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Antimatter

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Electron-positron annihilation
simplified

Antimatter is composed of antiparticle "partners" of corresponding "ordinary" matter particles. Antiparticles are generated in particle accelerators (total production of which is a few nanograms), in natural cosmic ray collisions and in some radioactive decay. No macroscopic amount of antimatter has ever been assembled due to the extreme cost and difficulty of production and handling. Theoretically, a particle and its anti-particle have the same mass, but opposite electric charge, and other differences in quantum numbers. Interaction between any particle and its anti-particle leads to mutual annihilation, with the emission of gamma rays, neutrinos, and sometimes less-massive particles and their respective antiparticles. The majority of this annihilation energy is ionizing radiation. Man has bound together a tiny fraction of produced antimatter particles to form antimatter atoms, e.g., a positron and an antiproton to form an atom of antihydrogen. The most complex, artificially produced anti-nuclei is antihelium. Theory allows anti-atoms corresponding to the known chemical elements. The baryon asymmetry of matter and antimatter in the observable universe, to include hypothesized baryogenesis, is one of the great unsolved problems in physics.

Quotes

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  • The interpretation of these tracks as due to protons, or other heavier nuclei, is ruled out on the basis of range and curvature. Protons or heavier nuclei of the observed curvatures could not have ranges as great as those observed. The specific-ionization is close to that for an electron of the same curvature, hence indicating a positively-charged particle comparable in mass and magnitude of charge with an electron.
    • Carl David Anderson, "The Apparent Existence of Easily Deflectable Positives" (1932) Science Vol. 76 (1967) pp. 238–239.
  • In his theory of beta reactivity Fermi introduced a new type of interactions among elementary particles, which today we call "weak interactions". Many new manifestations of weak interactions, which could be interpreted using Fermi's 1933 theory, were found in the following decades. The study of weak interactions has led to surprising discoveries, among which the violation of specular symmetry (known as parity symmetry or P symmetry), and the violation of time reversal symmetry (T symmetry) and of the symmetry between matter and antimatter (CP symmetry).
    • Nicola Cabibbo, "Weak Interactions," Enrico Fermi: His Work and Legacy (2001) ed., Carlo Bernardini, Luisa Bonolis.
  • Dear Millikan,
    I have just received a letter from Rutherford which contains some of Blackett's work which may interest you and Anderson. It is that they have capitulated on the question of positive electrons and agree with Anderson that there are present in large numbers among the tertiary or quartinary (or whatever they are) ionizing particles seen in a Wilson photograph of the cosmic ray effects particles of positive charge and electronic mass. ...I take it that Blackett has collected so many photographs of such tracks as those earlier ones of Anderson that he can no longer resist this devastatingly interesting conclusion. Blackett's photos will come out in P.R.S. (Proceedings of the Royal Society) in March.
    I have a lecture to deliver.
  • The annihilation of positrons with electrons from biological tissues constitutes the basis of Positron Emission Tomography (PET)... widely used in nuclear medicine... [S]ubstances called radiotracers and radiopharmaceuticals are injected into the patient. These are chemical compounds in which one or more atoms have been replaced by short-lived, positron-emitting, radioisotope of elements that are abundant in the body, like Carbon-11... Nitrogen-13... Oxygen-15... and Fluor-18... the latter... for the localization and monitoring of tumors... Since these isotopes are short-lived... they must be produced just before being injected... To do this, the corresponding [common] elements are bombarded with protons... from a small accelerator. ...[I]nside the PET scanner ...a series of detector rings ...record the gamma radiation emitted when the positrons are annihilated inside the body. ...[T]he recorded signals are used to make a series of slices that combine to for a 3-D image. ...[T]hey allow doctors to assess the condition of organs and tissues as they can monitor blood flow and many bodily and metabolic processes, including neuronal transmission.
    • Beatriz Gato-Rivera, Antimatter: What It Is and Why It's Important in Physics and Everyday Life (2021) p. 254.
  • If the Standard Model describes the world successfully, how can there be physics beyond it, such as supersymmetry? There are two reasons. First, the Standard Model does not explain aspects of the study of the large-scale universe, cosmology. For example, the Standard Model cannot explain why the universe is made of matter and not antimatter, nor can it explain what constitutes the dark matter of the universe. Supersymmetry suggests explanations for both of these mysteries. Second, the boundaries of physics have been changing. Now scientists ask not only how the world works (which the Standard Model answers) but why it works that way (which the Standard Model cannot answer). Einstein asked "why" earlier in the twentieth century, but only in the past decade or so have the "why" questions become normal scientific research in particle physics rather than philosophical afterthoughts.
    • Gordon L. Kane, Supersymmetry and Beyond: From the Higgs Boson to the New Physics (2013) p. 10.
  • In later years, the advent of a new elementary particle would scarcely ruffle the intellectual sensibilities of the world's physicists; in 1932, Anderson's announcement of the positron ran into a wall of resistance. If the neutron had resolved many long-standing difficulties of nuclear theory, the positron seemed to complicate matters. It is said that Neils Bohr dismissed Anderson's finding out of hand, and when in the fall of 1932 Millikan discussed the positron in a lecture at the Cavendish, various members of the audience suggested that Anderson had doubtless become tangled in some fundamental interpretive error. But not all of Rutherford's physicists were prepared to ignore Anderson's claims, especially not the resident Cavendish expert on cloud chambers, Patrick M. S. Blackett.
    • Daniel Kevles, The Physicists (1971) p. 233 (in the 1979 edition).
  • The Doctor: Here on Zeta Minor is the boundary between existence as you know it and the other universe which you just don't understand. From the beginning of time it has existed side by side with the known universe. Each is the antithesis of the other. You call it "nothing", a word to cover ignorance. And centuries ago scientists invented another word for it. "Antimatter", they called it. And you, by coming here, have crossed the boundary into that other universe to plunder it. Dangerous.
  • It was fortunate that Alan Guth did his work at the same time that another idea came into fashion, which was the theory that we could understand why the universe contains matter and not antimatter in terms of some asymmetry, some favoritism for matter over antimatter in the early universe; it's no good having a scheme that can inflate the universe to enormous dimension of it's not possible to create matter to fill that large universe.
    • Martin Rees, Third Culture: Beyond the Scientific Revolution (1996) ed. John Brockman.

