elementary particles: Carriers of the Basic Forces elementary particles Carriers of the Basic Forces The elementary particles of matter interact with one another through four distinct types of force : gravitation , electromagnetism, and the forces from strong interactions and weak interactions . A given particle experiences certain of these forces, while it may be immune to others. The gravitational force is experienced by all particles. The electromagnetic force is experienced only by charged particles, such as the electron and muon. The strong nuclear force is responsible for the structure of the nucleus , and only particles made up of quarks participate in the strong nuclear interaction or force. Other particles, including the electron, muon, and the three neutrinos, do not participate in the strong nuclear interactions but only in the weak nuclear interactions associated with particle decay. Each force is carried by an elementary particle. The electromagnetic force, for instance, is mediated by the photon , the basic quantum of electromagnetic radiation. The strong force is mediated by the gluon, the weak force by the W and Z particles , and gravity is thought to be mediated by the graviton. Quantum field theory applied to the understanding of the electromagnetic force is called quantum electrodynamics , and applied to the understanding of strong interactions is called quantum chromodynamics .

In 1979 Sheldon Glashow, Steven Weinberg, and Abdus Salam were awarded the Nobel Prize in Physics for their work in demonstrating that the electromagnetic and weak forces are really manifestations of a single electroweak force. A unified theory that would explain all four forces as manifestations of a single force is being sought. Sections in this article: Elementary Particles Elementary Particles : particle physics is the search for the fundamental building blocks of Nature, a reductionist goal elementary particles should be structureless, resulting in simple interactions One of the primary goals in modern physics is to answer the question " What is the Universe made of? " Often that question reduces to "What is matter and what holds it together?" This continues the line of investigation started by Democritus, Dalton and Rutherford. Modern physics speaks of fundamental building blocks of Nature, where fundamental takes on a reductionist meaning of simple and structureless. Many of the particles we have discussed so far appear simple in their properties. All electrons have the exact same characteristics (mass, charge, etc.), so we call an electron fundamental because they are all non-unique. The search for the origin of matter means the understanding of elementary particles .

And with the advent of holism, the understanding of elementary particles requires an understanding of not only their characteristics, but how they interact and relate to other particles and forces of Nature, the field of physics called particle physics . more advanced technology lead to the discovery of hundreds of new particles, forcing the search for some underlying principles to unite the chain of particles to something simpler The study of particles is also a story of advanced technology begins with the search for the primary constituent. More than 200 subatomic particles have been discovered so far, all detected in sophisticated particle accelerators. However, most are not fundamental, most are composed of other, simpler particles. For example, Rutherford showed that the atom was composed of a nucleus and orbiting electrons. Later physicists showed that the nucleus was composed of neutrons and protons. More recent work has shown that protons and neutrons are composed of quarks.

the two most fundamental types of particles are quarks and leptons the quarks and leptons are divided into 6 flavors corresponding to three generations of matter quarks (and antiquarks) have electric charges in units of 1/3 or 2/3's A quark is any of a group of subatomic particles believed to be among the fundamental constituents of matter. In much the same way that protons and neutrons make up atomic nuclei, these particles themselves are thought to consist of quarks. Quarks constitute all hadrons (baryons and mesons)--i.e., all particles that interact by means of the strong force, the force that binds the components of the nucleus. According to prevailing theory, quarks have mass and exhibit a spin (i.e., type of intrinsic angular momentum corresponding to a rotation around an axis through the particle). Quarks appear to be truly fundamental. They have no apparent structure; that is, they cannot be resolved into something smaller. Quarks always seem to occur in combination with other quarks or antiquarks, never alone. For years physicists have attempted to knock a quark out of a baryon in experiments with particle accelerators to observe it in a free state but have not yet succeeded in doing so.

Throughout the 1960s theoretical physicists, trying to account for the ever-growing number of subatomic particles observed in experiments, considered the possibility that protons and neutrons were composed of smaller units of matter. In 1961 two physicists, Murray Gell-Mann of the United States and Yuval Ne`eman of Israel, proposed a particle classification scheme called the Eightfold Way, based on the mathematical symmetry group SU(3), that described strongly interacting particles in terms of building blocks. In 1964 Gell-Mann introduced the concept of quarks as a physical basis for the scheme, adopting the fanciful term from a passage in James Joyce's novel Finnegans Wake. (The American physicist George Zweig developed a similar theory independently that same year and called his fundamental particles "aces.") Gell-Mann's model provided a simple picture in which all mesons are shown as consisting of a quark and an antiquark and all baryons as composed of three quarks. It postulated the existence of three types of quarks, distinguished by distinctive "flavours." These three quark types are now commonly designated as "up" (u), "down" (d), and "strange" (s).

Each carries a fractional electric charge (i.e., a charge less than that of the electron). The up and down quarks are thought to make up protons and neutrons and are thus the ones observed in ordinary matter. Strange quarks occur as components of K mesons and various other extremely short-lived subatomic particles that were first observed in cosmic rays but that play no part in ordinary matter. Most problems with quarks were resolved by the introduction of the concept of color, as formulated in quantum chromodynamics (QCD). In this theory of strong interactions, developed in 1977, the term color has nothing to do with the colors of the everyday world but rather represents a special quantum property of quarks. The colors red, green, and blue are ascribed to quarks, and their opposites, minus-red, minus-green, and minus-blue, to antiquarks. According to QCD, all combinations of quarks must contain equal mixtures of these imaginary colors so that they will cancel out one another, with the resulting particle having no net color. A baryon, for example, always consists of a combination of one red, one green, and one blue quark.

The property of color in strong interactions plays a role analogous to an electric charge in electromagnetic interactions. Charge implies the exchange of photons between charged particles. Similarly, color involves the exchange of massless particles called gluons among quarks. Just as photons carry electromagnetic force, gluons transmit the forces that bind quarks together. Quarks change their color as they emit and absorb gluons, and the exchange of gluons maintains proper quark color distribution. leptons are a separate class since they do not interact with quarks by the strong force leptons have charges in units of 1 or 0 Leptons are any member of a class of fermions that respond only to electromagnetic, weak, and gravitational forces and do not take part in strong interactions. Like all fermions, leptons have a half-integral spin. (In quantum-mechanical terms, spin constitutes the property of intrinsic angular momentum.) Leptons obey the Pauli exclusion principle, which prohibits any two identical fermions in a given population from occupying the same quantum state. Leptons are said to be fundamental particles; that is, they do not appear to be made up of smaller units of matter. Leptons can either carry one unit of electric charge or be neutral.

