Antiparticles

9 Key excerpts on "Antiparticles"

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  Superstrings and Other Things

  A Guide to Physics, Second Edition

  • Carlos Calle(Author)
  • 2009(Publication Date)
  • CRC Press

    (Publisher)

  These Antiparticles constitute a new kind of matter, antimatter , identi-cal to ordinary matter except that if the particle has electric charge, its antiparticle has the opposite charge. When a particle encounters its antiparticle, they annihi-late each other, disappearing in a burst of photons in a process called pair annihi-lation (Figure 24.1a). The inverse process, in which high energy photons create a (a) p (6) π – (5) π – (4) p – (7) π + (2) π + (8) Ξ – (3) p p – (I) — Ξ 0 (10) π 0 (12) Л (9) Л (II) (b) FIGURE 24.1 (a) Pair annihilation and (b) pair production. (Courtesy of Brookhaven National Laboratory and Lawrence Berkeley Laboratory.) Elementary Particles 441 positron–electron pair, is also possible (Figure 24.1b). This process is called pair production . Recall that, according to the special theory of relativity, an electron has a rest energy m 0 c 2 . Thus, photons of energy hf equal to at least 2 m 0 c 2 can produce an electron and a positron. THE FUNDAMENTAL FORCES Before we continue delving more deeply into the structure of matter, let us sum-marize what we have learned so far about the known interactions in nature. All the variety of phenomena in the universe, from the explosion of a supernova in another galaxy to the falling of a leaf during an autumn afternoon on earth, from the appear-ance of a dark spot on the atmosphere of a distant star, the explosion of a volcano on Io or the collapse of a distant red giant to the whirr of the wings of a hummingbird, all of these events can be ultimately explained by only four fundamental forces. Everything that happens anywhere in the universe is ultimately controlled by the operation of these four forces. Understanding the properties of these forces—gravity, electromagnetism, and the strong and weak nuclear forces—is, perhaps, the most important task in physics today. Gravity • , the first one of the four forces to be discovered, was formulated by Newton in his universal law of gravitation.

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  Modern Physics

  • Gary N. Felder, Kenny M. Felder(Authors)
  • 2022(Publication Date)
  • Cambridge University Press

    (Publisher)

  The antielectron is called a “positron,” but most Antiparticles have no special name and are just given the prefix “anti.” Antiparticles are written with a bar, e.g. n for an antineutron. Alternatively, a charged antiparticle may be indicated by specifying its charge, such as p − for an antiproton. 2 A few particles such as photons are their own Antiparticles. You could equivalently say that photons have no antiparticle since photons do not annihilate when they meet. (What would they produce?) For technical reasons in quantum field theory, we say instead that the photon is its own antiparticle. We call the particles around us “matter” and their exotic partners that we produce in a lab “antimatter,” but the laws of physics are almost exactly the same for both. So there might be parts of the universe made of what we call antimatter, and scientists there would presumably use the opposite names from what we do. We have good reason to believe, however, that all of the galaxies we can see are made of what we call matter. Conservation Laws and Other Particle Categories We have said several times that the mid-twentieth century was characterized by the discovery of an onerous bestiary of particles and properties. 2 A positron can be written as e + but never as p + since p refers to a proton. 13.1 Forces and Particles 601 Nowadays, atomic particles appear suddenly, out of the blue, doing somersaults. The physics of yesteryear was a bit like ballroom dancing to Mozart, while now it’s more like a fairground with halls of mirrors, labyrinths, target-shooting booths and men hawking phenomena. – Ernesto Sabato, “Física escandaloso” in Uno y el Universo (1945) A lot of these discoveries came from conservation laws. In addition to the three classical conservation laws (energy, momentum, and angular momentum), particle physics introduces new quantities that must also be conserved.