The Quantum Theory of the Electron (Jan 2, 1928)

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Paul Dirac, Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences. 117 No. 778, pp. 610–624
  • The new quantum mechanics, when applied to the problem of the structure of the atom with point-charge electrons, does not give results in agreement with experiment. The discrepancies consist of "duplexity" phenomena, the observed number of stationary states for an electron in an atom being twice the number given by the theory. ...It appears that the simplest Hamiltonian for a point-charge electron satisfying the requirements of both relativity and the general transformation theory leads to an explanation of all duplexity phenomena without further assumption.
  • The wave equation... refers equally well to an electron with charge e as to one with charge -e. If one considers for definiteness the limiting case of large quantum numbers one would find that some of the solutions of the wave equation are wave packets moving in the way a particle of charge -e would move on the classical theory, while others are wave packets moving in the way a particle of charge e would move classically. ...the electron suddenly changing its charge from -e to e ...has not been observed. The true relativity wave equation should thus be such that its solutions split up into two non-combining sets, referring respectively to the charge -e and the charge e. ...The resulting theory is therefore still only an approximation, but it appears to be good enough to account for all the duplexity phenomena without arbitrary assumptions.

The Positive Electron (Feb 28, 1933)

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Carl David Anderson, Physical Review, Vol. 43.
  • On August 2, 1932, during the course of photographing cosmic-ray tracks produced in a vertical Wilson chamber (magnetic field of 15,000 gauss) designed in the summer of 1930 by Professor R. A. Millikan and the writer, the tracks... seemed... interpretable only on the basis of the existence in this case of a particle carrying a positive charge but having a mass of the same order of magnitude as that normally possessed by a free negative electron.
  • In the course of the next few weeks other photographs were obtained which could be interpreted logically only on the positive-electron basis, and a brief report was then published with due reserve in interpretation in view of the importance and striking nature of the announcement.

Theory of Electrons and Positrons (Dec 12, 1933)

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Paul Dirac, Nobel Prize lecture
  • [O]ur equations allow of two kinds of motion for an electron, only one of which corresponds to what we are familiar with. The other corresponds to electrons with a very peculiar motion such that the faster they move, the less energy they have, and one must put energy into them to bring them to rest.
  • [W]e find from the theory that if we disturb the electron, we may cause a transition from a positive-energy state of motion to a negative-energy one, so that, even if.. all.. electrons in the world.. started.. in positive-energy states, after a time some... would be in negative-energy states. ...[B]ehaviour of these states in an electromagnetic field shows that they correspond to the motion of an electron with a positive charge ...a positron. One might... assume that electrons in negative-energy states are just positrons, but ...observed positrons ...do not have negative energies.
  • We make use of the exclusion principle of Pauli... there can be only one electron in any state of motion. We... make the assumptions that in the world as we know it, nearly all the states of negative energy for the electrons are occupied... any unoccupied negative-energy state, being a departure from uniformity, is observable and is just a positron.
  • An unoccupied negative-energy state, or hole... will have a positive energy, since it is a place where there is a shortage of negative energy. A hole is... just like an ordinary particle, and its identification with the positron... the most reasonable way of getting over the difficulty of... negative energies...
  • On this view the positron is just a mirror-image of the electron, having exactly the same mass and opposite charge. This has already been roughly confirmed by experiment. The positron should also have similar spin properties to the electron, but this has not yet been confirmed...
  • [W]e should expect an ordinary electron, with positive energy, to be able to drop into... and fill up this hole, the energy being liberated in the form of electromagnetic radiation. This would mean... an electron and a positron annihilate one another. The converse... creation of an electron and a positron from electromagnetic radiation, should also be able to take place. Such... appear to have been found experimentally, and are... being more closely investigated...
  • [I]t is probable that negative protons can exist, since as far as the theory is yet definite, there is a complete and perfect symmetry between positive and negative electric charge, and if this symmetry is really fundamental in nature, it must be possible to reverse the charge on any kind of particle. ...[N]egative protons would... be much harder to produce... since a much larger energy would be required, corresponding to... larger mass.
  • We must regard it rather as an accident that the Earth (and presumably the whole solar system), contains a preponderance of negative electrons and positive positrons. It is quite possible that for some of the stars it is the other way about... built up mainly of positrons and negative protons. ...[T]here may be half the stars of each kind. The two... would both show exactly the same spectra... there would be no way of distinguishing them...