The charged leptons are the electrons, muons, and taus. Each of these types has a negative charge and a distinct mass. Electrons, the lightest leptons, have a mass only 0.0005 that of a proton. Muons are heavier, having more than 200 times as much mass as electrons. Taus, in turn, are approximately 3,700 times more massive than electrons. Each charged lepton has an associated neutral partner, or neutrino (i.e., electron-, muon-, and tau-neutrino), that has no electric charge and no significant mass. Moreover, all leptons, including the neutrinos, have antiparticles called antileptons. The mass of the antileptons is identical to that of the leptons, but all of the other properties are reversed. the up and down quark, electron and neutrino (leptons) work together to form normal, everyday matter note that for every quark or lepton there is a corresponding antiparticle. For example, there is an up antiquark, an anti-electron (called a positron) and an anti-neutrino The electron is the lightest stable subatomic particle known.

It carries a negative charge which is considered the basic charge of electricity. An electron is nearly massless. It has a rest mass of 9.1x10-28 gram, which is only 0.0005 the mass of a proton. The electron reacts only by the electromagnetic, weak, and gravitational forces; it does not respond to the short-range strong nuclear force that acts between quarks and binds protons and neutrons in the atomic nucleus. The electron has an antimatter counterpart called the positron. This antiparticle has precisely the same mass and spin, but it carries a positive charge. If it meets an electron, both are annihilated in a burst of energy. Positrons are rare on the Earth, being produced only in high-energy processes (e.g., by cosmic rays) and live only for brief intervals before annihilation by electrons that abound everywhere. The electron was the first subatomic particle discovered. It was identified in 1897 by the British physicist J.J. Thomson during investigations of cathode rays. His discovery of electrons, which he initially called corpuscles, played a pivotal role in revolutionizing knowledge of atomic structure.

Under ordinary conditions, electrons are bound to the positively charged nuclei of atoms by the attraction between opposite electric charges. In a neutral atom the number of electrons is identical to the number of positive charges on the nucleus. Any atom, however, may have more or fewer electrons than positive charges and thus be negatively or positively charged as a whole; these charged atoms are known as ions. Not all electrons are associated with atoms. Some occur in a free state with ions in the form of matter known as plasma. Fundamental Forces : Matter is effected by forces or interactions (the terms are interchangeable) there are four fundamental forces in the Universe: gravitation (between particles with mass) electromagnetic (between particles with charge/magnetism) strong nuclear force (between quarks) weak nuclear force (that changes quark types) The first two you are familiar with, gravity is the attractive force between all matter, electromagnetic force describes the interaction of charged particles and magnetics. Light (photons) is explained by the interaction of electric and magnetic fields. The strong force binds quarks into protons, neutrons and mesons, and holds the nucleus of the atom together despite the repulsive electromagnetic force between protons.

The weak force controls the radioactive decay of atomic nuclei and the reactions between leptons (electrons and neutrinos). Current physics (called quantum field theory) explains the exchange of energy in interactions by the use of force carriers, called bosons. The long range forces have zero mass force carriers, the graviton and the photon. These operate on scales larger than the solar system. Short range forces have very massive force carriers, the W+, W- and Z for the weak force, the gluon for the strong force. These operate on scales the size of atomic nuclei. So, although the strong force has the greatest strength, it also has the shortest range. Bosons (Force Carriers): certain particles play and important role in the transfer of force, the bosons or force carriers the use of virtual particles to carry force resolves the action at a distance problem Bosons are the particles which transmits the different forces between the matter particles, they normally have a whole number spin, 0, 1 or 2. And Fermions which are matter particles they often have spin 1/2. Real particles are the ones you are familiar with, all Fermions are real particles. The Bosons can sometimes be virtual and sometimes real.

Virtual particles are the particles which transmits the force between the particles, e.g. virtual photon carries the electromagnetic force between e.g. electrons. They are called virtual particles because they can't be directly detected, you can't 'see' them so to speak. But their effect can be noticed, by e.g. the actual forces between particles. Baryons and Mesons: the large number of new particles discovered in the 1950's is resolved by quark model quarks are fundamental building blocks to baryons and mesons, coming together as triplets or pairs Quarks combine to form the basic building blocks of matter, baryons and mesons. Baryons are made of three quarks to form the protons and neutrons of atomic nuclei (and also anti-protons and anti-neutrons). Mesons, made of quark pairs, are usually found in cosmic rays. Notice that the quarks all combine to make charges of -1, 0, or +1.

quarks have 1/3 charge and bind through the exchange of gluons of the strong force Thus, our current understanding of the structure of the atom is shown below, the atom contains a nucleus surrounded by a cloud of negatively charged electrons. The nucleus is composed of neutral neutrons and positively charged protons. The opposite charge of the electron and proton binds the atom together with electromagnetic forces. the many particles of atomic nuclei become a simple combination of quarks unlike electric charge, quarks bind by exchanging color charge of three colors, blue, red and green gluons carry color to convert quarks due to their fractional charge nature, quarks cannot exist in isolation the strong force binds quarks like a rubber band force The protons and neutrons are composed of up and down quarks whose fractional charges (2/3 and -1/3) combine to produce the 0 or +1 charge of the proton and neutron. The nucleus is bound together by the nuclear strong force (that overcomes the electromagnetic repulsion of like-charged protons) Quarks in baryons and mesons are bound together by the strong force in the form of the exchange of gluons.

Much like how the electromagnetic force strength is determined by the amount of electric charge, the strong force strength is determined by a new quantity called color charge. Quarks come in three colors, red, blue and green (they are not actually colored, we just describe their color charge in these terms). So, unlike electromagnetic charges which come in two flavors (positive and negative or north and south poles), color charge in quarks comes in three types. And, just to be more confusing, color charge also has its anti-particle nature. So there is anti-red, anti-blue and anti-green. Gluons serve the function of carrying color when they interact with quarks. Baryons and mesons must have a mix of colors such that the result is white. For example, red, blue and green make white. Also red and anti-red make white. There can exist no free quarks, i.e. quarks by themselves. All quarks must be bound to another quark or antiquark by the exchange of gluons. This is called quark confinement. The exchange of gluons produces a color force field, referring to the assignment of color charge to quarks, similar to electric charge.

The color force field is unusual in that separating the quarks makes the force field stronger (unlike electromagnetic or gravity forces which weaken with distance). Energy is needed to overcome the color force field. That energy increases until a new quark or antiquark is formed (energy equals mass, E=mc2). if energy is used to split a quark pair, new quarks are produced, this is how matter was produced when the Universe formed Two new quarks form and bind to the old quarks to make two new mesons. Thus, none of the quarks were at anytime in isolation. Quarks always travel in pairs or triplets. Particles and Forces | Physics For Idiots Particles and Forces 5.5 Are There Really 4? Before I get into the different types of particle there’s a bit more back story you need. All particles have a property called spin. The spin of a particle has a fixed value that depends only on the type of particle. Spin can also have direction, up or down and the particle carrying the spin can have a handedness, left or right. This gives four possible combinations shown below I admit, this looks quite confusing but it can be possibly simplified with the help of your own left and right hands.

