is consistent with the particle theory, which proposes that light particles must always travel in straight lines. If the particles encounter the edge of a barrier, then they will cast a shadow because the particles not blocked by the barrier continue on in a straight line and cannot spread out behind the edge. On a macroscopic scale, this observation is almost correct, but it does not agree with the results obtained from light diffraction experiments on a much smaller scale. When light is passed through a narrow slit, the beam spreads and becomes wider than expected. This fundamentally important observation lends a significant amount of credibility to the wave theory of light. Like waves in water, light waves encountering the edge of an object appear to bend around the edge and into its geometric shadow, which is a region that is not directly illuminated by the light beam. This behavior is analogous to water waves that wrap around the end of a raft, instead of reflecting away. Almost a hundred years after Newton and Huygens proposed their theories, an English physicist named Thomas Young performed an experiment that strongly supported the wave-like nature of light.

Because he believed that light was composed of waves, Young reasoned that some type of interaction would occur when two light waves met. In order to test this hypothesis, he used a screen containing a single, narrow slit to produce a coherent light beam (containing waves that propagate in phase) from ordinary sunlight. When the sun's rays encounter the slit, they spread out or diffract to produce a single wavefront. If this front is allowed to illuminate a second screen having two closely spaced slits, two additional sources of coherent light, perfectly in step with each other are produced (see Figure 6). Light from each slit traveling to a single point halfway between the two slits should arrive perfectly in step. The resulting waves should reinforce each other to produce a much larger wave. However, if a point on either side of the central point is considered, then light from one slit must travel much farther to reach a second point on the opposite side of the central point. Light from the slit closer to this second point would arrive before light from the distant slit, so the two waves would be out of step with each other, and might cancel each other to produce darkness.

Interactive Tutorial Particle and Wave Diffraction Particles and waves should behave differently when they encounter the edge of an object and form a shadow. This interactive tutorial explores how particles and waves behave when diffracted by an opaque surface. As he suspected, Young discovered that when the light waves from the second set of slits are spread (or diffracted), they meet each other and overlap. In some cases, the overlap combines the two waves exactly in step. However, in other cases, the light waves are combined either slightly or completely out of step with each other. Young found that when the waves met in step, they added together by a process that has come to be termed constructive interference. Waves that meet out of step will cancel each other out, a phenomenon known as destructive interference. In between these two extremes, various degrees of constructive and destructive interference occur to produce waves having a wide spectrum of amplitudes. Young was able to observe the effects of interference on a screen placed at a set distance behind the two slits. After being diffracted, the light that is recombined by interference produces a series of bright and dark fringes along the length of the screen.

Although seemingly important, Young's conclusions were not widely accepted at the time, primarily because of the overwhelming belief in the particle theory. In addition to his observations on light interference, Young postulated that light of different colors was composed of waves having different lengths, a fundamental concept that is widely accepted today. In contrast, the particle theory advocates envisioned that various colors were derived from particles having either different masses or traveling at different speeds. The interference effect is not restricted to light. Waves produced on the surface of a pool or pond will spread in all directions and undergo an identical behavior. Where two waves meet in step, they will add together to make a larger wave by constructive interference. Colliding waves that are out of step will cancel each other via destructive interference and produce a level surface on the water. Even more evidence for a wave-like nature of light was uncovered when the behavior of a light beam between crossed polarizers was carefully examined (Figure 7). Polarizing filters have a unique molecular structure that allows only light having a single orientation to pass through. In other words, a polarizer can be considered a specialized type of molecular Venetian blind having tiny rows of slats that are oriented in a single direction within the polarizing material.

If a beam of light is allowed to impact a polarizer, only light rays oriented parallel to the polarizing direction are able to pass through the polarizer. If a second polarizer is positioned behind the first and oriented in the same direction, then light passing through the first polarizer will also pass through the second. Interactive Tutorial The Double Slit Experiment Explore how light waves diffracted by a twin-slit apparatus can recombine through interference to produce a series of dark and light fringes on a reflective screen. The tutorial enables visitors to adjust the slit distances and alter the resulting interference patterns. However, if the second polarizer is rotated at a small angle, the amount of light passing through will be decreased. When the second polarizer is rotated so the orientation is perpendicular to that of the first polarizer, then none of the light passing through the first polarizer will pass through the second. This effect is easily explained with the wave theory, but no manipulation of the particle theory can explain how light is blocked by the second polarizer. In fact, the particle theory is also not adequate to explain interference and diffraction, effects that would be later found to be manifestations of the same phenomenon.