Strange Particles (1963)

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by Robert Kemp Adair & ‎Earle Cabell Fowler
  • It seems probable that the interactions between elementary particles can be completely described by symmetry properties and conservation laws and by dimensionless numbers representing interaction strengths. Similarly, we might expect that the elementary particles, as quanta of these interactions, may be described in the same in terms... At the present... however, our description... must also include the mass, and in some cases, the magnetic moment, although in principle these are probably derivable from interaction strengths and symmetries. ...Symmetries usually result in conservation laws. ...Invariance under space inversion results in ...the conservation of parity. Let us also consider invariance under time reversal, and invariance under charge conjugation, the change of particles to antiparticles.
  • These three invariances are not independent. In the framework of local field theory, invariance under proper Lorentz transformation leads to the invariance of all interactions under combined operations CPT, where C is the charge conjugation operator, changing particles to antiparticles, P, the parity space inversion operator, changing to , and T is the time reversal operator, changing t to -t. The equality of the masses and lifetimes of the particles and their antiparticles follows from this theorem.
  • It appears that the strong interactions and electromagnetic interactions are invariant with respect to C, P, and T separately, while the weak interactions do not conserve P or C. All experimental results are consistent with the assumption the T invariance holds true for all interactions; consequently, from the CPT theorem, weak interactions must be invariant under CP. One could not, then, determine if the photographed scene were a scene of particles viewed normally, or a scene of antiparticles projected in a mirror.

Violation of CP invariance, C asymmetry, and Baryon asymmetry of the Universe (Sept 23, 1966)

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by Andrei Sakharov, Pis'maZh. Eksp. Teor. Fiz. 5 (1967) pp. 32-35; [JETP Lett.5 (1967) pp. 24-27, Also S7, pp. 85-88]; Usp. Fiz. Nauk 161, pp. 61-64 (May 1991) [Sov. Phys. Usp. 34 (5) (May, 1991)]
  • The theory of the expanding universe, which presupposes a superdense initial state of matter, apparently excludes the possibility of macroscopic separation of matter from antimatter; it must therefore be assumed that there are no antimatter bodies in nature, i.e., the universe is asymmetrical with respect to the number of particles and antiparticles (С asymmetry). In particular, the absence of antibaryons and the proposed absence of baryonic neutrinos implies a nonzero baryon charge (baryonic asymmetry).
  • We can visualize that neutral spinless maximons (or photons) are produced at t < 0 from contracting matter having an excess of antiquarks, that they pass "one through the other" at the instant t = 0 when the density is infinite, and decay with an excess of quarks when t > 0, realizing total CPT symmetry of the universe. All the phenomena at t < 0 are assumed in this hypothesis to be CPT reflections of the phenomena at t > 0.
  • The strong violation of the baryon charge during the superdense state and the fact that the baryons are stable in practice do not contradict each other. ...The baryon charge is violated if the interaction... is supplemented with a three-boson interaction leading to virtual processes ...we find the decay probability ...The lifetime of the proton turns out to be very large (more than 1050 years), albeit finite.

"The Golden Age of Theoretical Physics" (1972)

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:P.A.M. Dirac’s Scientific Work from 1924 to 1933 by Jagdish Mehra, for the Dirac Festschrift (Sept, 1972) Trieste Symposium & Aspects of Quantum Theory (1972) ed. A. Salam & E. P. Wigner. A source.
  • "I felt that writing this paper on the electron was not so difficult as writing the paper on the physical interpretation."
  • "It was an imperfection of the theory and I didn't see what could be done about it. It was only later that I got the idea of filling up all the negative energy states."
    • quoting Paul Dirac
  • "I felt right at the start that the negative energy electrons would have the same rest mass as the ordinary electrons ...I hoped that there was some lack of symmetry somewhere which would bring in the extra mass for the positively charged ones. I was hoping that in some way the Coulomb interaction might lead to such an extra mass, but I couldn't see how it could be brought about."
    • quoting Paul Dirac
  • "It thus appears that we must abandon the identification of the holes with protons and must find some other interpretation for them. A hole, if there were one [in the world], would be a new kind of particle, unknown to experimental physics, having the same mass and opposite charge to an electron. We may call such a particle an anti-electron. We should not expect to find any of them in Nature, on account of the rapid rate of recombination with electrons, but if they could be produced experimentally in high vacuum they would be quite stable and amenable to observation. An encounter between two hard γ-rays (of energy of at least half a million volts) could lead to the creation simultaneously of an electron and anti-electron. This probability [of the creation of a pair] is negligible, however, with the intensities of γ-rays at present available."
  • Then on 2 August 1932 there came along the discovery of the positron by C. Anderson. ...For Dirac it meant the satisfaction that his equation predicted the situation correctly as he had hoped. His work had also provided the first example in the history of physics where the existence of a new particle was predicted on a purely theoretical basis.