If you get your right hand in a grip position with your thumb sticking straight up then your fingers represent the handedness of the particle, or direction of spin, and your thumb represents whether the particle is orientated up or down. For example, if a clock is thrown with its face directed forwards it’s Left-handed. In order to represent its clock hands motion with your gripped fingers and the overall clock flying through the air motion with your thumb you have to use your left hand. Types of Particle Throughout the whole of the known universe there are only 2 types of particle. Particles that make up matter, and particle that carry force. They are the only 2 types found so far. Now you may be thinking that, yes there may only be two categories but I bet they’re filled with hundreds of different subgroups and types. Thankfully this isn’t the case, particles follow specific rules and once you known them everything gets a lot easier. The two groups are called Fermions and Bosons Fermions Fermions are all particles that make up matter. The name comes from the fact that all particles of matter follow a certain set of laws called Fermi-Dirac Statistics, developed by Enrico Fermi and Paul Dirac in 1926.

All fermions in existence possess half integer spin i.e. 1/2, 3/2, 5/2 etc, for example every electron in the universe possesses a spin of 1/2. Fermions also obey the Pauli exclusion principle. This sounds complicated but it’s relatively simple to describe. What it means is that only certain combinations of matter can exist in the same space, more specifically it states that No two identical fermions may occupy the same quantum state simultaneously For example take Helium. It’s got a lowest energy shell for the electrons. You can put one electron in easy, however the Pauli Exclusion principle says 2 electrons can’t occupy the same quantum state so the second one has to have the opposite spin. This then allows the 2 electrons because spin is part of the quantum state of the electron, so the two electrons are occupying different quantum states. The spin however can only be one of two things, up or down (+1/2 or -1/2). If for example you had a lithium atom, which has three electrons then the third electron can’t fit into the 1st shell. So to fit it in it has to move up to the next shell.

The entire Periodic table is built up from this principle. There are two different types of fermions, Leptons and Quarks. Leptons There are six sub-atomic particles that make up the leptons; the Electron and the Electron Neutrino, the Muon and Muon Neutrino (which are basically heavier versions of the Electron and the Electron Neutrino), and the Tau and the Tau Neutrino (which are heavier versions still). The electron, muon and tau all have charges of -1 whereas all the neutrinos have charges of 0 (1) Quarks Quarks are the other type of matter particle along with the leptons. Like the leptons there are six quarks, grouped in 3 sets of 2, with each successive group basically just a heavier version of the previous. Like the leptons the quarks in each set have a charge difference of 1, but instead of nice whole numbers the charges of quarks come in fractions of e. The six quarks are named Up, Down, Charmed, Strange, Top and Bottom (2) Hadrons, Mesons and Baryons Physicists seem to love their labels and groups.

As soon as you put quarks together in groups then the resultant particles are called Hadrons. But the names and classes don’t stop there. If you make a Hadron out of 2 quarks it’s called a Meson and if you make a Hadron out of 3 quarks it called a Baryon. The reason you get groupings of 2 or 3 quarks is because of their colour. Quarks can be red, green or blue and anti quarks can be anti-red anti-green and anti-blue. The particles aren’t actually coloured (for one thing they’re much smaller than the wavelength of visible light). The “colours” are just labels. Quarks have a property that can take 3 distinct values, so physicists called those values red, green and blue. Quarks exist in groups that have no overall colour charge, so you can get groups that are red+blue+green=white, anti-red+anti-blue+anti-green=white, red+anti-red=white, blue+anti-blue=white or green+anti-green=white, i.e. either 3 quarks or anti quarks together or 1 quark and 1 anti quark together; Baryons and Mesons.

Particles like the proton and neutron are examples of Baryons as they are comprised of 3 quarks, while particles like the and are Mesons as they are made from a quark and an anti quark, however all four of them are types of Hadrons. Bosons Bosons are the particles that carry force. They are characterised by having whole integer spin e.g. -1, 0, 1, and don’t obey the Pauli Exclusion Principle, so you can have loads of them in the same space. Each of the fundamental forces of nature has its own Bosons. For Electromagnetism the force carrier is the Photon. They are sometimes called virtual photons as they only exist for very small intervals of time or space. If an electron gets near another electron it emits a virtual photon which is absorbed by the second electron and lets it know it need to move away. This is a Feynman Diagram, named after the amazing physicist Richard Feynman. Its an easy way of describing or visualising particle interactions. Particles are represented as lines, either straight or wavy, and interactions are depicted as a vertex of the lines. Most of the time the lines will have arrows to show more specifically how the particles are moving.

In the above example two electrons move towards each other, then we have the interactions with the boson of the electromagnetic force, then they move away from each other. For The Strong Nuclear Force the boson is the Gluon. It has zero rest mass and zero charge. Despite there only being one boson for this force it can come in different Colours. For Gravity the boson is theorised to be the Graviton. Its is thought to have zero rest mass and zero charge but has not been discovered yet. The Weak Nuclear Force looks like the odd one out. It has three bosons, the W , W and the Z . None of them are massless like the photon, on average they’re about half the mass of a caffeine molecule. Forces What is stuff made of? Compounds, molecules, atoms, electrons and quarks. At the moment you can go no further. Forces are the same, there are loads of them about but really there just combinations of 4 fundamental forces. This isn’t enough for most physicists (myself included) and so research has been going on for a long time now to try and find just one description of all of them.

James Clerk Maxwell did it with Electricity and Magnetism, so why cant we do it with the rest. Actually we partially have, but more on that later . For now here are the 4 forces. Gravity Gravity is the weakest of all the forces, which seems odd at first. It holds planets together and holds them in their orbits. It is also the longest ranged force mainly because it is always attractive. You can easily overcome gravity just by jumping, that’s how weak it is. Gravity is felt by anything with mass. If it has mass, gravity can act on it. Gravity works via the following law It’s an inverse square law so it gets weaker the further you move away, it also gets stronger for objects of larger mass. Electromagnetism Electromagnetism is 1 trillion, trillion, trillion times stronger than gravity. However unlike gravity which is always attractive, electromagnetism can be both attractive and repulsive. This is because there are 2 types of electromagnetic “matter”, positive charge and negative charge. Electromagnetism follows the following law You can see that its very similar to the law for gravity. It’s inversely proportional to distance and is stronger for objects of larger charge.

It’s a long ranged force, however the mix of positive and negative charge cancel each other so it’s hardly ever felt on large scale, unlike gravity. Weak Nuclear Force Weak Nuclear Force 10 trillion, trillion times stronger than gravity. The weak nuclear force is responsible for all three types of nuclear decay; Alpha, Beta or Gamma. Alpha decay is the emission of a helium nucleus from an atom, Beta decay is when an electron or positron is emitted from an atom, and Gamma decay is the emission of a high energy photon from an atom. The weak nuclear force is the odd one out of all the forces. Firstly because of its bosons. The weak force has three bosons unlike the others which only have one each. The bosons are also unlike the others as they have charge and mass, so much mass in fact that they are heavier that atoms of Rubidium! This is why the force only acts over small distances. In one type of decay an Up quark can emits a W , that’s a particle emitting something that is 40,000 times heavier! The weak force is also different as it only affects left handed particles or right handed antiparticles with flavour.