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  Developments in Modern physics

  • Nelson Boli´var(Author)
  • 2023(Publication Date)
  • Arcler Press

    (Publisher)

  • Mesons: These had been initially intermediate-mass particles between the proton and the electron. Furthermore, several enormous mesons have subsequently been discovered, rendering the original description obsolete. Mesons are currently defined as particles whose disintegrated products do not contain baryons (Recami and Rodrigues, 1985). We shall see that mesons are Elementary Particle Physics 91 formed of a pair of quarks and antiquarks. All mesons possess intrinsic spin, or 0, 1, 2, 3, etc. The mass of a meson grows as its spin rises. In Table 4.1, several fundamental particles are listed. Table 4.1. Some of the Basics Fundamental Particles Leptons muon electron neutrinos tauon μ – e – v e , v μ , v τ τ – Hadrons – Mesons pi rho eta omega delta phi π ρ η Ω ∆ φ Baryons neutron proton lambda delta Hyperon Sigma Omega n p λ ∆ Λ Σ Ω Paul Dirac used special relativity and quantum theory to create a relativistic theory of the electron in 1928. The calculations anticipated two energy levels for every electron, which was a surprise outcome of the merger. One is linked to the electron, while the other is linked to a particle that is identical to the electron apart from that it has a +ve charge. The antielectron, or positron, was the name given to this article. It was the earliest forecast of antimatter’s presence. In 1932, the positron was discovered (Castell, 1966; Kopylov and Podgoretsky, 1975). There must be an antiparticle for each particle found in nature. The antiproton is the proton’s antiparticle. It possesses all of the properties of a proton except for the fact that this is negatively charged. The π 0 mesons and the photon are examples of totally neutral particles that have their Antiparticles. Antiparticles are represented by a bar over the particle sign. As a result, p, and n are antiprotons and antineutrons, respectively.

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  Phenomenology of Particle Physics

  • André Rubbia(Author)
  • 2022(Publication Date)
  • Cambridge University Press

    (Publisher)

  1 Introduction and Notation A particle (from the Latin particula, little part) is a minute portion of matter. 1.1 Subatomic Particles When we first observe the Universe, it might appear to us as a very complex object. One of the primary goals of the philosophy of Nature (or simply Physics) is to “reduce” (“simplify”) this picture in order to find out what the most fundamental constituents of matter (i.e., the atoms from the Greek word indivisible ) are and to understand the basic forces by which they interact in the otherwise void space, along the line of thinking of Demokritos 1 who wrote “Nothing exists except atoms and empty space.” In this context, subatomic particles are physical objects smaller than atoms. In particle physics, particles are objects that are localized in space and that are characterized by intrinsic properties. As we will see later, the set of intrinsic properties used to classify particles is chosen from those which behave in a well-defined way under the action of a transformation. As a matter of fact, we would expect some of these properties not to change at all under particular transformation. For instance, we expect some of its properties to be independent of the velocity of the particle or the direction in which it is traveling. This procedure of classification was actually initiated by Eugene Wigner 2 in his seminal paper of 1939 [1]. We will come back to this later. Elementary particles are particles which, according to current theories, are not made of other particles, or we should rather say, whose substructure, if any, is unknown. They are thus considered as point-like objects. Composite particles, on the other hand, are composed of other particles, in general of elementary particles, and are thus extended in space. All subatomic particles are classified according to their properties and given common names such as electrons, muons, taus, protons, neutrons, neutrinos, etc.

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  Third Thoughts

  The Universe We Still Don’t Know

  • Steven Weinberg(Author)
  • 2018(Publication Date)
  • Belknap Press

    (Publisher)

  Neutrinos and antineutrinos are nearly massless particles without electric charge. What Is an Elementary Particle? = 77 of principle, known as “nuclear democracy,” which held that any particle may be considered to be a bound state of any other particles that have the appropriate charge, spin, etc. This view was reflected decades later in a 1975 talk to the German Phys-ical Society by Werner Heisenberg, who reminisced that: This new situation was confirmed again and again in the experiments of the1950’s and 1960’s; many new particles of various lifetimes were discovered and there was no an-swer to the question, “What do these particles consist of?” A proton could be obtained from a neutron and a pion, or from a Lambda hyperon and a kaon, or from two nucleons and one antinucleon, and so on. Could we therefore simply say a proton consists of continuous matter? Such a statement would be neither right nor wrong: There is no difference in principle between elementary particles and compound sys-tems. This is probably the most important experimental re-sult of the last fifty years. 7 Long before Heisenberg reached this rather exaggerated conclusion, a different sort of definition of elementary particle had become widespread. From the perspective of quantum field theory, as developed by Heisenberg, Pauli, Victor Weiss-kopf and others in the period 1926–1934, the basic ingredients of nature are not particles but fields. Particles like the electron and photon are bundles of energy of the electron field and the electromagnetic field, respectively. It is natural to define an el-ementary particle as one whose field appears in the funda-mental field equations—or, as theorists usually formulate these theories, in a quantity known as the Lagrangian of the theory, from which the field equations can be derived. It doesn’t matter 7. Werner Heisenberg, “The Nature of Elementary Particles,” Physics Today 29, no. 3 (March 1976): 33.