The Concept of Particle Creation before and after Quantum Mechanics (1976)

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by Joan Bromberg, Historical Studies in the Physical Sciences, ed. Russell McCormmach, Vol. 7, pp. 161-182.
  • [C]reation and annihilation concepts antedate quantum mechanics. The concept of annihilation of pairs of oppositely charged, elementary particles... dates from the turn of the twentieth century. It became important in astrophysics about 1924... The annihilating pairs were first positive and negative electrons, later protons and electrons, and finally, starting in 1931, electrons and anti-electrons. ...In Dirac's "hole" theory of 1930... pair annihilation was neither novel nor central. Dirac's object was to deal with a difficulty... that the theory allowed electrons to make transitions to negative energy. ...interpreting electrons in states of negative energy as unobservable, and empty negative-energy states, or "holes" as protons. As a by-product, when an electron jumped into a vacant negative-energy state, an electron and a proton disappeared together into radiation. Since pair annihilation was already an accepted concept, this... was admissible.
  • Dirac's... paper, "A Theory of Electrons and Protons," makes it clear that his primary purpose was to deal with the negative energy difficulty, and his secondary purpose... was to present a theory of protons. ...[T]he chief novelty ...was the identification of the proton with the absence of the electron, whereas the concept of pair annihilation was not a novelty ...He began by stating the difficulty: relativistic theories of the electron all yield solutions in which the electron has a negative total energy, and quantum mechanical relativistic theories... permit the electron to make transitions from states of positive energy to these states of negative energy. He then argued... that these states, and the transistions to them, cannot be disregarded as nonphysical...

Mirror Matter: Pioneering Antimatter Physics (1988)

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by Robert L. Forward & Joel Davis
  • Since every particle needed to make up atoms has its antiparticle, it is conceivable... to combine positrons and antiprotons to make antihydrogen... Then one could use antineutrons to make heavier forms of antihydrogen such as antideuterium (an antinucleus containing one antiproton and one antineutron, with a positron in orbit) and antitritium (one antiproton and two antineutrons).
  • A few hundred heavy nuclei of antideuterium, antitritium, and antihelium-3 have been observed... Sadly, they have been unable to keep these antimatter fragments under control long enough to add positrons and make neutral antiatoms...
  • When a matter particle and its mirror antimatter twin are brought into contact, the two annihilate each other. The mass of both is totally converted into energy. The amount of energy... Einstein's ... The annihilation of a gram of matter and antimatter would produce the energy of a 20-kiloton nuclear bomb, the size... dropped on Japan.
  • The word "antimatter"... Strictly speaking, it's not... accurate... Antimatter is not "negative matter." It does not have negative mass, or negative spin, or negative (anti-) gravity (...scientists are... running experiments to see if antiprotons have the same kind of gravity as protons). ...One researcher has suggested replacing the... "anti-" with "co-,"... co-matter, co-protons,... Another... suggested... "exo-"... "Exo" in Greek means "outside." Other suggestions... "ob-" (obmatter, obproton) and "contra-" (contramatter and contraproton). None... ever caught on... Hannes Alfven in... Worlds-Antiworlds... said... let's coin a new word for "ordinary" matter... the word koinomatter... after the Greek word koinos, meaning common or well-known. ..."Matter will remain "matter" and "antimatter"... "antimatter"... However... "mirror matter" is the most accurate and unbiased term.
  • Mirror matter is, first and foremost, matter. ...[A]ll mirror matter is still matter.
  • A positron, Feynman has written, can... be thought of as an electron moving backwards in time! An electron doing such... would be indistinguishable from an electron moving with a positive charge. This would also be essentially true for any other mirror matter particle or object.
  • An even more bizarre extension of the Feynman model has been suggested by John Wheeler. ...[A]ll the electrons and positrons in the universe are just one single electron seen at different portions of a single long electron path!
    This... explains why all electrons have exactly the same charge.
  • In 1932 Millikan and Anderson were investigating cosmic rays, and they had built a large cloud chamber... When subatomic particles passed through... they left ghostly vapor trails in the supersaturated air... They placed powerful magnets around it to blanket the interior... with a magnetic field. ...[C]osmic rays ...were bent by the field ...[T]he direction and thickness of the paths... revealed the mass of the particles—and their charge. Anderson... noticed that some of the trails were... like... electrons, but were curved by the magnetic field in the opposite direction. At this point Anderson was not aware of Dirac's prediction ...After nearly a year of effort ...he ...identified ...pair production of electrons and antielectrons from the impact of cosmic rays.