Strong Nuclear Force Inside a nucleus you have protons and neutrons. Due to the electromagnetic force however all of the protons in the nucleus are pushing each other apart trying to break free, the thing that holds them together is the Strong Nuclear force. Its 100 times stronger that EM, an affects all particles with colour. The Strong Nuclear Force gets stronger with distance however is a very small ranged force only acting over a range of 10 m Are There Really 4? It turn out that most of the forced seem to be just different aspects of the same thing. Electromagnetism and Weak Nuclear Forces have shown that at high enough energies the two forces are the same, called the Electroweak interaction. Above the unification energy of about 100 GeV or 1015 kelvin, they would merge into a single Electroweak force. Work is currently being done on adding the Strong force and then hopefully gravity. A site to help make science simpler. Search for: Exchange Particles Go Back Feynman Diagrams Feynman diagrams are graphical ways to represent exchange forces . Each point at which lines come together is called a vertex, and at each vertex one may examine the conservation laws which govern particle interactions.

Each vertex must conserve charge, baryon number and lepton number. Developed by Feynman to decribe the interactions in quantum electrodynamics (QED), the diagrams have found use in describing a variety of particle interactions. They are spacetime diagrams, ct vs x. The time axis points upward and the space axis to the right. (Particle physicists often reverse that orientation.) Particles are represented by lines with arrows to denote the direction of their travel, with antiparticles having their arrows reversed. Virtual particles are represented by wavy or broken lines and have no arrows. All electromagnetic interactions can be described with combinations of primitive diagrams like this one. Only lines entering or leaving the diagram represent observable particles. Here two electrons enter, exchange a photon, and then exit. The time and space axes are usually not indicated. The vertical direction indicates the progress of time upward, but the horizontal spacing does not give the distance between the particles. Other electromagnetic process can be represented, as in the examples below. A backward arrow represents the antiparticle, in these cases a positron. Keep in mind that time progresses upward, and that a downward arrow is not a particle progressing downward, but an antiparticle progressing upward (forward in time).

After being introduced for electromagnetic processes, Feynman diagrams were developed for the weak and strong interactions as well. Forms of primitive vertices for these three interactions are Particle interactions can be represented by diagrams with at least two vertices. They can be drawn for protons, neutrons, etc. even though they are composite objects and the interaction can be visualized as being between their constituent quarks. Feynman diagrams for: Particle physics: A reminder of the beauty we know : Nature : Nature Research Physics The most precise measurement so far of the mass of the W boson is reported by researchers in the CDF Collaboration (Aaltonen et al. 1 ) at the Tevatron collider, Fermilab, near Chicago, in Physical Review Letters. The W boson is the particle that carries the weak nuclear force, which is responsible for radioactive β-decay. The particle decays in less than one billion-trillionth of a second, and many of the things that it decays to are hard to detect.

The measurement of its mass is an experimental tour de force, a consistency test for the standard model of particle physics, and a pointer that will aid the discovery of new phenomena at the Large Hadron Collider (LHC) at CERN, Europe's particle-physics laboratory near Geneva, Switzerland. The 'standard model' is the rather prosaic name given to the collection of quantum field theories that describes the Universe at the shortest distances and highest energies. You will often hear particle physicists hoping for physics 'beyond the standard model' because an anomalous experimental result might help to resolve some of the issues left unexplained by the model. These problems — including why the Universe contains more matter than antimatter, and where gravity fits in — are discussed so often that it is easy to forget how accurate and economical the standard model is. In the model, the fundamental forces are carried by particles known as gauge bosons. The photon, for example, is the gauge boson that carries the electromagnetic force, whereas the W and Z bosons mediate the weak force. Bosons are particles that have an integer quantum of intrinsic angular momentum, otherwise known as spin. 'Gauge' means that the particles are generated by mathematical symmetries.

Symmetry is a central feature in the standard model. For the model to work in its most symmetric form, the W and Z bosons should, like the photon, have zero mass. This is not the case. Yet without symmetry, the standard model loses all predictive power — a finding 2 that earned Gerardus 't Hooft and Martinus Veltman the Nobel Prize in Physics in 1999. The way out of this impasse could be supplied by the discovery of the Higgs boson. Its existence would indicate that symmetry remains in the standard model at high energies, even though it is manifestly broken in the low-energy Universe in which we live. The non-zero mass of the W boson is intimately connected with the Higgs boson, with the origins of mass in general and with our understanding of physics in terms of quantum field theories. It is a quantity well worth measuring precisely — just as the Tevatron experimenters 1 have done. This was a hugely challenging analysis. The Tevatron ( Fig.

1 ), which recently ceased operation, was a high-energy collider that stored protons and antiprotons, accelerated them to high energies — almost 1 teraelectronvolt (1012 eV) — and forced them into head-on collisions. The energy and frequency of the collisions were sufficient to produce large numbers of W bosons. W bosons decay rapidly, and their decay products could generally be detected in the Collider Detector at Fermilab (CDF), or in the rival detector D0, which is also located at the Tevatron. In the CDF analysis, the researchers used 1,094,834 W-boson decays to measure the W-boson mass. Figure 1: The Tevatron collider. Aaltonen et al. 1 have used the Tevatron — a 6.3-kilometre-long circular particle accelerator and collider — at Fermilab in Batavia, Illinois, to measure the mass of the W boson to high precision. subatomic particle - Elementary particles | physics | Britannica.com Elementary particles CP violation Electrons and quarks contain no discernible structure; they cannot be reduced or separated into smaller components.

It is therefore reasonable to call them “elementary” particles, a name that in the past was mistakenly given to particles such as the proton, which is in fact a complex particle that contains quarks. The term subatomic particle refers both to the true elementary particles, such as quarks and electrons, and to the larger particles that quarks form. Although both are elementary particles, electrons and quarks differ in several respects. Whereas quarks together form nucleons within the atomic nucleus, the electrons generally circulate toward the periphery of atoms. Indeed, electrons are regarded as distinct from quarks and are classified in a separate group of elementary particles called leptons . There are several types of lepton, just as there are several types of quark (see below Quarks and antiquarks ). Only two types of quark are needed to form protons and neutrons, however, and these, together with the electron and one other elementary particle, are all the building blocks that are necessary to build the everyday world. The last particle required is an electrically neutral particle called the neutrino . Neutrinos do not exist within atoms in the sense that electrons do, but they play a crucial role in certain types of radioactive decay.