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  College Physics

  • Paul Peter Urone, Roger Hinrichs(Authors)
  • 2012(Publication Date)
  • Openstax

    (Publisher)

  In this case, an electron and a positron convert all their mass into two identical energy rays, which move away in opposite directions to keep total momentum zero as it was before. Similar annihilations occur for other combinations of a particle with its antiparticle, sometimes producing more particles while obeying all conservation laws. Chapter 33 | Particle Physics 1327 Table 33.2 Selected Particle Characteristics [4] Category Particle name Symbol Antiparticle Rest mass (MeV / c 2 ) B L e L μ L τ S Lifetime [5] (s) Gauge Photon γ Self 0 0 0 0 0 0 Stable W W + W − 80.39×10 3 0 0 0 0 0 1.6×10 −25 Bosons Z Z 0 Self 91.19×10 3 0 0 0 0 0 1.32×10 −25 Electron e − e + 0.511 0 ±1 0 0 0 Stable Neutrino (e) ν e v ¯ e 0(7.0eV) [6] 0 ±1 0 0 0 Stable Muon µ − µ + 105.7 0 0 ±1 0 0 2.20×10 −6 Neutrino (µ) v µ v - µ 0( < 0.27) 0 0 ±1 0 0 Stable Tau τ − τ + 1777 0 0 0 ±1 0 2.91×10 −13 Leptons Neutrino (τ) v τ v - τ 0( < 31) 0 0 0 ±1 0 Stable Hadrons (selected) π + π − 139.6 0 0 0 0 0 2.60 × 10 −8 Pion π 0 Self 135.0 0 0 0 0 0 8.4 × 10 −17 K + K − 493.7 0 0 0 0 ±1 1.24 × 10 −8 Kaon K 0 K - 0 497.6 0 0 0 0 ±1 0.90 × 10 −10 Mesons Eta η 0 Self 547.9 0 0 0 0 0 2.53 × 10 −19 (many other mesons known) Proton p p - 938.3 ± 1 0 0 0 0 Stable [7] Neutron n n - 939.6 ± 1 0 0 0 0 882 Lambda Λ 0 Λ - 0 1115.7 ± 1 0 0 0 ∓1 2.63 × 10 −10 Σ + Σ - − 1189.4 ± 1 0 0 0 ∓1 0.80 × 10 −10 Σ 0 Σ - 0 1192.6 ± 1 0 0 0 ∓1 7.4 × 10 −20 Sigma Σ − Σ - + 1197.4 ± 1 0 0 0 ∓1 1.48 × 10 −10 Ξ 0 Ξ - 0 1314.9 ± 1 0 0 0 ∓2 2.90 × 10 −10 Xi Ξ − Ξ + 1321.7 ± 1 0 0 0 ∓2 1.64 × 10 −10 Baryons Omega Ω − Ω + 1672.5 ± 1 0 0 0 ∓3 0.82 × 10 −10 (many other baryons known) All known leptons are listed in the table given above. There are only six leptons (and their Antiparticles), and they seem to be fundamental in that they have no apparent underlying structure. Leptons have no discernible size other than their wavelength, so 4. The lower of the ∓ or ± symbols are the values for Antiparticles.

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  Turning Points in Physics

  A Series of Lectures Given at Oxford University in Trinity Term 1958

  • R.J. Blin-Stoyle, D. ter Haar, K. Mendelssohn, J. De Boer, H. Brinkman, H. B. G. Casimir(Authors)
  • 2016(Publication Date)
  • North Holland

    (Publisher)