The Autobiography of Carl David Anderson (1999)

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: the Youngest Man to Win the Nobel Prize, by Carl David Anderson & David A. K. Anderson, ed. Richard J. Weiss.
  • The first results from the magnet cloud chamber in 1931 and 1932 were dramatic and completely unexpected. An approximately equal number of particles of positive and negative charge were observed, whereas, according to the theories known at the time, one would expect to see only ordinary electrons (all of negative charge). The presence of such an abundance of particles of positive charge was perplexing—something new and mysterious must be ocurring.
  • Seth Neddermeyer joined me... and I assigned him to the task of continuing the curvature measurements... As more data accumulated... practically all of the low-velocity cases of positive charge were particles... whose mass seemed to be too small to permit their interpretation as protons. The alternative explanations... were that these particles were either ordinary electrons (of negative charge) moving upward, or some unknown lightweight particles of positive charge moving downward. In the spirit of scentific conservatism I tended... toward the former... [T]he chief... repeatedly pointed out that cosmic ray particles travel downward, and not upward, except in extremely rare circumstances, and that these... must be downward-moving protons. This point of view was difficult for me to accept... since in nearly all cases the density of the... tracks... was too low for particles of proton mass.
    To resolve this apparent paradox, a lead plate was inserted across the center of the cloud chamber... [A] fine example was obtained in which a low-energy lightweight particle of positive charge was observed to traverse the plate... This particle came in from the bottom of the chamber, passed through the lead plate and went out near the top of the chamber. ...[I]ts track... was more curved above the plate... this meant it was going slower... therefore, it must have passed through the plate traveling upward.
  • I knew it could not have been a proton. Since a proton is 1800 times as heavy as an electron it would have produced a much thicker line [trail]... [I]t could not have been a neutron since neutrons have no electric charge and, therefore, are incapable of producing any kind of line... [T]he line was exactly what would have been produced by an ordinary electron except that electrons had always been found to have a negative electric charge and, therefore, should have turned to the right. This one turned to the left... an electron with a positive charge ...a positive electron!
  • Ionization and curvature measurements clearly showed this particle to have a mass much smaller than... a proton... a mass entirely consistent with an electron. ...[D]espite the strong admonitions of the Chief that upward-moving cosmic ray particles were very rare, this... was an example...

Antimatter: The Ultimate Mirror (2000)

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by Gordon Fraser
  • In the early 1950s... attention was focused on two new unstable, electrically neutral particles... tau and theta. ...[T]he tau and theta were 'strange'—they carried Gell-Mann's additional charge. They decayed in different ways, and had different parities... [T]he tau and theta had the same mass. ...Chen Ning ('Frank') Yang and Tsung-Dao Lee, thought it was bizarre for two apparently different particles to have the same mass, and suspected... two faces of the same particle, despite... different parities. ...[They] had to throw overboard ...apparently solid ...assumptions about quantum behaviour: ...[1] it would not be basically altered by left-right mirror reflection... [2] behaviour would not be altered by a mirror that reflected particles as antiparticles and vice-versa... [They] re-examined the evidence for both mirror symmetries, which everyone had assumed ...watertight ...showing that for particle decays this had never been proved conclusively.
  • Lee and Yang... suggested that the particle-antiparticle mirror could be flawed. ...[T]wo experiments—by Richard Garwin, Leon Lederman and Marcel Weinrich... and by Jerome Friedman and Val Telegdi...—looked at multiple particle transformations in which a pion decays into a muon, which in turn decays into an electron. ...[These] found that ...[f]or a positively charged pion, the muon's spin points backwards, against its direction of motion. [When t]he antiparticle... a negatively charged pion... decays, the muon emerges with its spin pointing in the direction of its motion. Looking in a mirror that changes particles into antiparticles, the antismoke comes down the chimney.
  • For the subnuclear world, the ordinary mirror has to be replaced by an extended mirror that carries out three reflections simultaneously—switching particle to antiparticle and vice-versa, changing left to right and vice-versa, and reversing the arrow of time. ...[R]espectively C (for charge), P (for parity) and T (for time). The CPT mirror changes Alice into a mirror-image Anti-Alice going backward in time.
  • Sakharov looked wryly at the composition of an average cubic metre of Universe. ...a billion quanta of radiation, one proton and no antiprotons. Tracking... to just after the Big Bang... [we] should have had... a billion antiprotons, and a billion and one protons. ...Why the odd proton? ...[A]ntimatter had slipped off the map of the Universe ...Sakharov put forward a three-point explanation.
  • [1] Big Bang... particle-antiparticle creation briefly got out of hand, more pairs being created than were reabsorbed back into radiation. ...[T]he present Universe is much larger than a sphere of light rays which started out from the Big Bang... Sometime in the past, the Universe... expanded faster than light... Most of the Universe we have not yet seen, despite traveling at [c]... not yet having had time to reach us. ...In the first fraction of a second... the Universe must have 'inflated' faster than the speed of light and particle-antiparticle pairs were produced faster than they could be reabsorbed.
  • [3] The proton... has to be slightly unstable... Sitting still, the quark-filled proton would have to disintegrate into electrons and other light particles. ...But ...the level of ...instability needed was so small as to be almost undetectable. ...[E]xperiments are trying to capture this effect...
  • The Big Bang should have been matter-antimatter symmetric. But the visible Universe... shows little sign of this primordial antimatter.
  • Paul Dirac, the spiritual father of antimatter, probably did not yet know very much about the Big Bang picture when he gave his Nobel lecture... and suggested that the Universe could contain both matter and antimatter without us knowing... If Dirac were right, the whole Universe should be a uniform mix... overall the two halves of the Universe should balance. Where is this antimatter?
  • Light antiparticles... as positrons, are common in cosmic rays. However, such... are usually from particle-antiparticle pairs produced... as primary cosmic ray particles collide with atmospheric gas or interstellar dust. ...'Fountains' of positrons... seen... peering into the center of our Galaxy... can be explained by violent cosmic processes spitting out... radiation...
  • Any antimatter stars... [w]hen such... died in supernova explosions, their... antinuclei would have been flung out... But the cosmic rays arriving... have revealed no signs of antimatter heavier than antiprotons.
  • [P]erhaps matter and antimatter are separated into distinct domains. Maybe... there is... an antidomain. ...Wherever and whenever the boundaries... briefly touched, pieces... would have mutually annihilated to give powerful bursts of... gamma rays. As the Universe... cooled these... would have... produced a dim but uniform... signal all over the sky.
  • If the initial Universe had contained widely space clusters of matter and antimatter, these would have left their... imprint on the Cosmic Background Radiation. The tiny ripples seen by COBE and other detectors are not compatible with separate domains of matter and antimatter... The Universe we can see looks to have been eternally free of nuclear antimatter.