In a basic process of one type of radioactivity , known as beta decay , a neutron changes into a proton. In making this change, the neutron acquires one unit of positive charge. To keep the overall charge in the beta-decay process constant and thereby conform to the fundamental physical law of charge conservation , the neutron must emit a negatively charged electron. In addition, the neutron also emits a neutrino (strictly speaking, an antineutrino), which has little or no mass and no electric charge. Beta decays are important in the transitions that occur when unstable atomic nuclei change to become more stable, and for this reason neutrinos are a necessary component in establishing the nature of matter. Similar Topics spin The neutrino, like the electron, is classified as a lepton. Thus, it seems at first sight that only four kinds of elementary particles—two quarks and two leptons—should exist. In the 1930s, however, long before the concept of quarks was established, it became clear that matter is more complicated. Spin The concept of quantization led during the 1920s to the development of quantum mechanics , which appeared to provide physicists with the correct method of calculating the structure of the atom.

In his model Niels Bohr had postulated that the electrons in the atom move only in orbits in which the angular momentum (angular velocity multiplied by mass) has certain fixed values. Each of these allowed values is characterized by a quantum number that can have only integer values. In the full quantum mechanical treatment of the structure of the atom , developed in the 1920s, three quantum numbers relating to angular momentum arise because there are three independent variable parameters in the equation describing the motion of atomic electrons. In 1925, however, two Dutch physicists, Samuel Goudsmit and George Uhlenbeck , realized that, in order to explain fully the spectra of light emitted by the atoms of alkali metals, such as sodium, which have one outer valence electron beyond the main core, there must be a fourth quantum number that can take only two values, −1/2 and +1/2. Goudsmit and Uhlenbeck proposed that this quantum number refers to an internal angular momentum, or spin , that the electrons possess. This implies that the electrons, in effect, behave like spinning electric charges. Each therefore creates a magnetic field and has its own magnetic moment .

The internal magnet of an atomic electron orients itself in one of two directions with respect to the magnetic field created by the rest of the atom. It is either parallel or antiparallel; hence, there are two quantized states—and two possible values of the associated spin quantum number . Britannica Stories Cheetahs Face Extinction Risk The concept of spin is now recognized as an intrinsic property of all subatomic particles. Indeed, spin is one of the key criteria used to classify particles into two main groups: fermions, with half-integer values of spin (1/2, 3/2,…), and bosons , with integer values of spin (0, 1, 2,…). In the Standard Model all of the “matter” particles (quarks and leptons) are fermions, whereas “force” particles such as photons are bosons. These two classes of particles have different symmetry properties that affect their behaviour. Antiparticles Space Objects: Fact or Fiction Two years after the work of Goudsmit and Uhlenbeck, the English theorist P.A.M. Dirac provided a sound theoretical background for the concept of electron spin .

In order to describe the behaviour of an electron in an electromagnetic field, Dirac introduced the German-born physicist Albert Einstein ’s theory of special relativity into quantum mechanics. Dirac’s relativistic theory showed that the electron must have spin and a magnetic moment, but it also made what seemed a strange prediction. The basic equation describing the allowed energies for an electron would admit two solutions, one positive and one negative. The positive solution apparently described normal electrons. The negative solution was more of a mystery; it seemed to describe electrons with positive rather than negative charge. The mystery was resolved in 1932, when Carl Anderson , an American physicist, discovered the particle called the positron . Positrons are very much like electrons: they have the same mass and the same spin, but they have opposite electric charge. Positrons, then, are the particles predicted by Dirac’s theory, and they were the first of the so-called antiparticles to be discovered. Dirac’s theory, in fact, applies to any subatomic particle with spin 1/2; therefore, all spin-1/2 particles should have corresponding antiparticles. Matter cannot be built from both particles and antiparticles, however.

When a particle meets its appropriate antiparticle, the two disappear in an act of mutual destruction known as annihilation . Atoms can exist only because there is an excess of electrons, protons, and neutrons in the everyday world, with no corresponding positrons, antiprotons, and antineutrons. Positrons do occur naturally, however, which is how Anderson discovered their existence. High-energy subatomic particles in the form of cosmic rays continually rain down on the Earth’s atmosphere from outer space, colliding with atomic nuclei and generating showers of particles that cascade toward the ground. In these showers the enormous energy of the incoming cosmic ray is converted to matter, in accordance with Einstein’s theory of special relativity , which states that E = mc2, where E is energy, m is mass, and c is the velocity of light. Among the particles created are pairs of electrons and positrons. The positrons survive for a tiny fraction of a second until they come close enough to electrons to annihilate . The total mass of each electron-positron pair is then converted to energy in the form of gamma-ray photons.

Connect with Britannica Facebook Twitter YouTube Instagram Pinterest Using particle accelerators , physicists can mimic the action of cosmic rays and create collisions at high energy (see the figure ). In 1955 a team led by the Italian-born scientist Emilio Segrè and the American Owen Chamberlain found the first evidence for the existence of antiprotons in collisions of high-energy protons produced by the Bevatron , an accelerator at what is now the Lawrence Berkeley National Laboratory in California. Shortly afterward, a different team working on the same accelerator discovered the antineutron . Since the 1960s physicists have discovered that protons and neutrons consist of quarks with spin 1/2 and that antiprotons and antineutrons consist of antiquarks. Neutrinos too have spin 1/2 and therefore have corresponding antiparticles known as antineutrinos. Indeed, it is an antineutrino , rather than a neutrino, that emerges when a neutron changes by beta decay into a proton. This reflects an empirical law regarding the production and decay of quarks and leptons: in any interaction the total numbers of quarks and leptons seem always to remain constant.

Thus, the appearance of a lepton —the electron—in the decay of a neutron must be balanced by the simultaneous appearance of an antilepton, in this case the antineutrino. Britannica Lists & Quizzes Editor Picks: Exploring 10 Types of Basketball Movies In addition to such familiar particles as the proton, neutron, and electron, studies have slowly revealed the existence of more than 200 other subatomic particles. These “extra” particles do not appear in the low-energy environment of everyday human experience; they emerge only at the higher energies found in cosmic rays or particle accelerators. Moreover, they immediately decay to the more-familiar particles after brief lifetimes of only fractions of a second. The variety and behaviour of these extra particles initially bewildered scientists but have since come to be understood in terms of the quarks and leptons. In fact, only six quarks, six leptons, and their corresponding antiparticles are necessary to explain the variety and behaviour of all the subatomic particles, including those that form normal atomic matter. electromagnetism - What force particle mediates electric fields and magnetic fields? - Physics Stack Exchange What force particle mediates electric fields and magnetic fields?