  We must then take the conservation of charge and also the quantization of charge as found on elementary particles as basic concepts which are not yet derived from more fun-damental postulates. 165 TOWARDS NEW CONCEPTS [Ch. VI, § 5 5. Conservation of Baryons: Anti-Particles Another quantity that appears to be conserved to a very high degree of precision concerns the number of baryons. A baryon is a neutron or a proton or a hyperon. A hyperon itself is an unstable elementary particle of mass greater than the neutron or proton which decays into various products one and only one of which is always finally a neutron or proton. To every baryon there corresponds an anti-baryon. The concept of anti-particle is due to Dirac and is a very simple one: an anti-particle is the reversal of all the revers-ible attributes of a particle. Reciprocally of course the par-ticle is the anti-particle of the anti-particle. For example an anti-proton has a negative charge whereas the proton has a positive charge, but the two have identical masses. Our conservation theorem is now that the sum of the number of all the baryons in the universe minus the sum of the number of all the anti-baryons is a constant. This concept is called the conservation of baryons, and simply says that single baryons or anti-baryons cannot disappear or appear. For example there seems to be nothing to prevent a proton from decaying spontaneously into, say, a positive electron and a gamma-ray (photon). This reaction would release a very great deal of energy and offhand we should have anticipated that because of this it would take place with very great ease. We in fact know from experiment that the lifetime of the proton against such a spontaneous disinte-gration into a positive electron and a gamma-ray is greater than io 21 years. This process would not conserve baryons, however, since the proton is not replaced in it by another baryon and it demonstrates the extraordinary degree of the conservation theorem in this particular example. 166

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  The Mystery of the Missing Antimatter

  • Helen R. Quinn, Yossi Nir(Authors)
  • 2010(Publication Date)
  • Princeton University Press

    (Publisher)

  The story of antimatter is a real departure from the old concept of persistence of matter. Massive particles appear from nowhere, given suffi- cient energy. They always appear in a paired arrangement, matching a particle with its Dirac-equation equal-mass partner. Such a new discovery needed new language—the new partner particles to the known matter particles were generically called antimatter. Slightly trickier usage evolved with regard to the word antiparticle; the new types of particles were the Antiparticles of their equal-mass partners; but the already known particles are also called the Antiparticles of their newly discovered antimatter doppel- ga ¨ngers. In inventing the term antimatter for the new types of massive particle in Dirac’s equation, physicists, probably without even thinking about it 48 c h a p t e r 4 much, were drastically redefining the word matter. Previously matter was all that had mass; mass being one of the indicators for substance. Now we have two types of substance, two classes of particles with mass: matter particles, the proton, neutron, and electron; and antimatter particles, their Dirac mirrors, the antiproton, antineutron, and antielectron (the first discovered antimatter particle, also called the positron, it would be the only antimatter particle to be given a special name of its own). The world of physics is a little like the land behind the looking glass. Physicists, like the Queen of Hearts playing croquet, feel justified in changing the rules of the game whenever they see a better set of rules. Of course, we are not quite as capricious about it as the queen. In fact, as the history of the Dirac equation showed, we physicists tend to be very stodgy and to hold onto our ideas as long as we can make them work, meanwhile speculating about what the next set of rules might be. But eventually nature forces our hand, by producing a result that is inconsistent with our theory.

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  Physics 1942 – 1962

  Including Presentation Speeches and Laureates' Biographies

  • Sam Stuart(Author)
  • 2013(Publication Date)
  • Elsevier

    (Publisher)

  Physics 1959 EMILIO GINO SEGRÈ OWEN CHAMBERLAIN «for their discovery of the antiproton» This page intentionally left blank Physics 1959 Presentation Speech by Professor E. Hulthén, Chairman of the Nobel Committee for Physics Your Majesties, Your Royal Highnesses, Ladies and Gentlemen. The notion of matter as something built up of very tiny and indivisible atoms is a heritage from classical times. Since, however, experimental re-search in our days has shown that the atoms in their turn are complicated structures, the notion of indivisibility has been transferred to the so-called elementary particles of which the atom is composed, in the hope of there-with having reached the ultimate limit for the division of matter. However, the different kinds of elementary particles showed an alarming tendency to increase in number - something which is at variance with the attractive idea that matter is built up of one or at most two kinds of par-ticles. Among the most successful and noteworthy attempts to interpret this sit-uation is Dirac's theory of particles and Antiparticles, which may be des-ignated, almost, as each other's mirror images. Both kinds of particles are conceived as arising through the formation of pairs and as reciprocally an-nihilating each other. The world in which we find ourselves belongs, by chance, to the one kind of particles, among which sporadically occurring Antiparticles are very quickly destroyed. On account of the mirror symmetry it would be very difficult to decide whether a remote star or galaxy belonged to the one or the other kind of matter. There were probably very few physicists who at first ascribed to this side of Dirac's otherwise very valuable theory any real import until, quite sud-denly and unexpectedly, the first antiparticle, the positive electron, was dis-covered by Anderson in cosmic radiation in the year 1931.

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