The Matter-Antimatter Asymmetry of the Universe (2002)

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by F.W. Stecker, Matter-Antimatter Asymmetry ed. L. Iconomidou-Fayard, J. Tran Thanh Van, pp. 5-14.
  • It became apparent that in a hot early epoch of the big bang there would exist a fully mixed dense state of matter and antimatter in the form of leptonic and baryonic pairs in thermal equilibrium with radiation. As the universe expanded and cooled this situation would result in an almost complete annihilation of both matter and antimatter.
    • 1 Introduction
  • Antinucleons "freeze out" of thermal equilibrium when the annihilation rate becomes smaller than the expansion rate of the universe. This would have occurred when the temperature of the universe dropped below ∼20 MeV. The predicted freeze out density of both matter and antimatter is only about 4×10-11 of the closure density of the universe...
    • 1 Introduction
  • Sakharov showed that three conditions are necessary in order to create the appropriately significant concentration of baryons in the early universe. They are:
    • Violation of Baryon Number, B
    • Violation of C and CP
    • Conditions in which Thermodynamic Equilibrium does not Hold
    • 2 The Sakharov Conditions (and Beyond).
  • If CPV is predetermined, then only matter will remain in the present universe. We can refer to this case as a "global" matter-antimatter asymmetry. If... CPV is the result of spontaneous symmetry breaking, domains of positive and negative CPV may result. In the case of spontaneous CPV, the Lagrangian is explicitly CP invariant, but at the symmetry breaking phase transition a CP invariant high temperature vacuum state undergoes a transition to a state where the vacuum solutions break CP either way. This mechanism may be compared to the spontaneous formation of ferromagnetic domains when a piece of unmagnetized iron cools below the critical temperature in the absence of a magnetic field. Although there is no preferred direction of magnetization, individual domains acquire random local directions of magnetization.
    • 4 A Locally Asymmetric Domain Cosmology
  • If the CP domain structure is stretched to astronomical size by a subsequent period of moderate inflation, then, following baryogenesis, baryons may survive as galaxies in some regions of the universe and antibaryons may survive as antigalaxies in other regions. In this case, we have a "local" matter-antimatter asymmetry instead of a global one. ...[i.e.,] a "locally asymmetric domain cosmology (LADC)." Following baryogenesis, the walls of the initially CP symmetric vacuum between the positive and negative CP domains must vanish because they are quite massive and could eventually dominate the evolution of the universe, in conflict with observations.
    • 4 A Locally Asymmetric Domain Cosmology
  • Antimatter galaxies will look exactly the same as matter galaxies. This is because the photon is its own antiparticle. However, we can look for other clues. Searches have been made for antimatter in the cosmic radiation and for the indirect traces of cosmic matter-antimatter annihilation in the extragalactic γ-ray background radiation.
    • 4 A Locally Asymmetric Domain Cosmology

Neutrino Models and Leptogenesis (Dec 2008)