The effects observed with polarized light were critical to the development of the concept that light consists of transverse waves having components that are perpendicular to the direction of propagation. Each of the transverse components must have a specific orientation direction that enables it to either pass through or to be blocked by a polarizer. Only those waves with a transverse component parallel to the polarizing filter will pass through, and all others will be blocked. By the middle of the 1800s, scientists were becoming increasingly convinced of the wave-like character of light, but there remained one overbearing problem. Exactly what is light? A breakthrough was made when it was discovered by English physicist James Clerk Maxwell that all forms of electromagnetic radiation represent a continuous spectrum, and travel through a vacuum at the same speed: 186,000 miles per second. Maxwell's discovery effectively nailed the coffin of the particle theory and, by the dawn of the twentieth century, it seemed that the basic questions of light and optical theory had finally been answered. A major blow to the wave theory occurred behind the scenes in the late 1880s when scientists first discovered that, under certain conditions, light could dislodge electrons from the atoms of several metals (Figure 8).

Although at first only a curious and unexplainable phenomenon, it was quickly discovered that ultraviolet light could relieve atoms of electrons in a wide variety of metals to produce a positive electrical charge. German physicist Philipp Lenard became interested in these observations, which he termed the photoelectric effect. Lenard used a prism to split white light into its component colors, and then selectively focused each color onto a metal plate to expel electrons. What Lenard discovered confused and amazed him. For a specific wavelength of light (blue, for example), the electrons produced a constant potential, or a fixed amount of energy. Decreasing or increasing the amount of light produced a corresponding increase or decrease in the number of electrons liberated, but each still maintained the same energy. In other words, electrons escaping their atomic bonds had energies that were dependent on the wavelength of light, not the intensity. This is contrary to what would be expected from the wave theory. Lenard also discovered a link between wavelength and energy: shorter wavelengths produced electrons having greater amounts of energy. The foundation for a connection between light and atoms was cast in the early 1800s when William Hyde Wollaston discovered that the sun's spectrum was not a continuous band of light, but contained hundreds of missing wavelengths.

Over 500 narrow lines corresponding to missing wavelengths were mapped by German physicist Joseph von Fraunhofer, who assigned letters to the largest gaps. Later, it was discovered that the gaps were produced from absorption of specific wavelengths by atoms in the sun's outer layer. These observations were some of the first links between atoms and light, although the fundamental impact was not understood at the time. In 1905, Albert Einstein postulated that light might actually have some particle characteristics, regardless of the overwhelming evidence for a wave-like nature. In developing his quantum theory, Einstein suggested mathematically that electrons attached to atoms in a metal can absorb a specific quantity of light (first termed a quantum, but later changed to a photon) and thus have the energy to escape. He also speculated that if the energy of a photon were inversely proportional to the wavelength, then shorter wavelengths would produce electrons having higher energies, a hypothesis borne in fact from the results of Lenard's research. Einstein's theory was solidified in the 1920s by the experiments of American physicist Arthur H. Compton, who demonstrated that photons had momentum, a necessary requisite to support the theory that matter and energy are interchangeable.

About the same time, French scientist Louis-Victor de Broglie proposed that all matter and radiation have properties that resemble both a particle and a wave. De Broglie, following Max Planck's lead, extrapolated Einstein's famous formula relating mass and energy to include Planck's constant: E = mc2 = hn where E is the energy of a particle, m the mass, c is the speed of light, h is Planck's constant, and n is the frequency. De Broglie's work, which relates the frequency of a wave to the energy and mass of a particle, was fundamental in the development of a new field that would ultimately be utilized to explain both the wave-like and particle-like nature of light. Quantum mechanics was born from the research of Einstein, Planck, de Broglie, Neils Bohr, Erwin Schr�dinger, and others who attempted to explain how electromagnetic radiation can display what has now been termed duality, or both particle-like and wave-like behavior. At times light behaves as a particle, and at other times as a wave.