6 The force carrier for magnetic fields and electric fields are supposedly photons . I don't get it: 1) Wouldn't that mean that a charged particle (e.g. an electron or even a polarized H2O molecule) would constantly be losing endergy from sending out photons? 2) Wouldn't that mean that an electric field is inseparable from a magnetic field, as photons have both - and that one can't have one without the other? 3) Would it be possible, then, to determine the wavelength of magnetic-field-mediating photons? If so, what is the wavelength - is it random or constant? 4) How can a photon (which has momentum) from one electrically charged particle to an oppositely charged particle cause these particles to be pulled toward each other - or how can a magnetic field cause an electrically charged moving particle to experience a force perpendicular to the source of the magnetic field if a particle with a non-zero mass moving between the two is the mediator of that force? If "virtual photons" are involved, please explain why they work differently from regular photons. Answer to 2.

Yes indeed both are one and the same thing, it just depends on what frame of reference you are looking from, one field changes into another as your frame changes accelerations. – Rijul Gupta Jan 18 '14 at 21:41 Related : physics.stackexchange.com/q/82405 – Rijul Gupta Jan 18 '14 at 21:43 What force particle mediates electric fields and magnetic fields? Photons, as you have suggested. 1) Wouldn't that mean that a charged particle (e.g. an electron or even a polarized H2O molecule) would constantly be losing energy from sending out photons? You must describe this process in a quantum field theory . Virtual photons emitted by charged particles are reabsorbed in a time consistent with uncertainty principle . Hence over some finite amount of time, energy is conserved. 2) Wouldn't that mean that an electric field is inseparable from a magnetic field, as photons have both - and that one can't have one without the other? You can already show this in classical electromagnetism - see Maxwell's equations . 3) Would it be possible, then, to determine the wavelength of magnetic-field-mediating photons?

If so, what is the wavelength - is it random or constant? The wavelength of a photon is related to it's energy, which is again related to the uncertainty principle. The longer the time borrowed from the vacuum, the lower the energy of the photon, so it has a longer wavelength. Hence the wavelength of virtual photons at large distances from the EM source is much longer than at short distances. 4) How can a photon (which has momentum) from one electrically charged particle to an oppositely charged particle cause these particles to be pulled toward each other - or how can a magnetic field cause an electrically charged moving particle to experience a force perpendicular to the source of the magnetic field if a particle with a non-zero mass moving between the two is the mediator of that force? This become less intuitive depending on your background. Richard Feynman introduced a trick which offers a way to imagine the process. Imagine the photon is emitted between opposite-charge particles in the future and travels 'backward in time'- therefore its momentum minus minus what it really is. This is explained in good detail here .

If "virtual photons" are involved, please explain why they work differently from regular photons Unlike 'real' photons (which have transverse polarisation), virtual photons have both transverse and longitudinal polarisations. The energy momentum four vector of the virtual photons, and generally all virtual particles, is not necessarily 0: virtual particles are off mass shell . This means that virtual photons may have non-zero mass - which means that they also have a longitudinal polarisation state. It is important to consider the extra polarisation in your calculations. Glossary | Elementary-Particle Physics | The National Academies Press Cancel Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages. Glossary . Accelerator. A device that increases the energy of charged particles such as electrons and protons. AGS. The Alternating Gradient Synchrotron, a 33-GeV proton accel- erator at the Brookhaven National Laboratory. Annihilatior'. See Antiparticle. Antimatter.

Matter composed of antiparticles, i.e., antiprotons, antineutrons, antielectrons, instead of, i.e., the ordinary protons, neutrons, electrons. Antiparticle. Each particle has a partner, called an antiparticle, which is identical except that all chargelike properties (electric charge, strangeness, charm. for example) are opposite to those of the particle. When a particle and its antiparticle meet, these properties cancel out in an explosive process called annihilation. The particle and antiparticle can then disappear and other particles be produced. Antiprotorl. The antiparticle partner of the proton. Astrophysics. Physics applied to astronomy and astronomical phe- nomena such as the evolution of stars and the formation of galaxies. Asymptotic freedom. The concept that the strong force between quarks gets weaker as the quarks get close together. Atom. The smallest unit of a chemical element, approximately 1/100.000,000 centimeter in size, consisting of a nucleus surrounded by electrons. Bar~'o''. A type of hadron.

The baryon family includes the proton, 213 214 GLOSSARY neutron, and those other particles whose eventual decay products include the proton. Baryons are composed of three-quark combina- tions. Beam. A stream of particles produced by an accelerator. Beauty . See Bottom. BEPC. A circular electron-positron collider with a total energy up to 6 GeV and high luminosity, under construction near Beijing, China. Beta decay. The decay of a particle or nucleus by the emission of an electron or positron through the weak interaction. Bevatron. A circular accelerator at the Lawrence Berkeley Labora- tory, Berkeley, California; previously used to accelerate protons up to 6 GeV and now part of a complex for accelerating nuclei. BNL. Brookhaven National Laboratory. Bottom. The distinguishing characteristic of the fifth type of quark, also called the b quark or beauty quark. Each quark is characterized by a number of properties including familiar ones like mass and electric charge, and less familiar ones that were arbitrarily given names like bottom and charm. Broken symmetry. The failure of a symmetry principle owing to the presence of an additional force or phenomenon. Bubble chamber.

A particle detector in which the paths of charged particles are revealed by a trail of bubbles produced by the particles as they traverse a superheated liquid. Hydrogen, deuterium, helium, neon, propane, and Freon liquids have been used for this purpose. Calorimeter. A particle detector in which the total energy carried by a particle or group of particles is measured. Cerenko`, counter. A detector of Cerenkov radiation, which is elec- tromagnetic radiation emitted by a charged particle when it passes through matter at a velocity exceeding that of light in that material. CERN. The European Center for Nuclear Research, located near Geneva, Switzerland, and supported by most of the nations of Western Europe. CESR. The Cornell Electron Storage Ring, an electron-positron collider with a maximum total energy of 16 GeV located at Cornell University. Charm. The distinguishing characteristic of the fourth type of quark, also called the c quark. Each quark is characterized by a number of properties, including familiar ones like mass and electric charge, and less familiar ones that were arbitrarily given names like charm and bottom. Charmonium.

The family of hadronic particles composed of a charm quark and an anticharm quark. GLOSSAR Y 2 15 Circular accelerator. An accelerator in which the particles move around a circle many times. being accelerated further in each revolution around the circle. Collider. When a high-energy particle collides with a stationary target' a large portion of the energy resides in the continuing forward motion. Only a small portion of the energy is available for creating new particles. In a collider. collisions take place between high- energy particles that are moving toward each other. In such an arrangement, most of the energy. is available for creating new particles. Colliding-bec~m ac c elevator. See Collider. Color. A property of quarks and gluons. analogous to electric charge, which describes how the strong force acts on a quart; or gluon. Conserl atiO'I lint'. A physical law that states that some quantity or property cannot be changed in a reaction. The law of conservation of energy states that the total energy cannot change in a reaction. Cosmic rays. Energetic particles such as protons that come from outside the Earth's atmosphere. Cosmolo<t,,!.