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by Sandy Sheung Che Law, Ph.D. Thesis, School of Physics, University of Melbourne. A source.
  • Neutrino properties can play a crucial role in determining the matter-antimatter asymmetry of the universe if thermal leptogenesis is the correct solution to the baryogenesis problem. Owing to this, the study of Neutrino models goes beyond the mere purpose of generating tiny neutrino masses, and it is natural to incorporate the puzzle of cosmic baryon asymmetry.
    • Abstract, p. i.
  • One of the most fundamental concepts in the study of physics is the idea of symmetry. Yet, Nature as we know it does not always seem to be perfectly symmetrical. ...[T]he principal theme for this current work is motivated by none other than the apparent asymmetry between matter and antimatter in the universe. Therefore, along with the appeal of symmetry, a major topic of interest is the mechanism of symmetry breaking or asymmetry creation.
    • 5 Conclusion, p. 153.
  • [I]t is quite fascinating that two seemingly unrelated problems—the tiny masses of light neutrino and the matter-antimatter asymmetry—may be explained by the mere introduction of heavy RH [right-hand] neutrinos to the SM. ...[T]he former may be explained by the Type I seesaw mechanism while thermal leptogenesis provides an attractive solution to the later. This... means that an intricate link between neutrino properties and the baryon asymmetry can be established. Consequently, it has been the purpose of this work to explore the implications of several different neutrino models in the leptogenesis context.
    • 5 Conclusion, pp. 153-154.
  • In the representative models... it has been found that successful leptogenesis is only possible in a very fine-tuned region of the parameter space. Specifically, one must select the case, as well as certain combinations of Dirac and Majorana phases in UPMNS such that a lepton asymmetry can be generated via either resonant of flavoured N2-leptogenesis. Further, it has been shown that although the case can yield a TeV scale RH neutrino, the probability of detecting it at the LHC or a next-generation collider such as the ILC is far too small.
    • 5 Conclusion, pp. 154-155.
  • [W]e investigated the effects of introducing an effective transition electromagnetic dipole moment [EMDM] operator between the LH light and the RH heavy neutrinos. ...As a result, a new scenario for leptogenesis whereby the lepton asymmetry is solely generated by the EMDM-type (instead of the usual Yukawa-mediated) interactions is possible. By exploring the key ingredients leading to CP violation, we have shown by explicit computations of the relevant diagrams in a toy model that, in principle, electromagnetic leptogenesis is a viable alternative for creating a lepton asymmetry. ...[T]here is no doubt that transition EMDM interactions between light and heavy neutrinos can have far-reaching consequences in the early universe.
    • 5 Conclusion, pp. 155-156.

Meeting the Demands of Reason (2009)

[edit]
: the Life and Thought of Andrei Sakharov by by Jay Bergman
  • Sakharov published other papers in cosmology. ...[T]he most far-reaching, innovative, and original... concerned "baryon asymmetry". "Baryons"... denote collectively not only protons and neutrons but also... unstable particles... created when protons and neutrons collide at extraordinarily high speeds. "Antibaryons"... carry the opposite electrical charge. When baryons and antibaryons collide, they annihilate each other, producing... exotic, unstable particles, such as pi-mesons, which are lighter than baryons, as well as radiation... "quanta"... or photons, which have no mass at all. The "background radiation" cosmologists discovered in the mid-1960s is... a remnant of the... annihilation of baryons and antibaryons... when the universe was created or shortly afterward. Baryons and antibaryons, in other words, are one form of matter and antimatter, respectively; electrons... and their opposite, positrons, are another.
  • Sakharov tried to explain why baryon asymmetry exists... how there came to be a surplus of baryons... The consensus... was that there had to be baryon symmetry when the universe began. But there was no consensus on how symmetry broke down. ...According to Sakharov, for baryon asymmetry... the universe at the quantum level... had to have, in Christopher Korda's words, "an intrinsic arrow of time." ...[P]hysicists ...refer to the sequence... Sakharov described as "the Sakharov conditions."
  • Sakharov's conclusion was that "baryon number"—the difference between baryons and antibaryons in the universe—was not constant, as most... believed. ...[B]aryons, and in particular protons, can decay, and it was Sakharov's concept of proton decay and how it comes about that proved to be perhaps the most remarkable of all his contributions to cosmology. ...D. S. Chernavski, went as far as to say that, by showing theoretically that the proton can disnintegrate, he revealed "the basis of the universe." Ironically, Sakharov's ideas on the subject did not attract much attention for about a decade. But the development of... gauge theories in the late 1970s sparked new interest... even though proton decay has yet to be confirmed experimentally.