This complementary, or dual, role for the behavior of light can be employed to describe all of the known characteristics that have been observed experimentally, ranging from refraction, reflection, interference, and diffraction, to the results with polarized light and the photoelectric effect. Combined, the properties of light work together and allow us to observe the beauty of the universe. Contributing Authors Kenneth R. Spring - Scientific Consultant, Lusby, Maryland, 20657. Michael W. Davidson - National High Magnetic Field Laboratory, 1800 East Paul Dirac Dr., The Florida State University, Tallahassee, Florida, 32310. National Academic Quiz Tournaments, LLC School You Gotta Know These Classes of Particles Physics and chemistry are often difficult subjects for quiz bowl teams if those classes are taught during the junior or senior years since many players will not have completed them before encountering the subject matter at tournaments. One high-yield area of physics to study is the nomenclature of various groups of particles. Some conventions: The mass of particles is usually given in mega-electronvolts (MeV), where an electron-volt is the energy acquired by an electron when it crosses a potential difference of one volt.

The energies are converted to masses by Einstein's famous equation E = mc2, where c is the speed of light. Charges are given in terms of the fundamental electric charge (the absolute value of the charge on an electron). Every kind of particle also has a corresponding anti-particle made of anti-matter; when it is said that there "six leptons," anti-particles are not counted (so, in some sense, there are twelve). Anti-particles have the same mass, but the opposite charge, of the original. There are no particles with negative mass. Note that in some rare situations, a particle can be its own anti-particle. Leptons are one of the classes of "fundamental particles" (meaning that they cannot be broken down into smaller particles). There are six "flavors" of leptons: the electron, the muon, the tauon, the electron neutrino (usually just called "the" neutrino), the muon neutrino, and the tauon neutrino. The three neutrinos are neutral (and were once thought to be massless), while the other three have a charge of -1.

All neutrinos are fermions and the total number of leptons is conserved (counting regular leptons as +1 particle and anti-leptons as -1 particle). The word "lepton" comes from the Greek for "light" (as in "not heavy"), even though the muon and tauon are fairly massive. Quarks are another class of fundamental particle. They also come in six flavors: up, down, charm, strange, top (sometimes, "truth"), and bottom (sometimes, "beauty"). The up, charm, and top quarks have a charge of +2/3, while the down, strange, and bottom have a charge of -1/3. All quarks are fermions and they combine in pairs to form mesons and in triples to form baryons. The enormous mass of the top quark (178 GeV) made it difficult to create in particle accelerators, but its discovery in 1995 confirmed an essential element of the "Standard Model" of particle physics. The name "quark" comes from the line "Three quarks for Muster Mark" in Finnegans Wake that appealed to Murray Gell-Mann.

The study of quarks (and the strong nuclear force) is quantum chromodynamics. Baryons are composite (i.e., non-fundamental) particles made from three quarks. The most common examples are the proton (two up quarks and one down quark) and the neutron (two down quarks and one up). All baryons are fermions. Quarks possess a characteristic called "color" (which has nothing to do with visual color) which can be either red, green, or blue (which are arbitrary names). A baryon must have one quark of each color so that the "total color" (analogous to mixing red, green, and blue light) is colorless (i.e., "white"). The word "baryon" comes from the Greek for "heavy." The total number of baryons is conserved (again, counting anti-baryons as -1). Mesons are composite particles generally made from a quark and an anti-quark. There are dozens of examples including the pion, kaon, J/Psi, Rho, and D. All mesons are bosons.

The quark and anti-quark must have the same color (such as red and anti-red) so that the resulting meson is colorless (or "white"). It is also possible to make mesons out of two (or more) quarks and the same number of anti-quarks, but this kind of particle (a "tetraquark") is rare, both in nature and in quiz bowl. Fermions are particles with half-integral spin. Spin is a form of "intrinsic angular momentum" which is possessed by particles as if they were spinning around their axis (but, in fact, they aren't). The values cited for spin are not (usually) the real magnitude of that angular momentum, but the component of the angular momentum along one axis. Quantum mechanics restricts that component to being n/2 times Planck's constant divided by 2 pi for some integer n. If n is even, this results in "integral" spin, if it is odd, it results in "half-integral" spin. Note that the exact value of the spin itself is a real number; it's the multiplier of h/2pi that determines whether it is "integral" or not.