The parts of astrophysics and astronomy having to do with the large-scale behavior of the universe and with its origin. CP violation. An experimentally discovered, but not understood, phenomenon in the decay of neutral K mesons that violates some previously held ideas about the connection of particles to antiparti- cles and about time reversal. Cryogenics. The science and technology of producing and using very low temperatures. even approaching absolute zero temperature. DES Y. Deutsches Electronen Synchrotron, the laboratory in Ham- burg, Federal Republic of Germany, and its 6-GeV circular electron accelerator. Deuterium. Heavy hydrogen, the nucleus of which contains one proton and one neutron. DORIS. An electron-positron collider at DESY with a maximum energy of about 10 GeV. Dozen. The distinguishing characteristic of one of the two lightest quarks, also called the d quark. The other light quark is the up or u quark. Drift chamber. A particle detector in which the paths of charged particles produce tracks of ionized gas. Electrical signals from those tracks are detected and recorded, allowing the reconstruction of the particle paths. Elec tromagnetic fore e or interaction.

The long-range force and inter- action associated with the electric and magnetic properties of 216 GLOSSARY . particles. This force is intermediate in strength between the weak and strong force. The carrier of the electromagnetic force is the photon. Electron. An elementary particle with a unit negative electrical charge and a mass 1/1840 that of the proton. Electrons surround an atom's positively charged nucleus and determine the atom's chemi- cal properties. Electrons are members of the lepton family. Electron volt. The amount of energy of motion acquired by an electron accelerated by an electric potential of one volt: MeV, million electron volts; GeV, billion electron volts; TeV, trillion electron volts. Electroweak force or interaction. The force and interaction that represents the unification of the electromagnetic force and the weak force. Elementary particle. A particle (piece of matter) that has no other kinds of particles inside of it and no subparts that can be identified. Hence the simplest kind of matter. Elemerztary-particle physics. The area of basic science whose goal is to determine and understand the structure and forces of the most basic constituents of matter and energy. Fermilab.

Fermi National Accelerator Laboratory. Flavor. A general name for the various kinds of quarks, such as up. down, and strange. Also sometimes applied to the various kinds of leptons. Gamma rays. A term used for the energetic photons that are emitted in the decay of unstable particles and nuclei. Gauge theory. A type of general theory of forces, modeled on the immensely successful modern theory of electromagnetism. Generation. The classification of the leptons and quarks into families according to a mass progression. The first generation consists of the electron and its neutrino and of the up and down quarks. The second generation consists of the muon and its neutrino and of the charm and strange quarks. The third generation consists of the tau and its neutrino and of the bottom and expected top quarks. GeV. (Giga electron volt) A unit of energy equal to one billion (109) electron volts. Gluon. A massless particle that carries the strong force. Grand unified theory (GUT,. A hoped-for unification of the electro- weak force with the strong force into a single gauge theory. Gravitational force or interaction.

The weakest of the four basic forces and the one responsible for the weight of matter and the motion of the stars and planets. GLOSSARY 217 Gra~'iton. A proposed massless particle that is assumed to carry the gravitational force. Hadron. A subnuclear. but not elementary' particle composed of quarks. The hadron family of particles consists of baryons and mesons. These particles all have the capability of interacting with each other via the strong force. HERA. An electron-proton circular collider being constructed at the DESY laboratory in the Federal Republic of Germany. Higgs mechanism and particle. A mechanism that may explain the origin and value of the mass of all or some of the elementary particles. The mechanism includes a proposed set of particles called -Higgs particles. High-energ~ physics. Another name for elementary-particle physics. This name arises from the high energies required for experiments in this field. IHEP. A 76-GeV circular proton accelerator in Serpukhov, USSR. intermediate vector boson. The general name for the W and Z part- icles that carry the weak force. invariance.

A property of physical laws and equations such that they do not change when changes are made in reference or coordinate systems. J. A particle made of a c quark (see Charm) and an anti-c-quark. It is also called the psi particle and is three times as massive as the proton. Jet. A narrow stream of hadrons produced in a very-high-energy collision. K meson or koon. The next to the lightest meson. It is the lightest hadron that contains a strange quark. KKK. A 12-GeV circular proton accelerator at Tsukuba, Japan. LAMPF. An 800-MeV linear proton accelerator at Los Alamos National Laboratory, used for nuclear and elementary-particle phys- iCS. LEP. A circular electron-positron collider with a maximum design energy of about 200 GeV being constructed at CERN, Switzerland. Lepton. A member of the family of weakly interacting particles, which includes the electron, muon, tau, and their associated neutri- nos and antiparticles. Leptons are not acted on by the strong force but are acted on by the electroweak and gravitational forces. Lifetime.

A measure of how long an unstable particle or nucleus exists on the average before it decays. Linac. An abbreviation for linear accelerator. Linear accelerator. In this type of accelerator, particles travel in a 218 GLOSSARY straight line and gain energy by passing once through a series of electric fields. L``mi'`osit~. A measure of the rate at which particles in a collider interact. The larger the luminosity the greater the rate of interaction. Magnet. A device that produces a magnetic field and thus causes charged particles to move in curved paths. Magnets are essential elements of all circular accelerators and colliders, as well as of many particle detectors. Magnetic mon`'pc~le. A hypothetical particle that would carry a single north or south magnetic pole. All known particles with magnetic properties carry both a north and a south magnetic pole. Mass. The measure of the amount of matter in a particle and an intrinsic property of the particle. Meson. Any strongly interacting particle that is not a baryon. Mesons are composed of quark-antiquark combinations. Me V. (Mega electron volt) A unit of energy equal to one million electron volts. Molec'`le.

A type of matter made up of two or more atoms. Muon. A particle in the lepton family with a mass 207 times that of the electron and having other properties similar to those of the electron. Muons may have positive or negative electric charge. Neutrino. An electrically neutral and massless particle in the lepton family. The only force experienced by neutrinos is the weak force. There are at least three distinct types of neutrinos, one associated with the electron, one with the muon, and one with the taut Neutron. An uncharged baryon with mass slightly greater than that of the proton. The neutron is a strongly interacting particle and a constituent of all atomic nuclei except hydrogen. An isolated neutron decays through the weak interaction to a proton, electron, and antineutrino with a lifetime of about 1000 seconds. Nucleon'. A neutron or a proton. Nucleus. The central core of an atom, made up of neutrons and protons held together by the strong force. Particle. A small piece of matter. An elementary particle is a particle so small that it cannot be further divided it is a fundamental constituent of matter. Particle detector.

A device used to detect particles that pass through it. PEP. An electron-positron circular collider with a maximum energy of 36 GeV, at SLAC. PETRA. An electron-positron circular collider with a maximum en- ergy of 46 GeV, at DESY, Hamburg, Federal Republic of Germany. GLOSSARY 219 Photon. A quantum of electromagnetic energy. A unique massless particle that carries the electromagnetic force. Pion. The lightest meson. Positron. The antiparticle of the electron. Proton. A baryon with a single positive unit of electric charge and a mass approximately 1840 times that of the electron. It is the nucleus of the hydrogen atom and a constituent of all atomic nuclei. PS. A circular proton accelerator with a maximum energy of 28 GeV at CERN, Switzerland. . Psi. A panicle made of a Cal quark (see Charm) and an anti-c -quark and three times as heavy as the proton. It is also called the J particle. Quantum chromodvnamics (QCD).

A theory that describes the strong force among quarks in a manner similar to the description of the electromagnetic force by quantum electrodynamics. Quantum electrodynamics (QEDJ. The theory that describes the electromagnetic interaction in the framework of quantum mechanics. The particle carrying the electromagnetic force is the photon. Quantum mechanics. The mathematical framework for describing the behavior of photons, molecules, atoms, and subatomic particles. According to quantum mechanics, the forces between these particles act through the exchange of discrete units or bundles of energy called quanta. Quarks. The family of elementary particles that make up the hadrons. The quarks are acted on by the strong, electroweak, and gravita- tional forces. Five are known, called up, down, strange, charm, and bottom. A sixth, called top, is expected to exist. Relativistic. The term that describes particles moving with velocities close to the velocity of light. Scattering. When two particles collide, they are said to scatter off each other during the collision. Scintillation counter. A panicle detector in which the passage of a charged particle produces a flash of light called scintillation light.

That light, when detected, records the time at which the particle passed through the counter. SLAC. Stanford Linear Accelerator Center in Stanford, California. Also refers to the electron linear accelerator there that is being rebuilt to have a total energy of 50 GeV. SLC. Stanford Linear Collider, a linear electron-positron collider with an initial total energy of about 100 GeV being constructed at SLAC. SPEAR. A circular electron-positron collider with a total energy of about 8 GeV at SLAC. r 220 GLOSSAR Y SppS. A circular proton-antiproton collider at CERN that uses the SPS accelerator there and has a total energy of about 600 GeV. SPS. A circular proton accelerator with a maximum beam energy of about 400 GeV at CERN. Switzerland. SSC. See S``perc o'~d``cting Sniper Collider. Standard model. A collection of established experimental knowledge and theories in particle physics that summarizes our present picture of that field. It includes the three generations of quarks and leptons. the electroweak theory of the weak and electromagnetic forces.

and the quantum chromodynamic theory of the strong force. It does not include answers to some basic questions such as how to unify the electroweak forces with the strong or gravitational forces. Storc'~,~e ritzy,>. An acceieratorlike machine composed of magnets ar- ranged in a ring used to store circulating particles or to act as a collider. Sometimes a synonym for a collider. Strangeness. The distinguishing characteristic of the third type of quark' also called the s quark. Each quark is characterized by a number of properties. including familiar ones like mass and electric charge and less familiar ones that were arbitrarily given names like charm and strangeness. Strange particle. The name given to panicles thought to contain just one s quark. The remaining quarks in strange particles are either u or d quarks. Strong force or interac lion. . The short-range force and interaction between quarks that Is carried by the gluon. The strong force also dominates the behavior of interacting mesons and baryons and accounts for the strong binding among nucleons. Superconducting magnet. See Superconductivity. Superconducting S`'per Collider (SSC'.

A design for a circular pro- ton-proton collider with a total energy that could be as high as 40 TeV being developed in the United States. Superconductivity. A property of some metals that when they are cooled to a temperature close to absolute zero, their electrical resistance becomes exactly zero. Magnets with superconducting coils can produce large magnetic fields while keeping size and power costs small. Supersymmetrv. A proposed theory of elementary particles in which a property of particles called spin is used. In most theories particles that differ in spin in some ways cannot be related. In this theory such particles can be related through a new proposed symmetry principle called supersymmetry. Symmetry. A general property of many objects and physical systems ClOSSARY 221 whereby the object or system appears unchanged when looked at from different reference frames or coordinate systems. For example, a tennis ball has spherical symmetry because it always looks the same to us no matter how we move around it. Synchrotro'`. A type of circular particle accelerator in which the frequency of acceleration is synchronized with the particle as it makes successive orbits. Synchrotron radiation. intense light or x rays emitted when electrons move in a circular orbit at relativistic speeds.

Target. The material. often liquid hydrogen. that is struck by the beam of high-energy particles in some types of elementary-particle physics experiments. Tail. An elementary particle in the lepton family with a mass 3500 times that of the electron but with similar properties. There are positive and negative tau particles. Tec hail olor. A proposed theory for explaining the masses of particles that postulates the existence of a new force. TeV. (Tera electron volt) A unit of energy equal to one thousand billion ~ 10'') electron volts. Tevatron. A complex of accelerator facilities and beam lines at Fermilab. The main facility is a circular proton accelerator with superconducting magnets (the first large accelerator to use such magnets) with a maximum energy of 1 TeV. An addition is being constructed so that this accelerator can be used as an antiprotoo- proton collider with a total energy of 2 TeV. On completion, this will be the highest-energy collider in the world. Top. The distinguishing characteristic of the expected sixth type of quark, also called the truth quark or t quark.

Each quark is characterized by a number of properties, including familiar ones like mass and electric charge and less familiar ones that were arbitrarily given names like bottom and top. TPC. (Time projection chamber) A particle detector in which the position of the track of ionized gas left by a charged particle is detected by the time it takes for the electrons in the gas to move to the ends of the chamber. TRISTAN. A circular electron-positron collider with a total energy of 60 to 70 GeV under construction at the KKK laboratory in Japan. Unified theories. Theories of forces in which the behavior of different kinds of forces is described by a unified or single set of equations and has a common origin. For example. the electric and magnetic forces are unified in the theory of electromagnetism. 222 GLOSSAR Y UNK. A complex of high-energy circular proton accelerators and colliders under construction at Serpukhov, USSR. Up. The distinguishing property of one of the two lightest quarks also called the u quark. The up and down quarks form the first quark generation. [epsilon. A meson made up of a b quark and an anti-lo-quark.

It Is approximately ten times as massive as the proton. VEPP4. A circular electron-positron collider with a total energy of up to 14 GeV at Novosibirsk, USSR. W. The charged particle that carries the weak force, also called an ~ntermediate-vector boson. Its mass is about 90 times the proton mass. Weak force or interaction. The force and interaction that is much weaker than the strong force, but stronger than gravity. It causes the decay of many particles and nuclei. It is carried by the W and Z particles. X rays. Photons produced when atoms in states of high energy decay to states of lower energy. Z. The neutral particle that carries the weak force, also called an intermediate-vector boson. It is slightly heavier than the W particle, with a mass about 100 times the proton mass. . .

Question: What name is given to the elementary particle that carries the electromagnetic force?
Answer: PHOTON