The most significant thing about fermions is that they are subject to the Pauli Exclusion Principle: No two fermions can have the same quantum numbers (i.e., same state). The name "fermion" comes from that of the Italian-American physicist Enrico Fermi. Bosons are particles with integral spin. All particles are either bosons or fermions. The spin of a composite particle is determined by the total spin (i.e., the component of its intrinsic angular momentum along one axis) of its particles. For instance, an alpha particle (two protons and two neutrons) has four half-integral spin values. No matter how they are added up, the result will be an integral spin value (try it!), so an alpha particle is a (composite) boson. The Pauli Exclusion Principle does not apply to bosons (in fact, bosons prefer to be in the same quantum state). The name "boson" comes from that of the Indian-American physicist Satyendra Nath Bose. Hadrons are any particles made out of quarks (alternatively, any particle affected by the strong nuclear force). Generally, this means the baryons and the mesons.

All hadrons are colorless (in the sense of the combined color of their constituent quarks). The name "hadron" comes from the Greek for "thick." Gauge bosons (sometimes called "vector bosons") are fundamental bosons that carry the forces of nature. That is, forces result from particles emitting and absorbing gauge bosons. The strong nuclear force is carried by gluons, the weak nuclear force is carried by the W, Z-, and Z+ particles, the electromagnetic force is carried by the photon, and gravity is carried by the (as yet unobserved) graviton. The name comes from the role of "gauge theories" in describing the forces (which are beyond the scope of this article). Gluons are the gauge bosons that carry the strong nuclear force and bind hadrons together. Gluons have no charge and no mass, but do have color (in the sense of quarks). This color cannot be observed directly because the gluons are part of the larger hadron. The name comes from their role in "gluing" quarks together. Partons are an older name that was used for the "internal parts" of hadrons before the discovery and widespread acceptance of the quark model.

Models based on partons are still used but, for the most part, it was determined that partons were quarks and the term is rarely used at the high school level except in historical contexts. 5.2 Light as a particle Light is also a particle. Scientists - PHSC - 108 View Full Document This is the end of the preview. Sign up to access the rest of the document. Unformatted text preview: 5.2 Light as a particle Light is also a particle. Scientists have given it the name photon , and it was listed among the fundamental particles in Table 3.1. To understand more about what we mean by a particle, we go back to Chapter 3, when we talked about an electron as a fundamental particle and a proton as a composite particle. In many ways, when we call something particle-like, we are saying that it can be counted. You could say that you got hit by one electron, or one proton, or three quarks. Another important property of a particle is that it moves in a straight line unless something forces it to change direction. Consider a rock.

It can be thought of as particle-like because if it is near the surface of the Earth, the pull of gravity will cause it to stay in place or fall. Alternatively, if it is far away from any planets or stars, it could travel forever in a straight line until it hit something. An electron moves like a rock traveling in space, but if it happens to be near a proton, it could move in a circle around it because of the electric force between the two (more about this in 5 - 6 Chapter 7). The same phenomenon occurs when the attraction of gravity causes the Earth to orbit around the Sun. Billiard balls on a pool table likewise demonstrate this property when they either move across the table in a straight line or are forced to change direction by bumping the side of the table or another ball. What is some evidence that light is particle-like? Let us get back to the light coming from our laser, as shown in Figure 5.2. As we dim the laser, we notice that the color of the light on the back wall stays the same and the spots remain in the same place. The difference is in the brightness of the spots, which get progressively dimmer.

From this, we learn that the laser is sending less light. If we dim things enough, we could count each particle of light after leaves the laser, hits one of the holes in the wall, changes direction, and strikes the back wall. The light is behaving like a particle. Another example of light acting like a particle is in Figure 5.2b, where each photon enters the spectrometer individually. In a very real sense, light hits your eye one photon at a time. Since this is a lot to take in, lets look at some more examples. A typical light bulb makes photons that hit our eyes. If it is a dim bulb, there are fewer photons. If there is no light at all, there are no photons, and the room will be dark. Darkness is the absence of photons. When you look at a light bulb, the light often looks white. What you are seeing is all of the different visible wavelength photons hitting your eyes. When you shine this same white light into a prism, it breaks the light into different colors. This is light acting as both a particle and a wave.

5 - 7 Ultimately, we say light is both a wave and a particle, and can be described as either because it has the properties of both. In many ways, this is the essence of quantum mechanics, because it has the properties of both.... Chapter 6 Gravity and General Relativity For more than seventy-five years, the theori ChapterOnGeneralRelativity

Question: What is the name given to fundamental particles of light?
Answer: PHOTONS