Antimatter (2009)

[edit]
by Frank Close
  • A helium nucleus contains two protons and two neutrons. Under suitable circumstances a proton can change into a neutron and emit energy some of which materializes as a positron, similar to what happens in the positron emitters of... medicine.
  • The positron finds itself in the heart of the sun, where there are lots of electrons and is instantly destroyed, turned into gamma rays. These try to rush away... but are interrupted by the crowd of electrically charged particles, electrons and protons... [R]epeatedly absorbed by electrons and then emitted with less energy... it will take a hundred thousand years before gamma rays... reach the surface... In doing so the rays lose lots of energy... changing from X-rays to ultra-violet and at last into the rainbow of colours that are visible... So daylight is the result of antimatter being produced in the heart of the sun and, in part, of its annihilation.
  • The laws of electricity and magnetism that underlie the existence of bulk matter don't care which bits... carry negative charge, and which... are positive. If we swapped all positives to negative, and all negatives to positive... resulting forces would be the same and the structures they built would... be unchanged. ...[T]o all outward appearances, nothing would appear different.
    Such a swapping of charges would turn what we know as matter into... antimatter. An anti-atom of antihydrogen would consist of a negative 'antiproton' encircled by a positively charged 'positron'. Paul Dirac... first predicted that such a mirror image of matter should exist.
  • [H]ow can an electron with negative electrical charge emerge from the energy in a puff of light, which has no... charge? This is where nature's two forms of matter enter the story. The negatively charged electron has a positively charged form... the positron. The energy of a photon, a particle of light, becomes trapped in these two complementary pieces of substance. This... can also happen in reverse: an electron and a positron can annihilate one another, their individual energies being taken by the photons that rush from the scene of destruction at the speed of light.
    The emergence of substance from pure energy... is almost biblical in scope. With antimatter... we make contact with the gods of creation.
  • In 1923 Dmitri Skobeltsyn... was investigating gamma rays... using a cloud chamber. ...The ...rays would knock electrons out of atoms... whose trails he could see... [I]n addition to knocking electrons out of the gas, they were ejecting them out of the walls of the chamber ...which interfered with the measurements... He... came up with the... idea of sweeping away the unwanted electrons by putting the chamber between the poles of a large magnet. ...[T]he clearer view revealed ...the magnetic forces seemed to make some of the 'electrons' curve 'the wrong way'.
    Today we know he was seeing positrons, but    ... [the] anomalous trails were a distraction from what he was trying to do. ...News about these images spread ...and five years later Skobeltsyn decided to show them at an international conference in Cambridge. ...[N]o one could offer an explanation. It was ironic that [this was]... the same year and... place that Dirac came up with his theoretical prediction of positrons... [A]s no one at the time had any reason to expect... positrons existed, he missed the big prize.
  • Blackett had been working with a cloud chamber in Rutherford's group... a chamber that was ready for action every ten seconds or so, and took photos on ordinary cinematograph film. ...[H]e accumulated ...pictures of trails made by alpha particles—a product of radioactive nuclear decays— ...bombarding nitrogen gas in the chamber. ...[I]n 1931 Giuseppe Occhialini arrived ...His specialty was detecting nuclear radiation using Geiger counters. ...Their big idea ...put one Geiger counter above a cloud chamber, and another... below. ...By connecting the Geiger counters to a relay ...a flash of light [and the cinematograph] captured the tracks of the cosmic rays on film. ...They noticed that ...a few tracks that appeared at first sight to be electrons, were ...curved the wrong way in the magnetic field. Blackett talked to Dirac about them... neither aware of the precious truth. ...It was only when they heard of Anderson's discovery that Blackett and Occhialini ...realized what they had.
  • [L]uckily... they had more... Many of the pictures showed up to twenty... tracks ...from a copper plate just above the chamber ...roughly half of the particles were negatively charged and the rest positively charged. Blackett and Occhialini realized... the appearance of equal numbers of positrons and electrons must be... the result of cosmic rays hitting the metal.
  • Albert Einstein's equation , implies that energy (E) can be converted into mass (m)—radiation into matter—and Blackett and Ochialini had for the first time demonstrated the creation of matter, and antimatter, from radiation; they had proved that Anderson's new particle was not some weird extraterrestrial interloper.
  • An ambitious plan took hold at Berkeley... to build an accelerator that would speed protons such that when smashed into a target, there would be enough energy to produce an antiproton. ...When energy turns into massive particles they emerge in pairs, a particle... matched with its antiparticle, so the BeVatron was built with enough power to produce an antiproton in conjunction with a proton... Several ideas on how to isolate the antiproton 'needle' from the particle 'haystack' were presented... A small team... of Owen Chamberlain, Emilio Segre, Clyde Wiegand, and Thomas Ypsilantis won the competition ...their idea worked ...and in 1955 they announced their discovery. One of the other teams led by Oreste Piccioni that had entered the competition also gained success... with the discovery of the antineutron in 1957. So thirty years after Dirac['s]... seminal prediction, the basic pieces of the antiworld were in place: positron, antiproton, and antineutron.
  • Since antimatter will destroy any material object, it must be kept in a cage without material walls. The solution... a vacuum that is better than in outer space with magnetic and electric fields that confine the antiparticles, positrons, or antiprotons, as circulating beams.
    That is in effect what is done at particle physics laboratories such as CERN...
  • Magnetic fields that had been able to focus positrons into stable orbits were unable to control the wild antiprotons... Budker's idea was to pass the antiprotons through clouds of cold electrons. Although electrons are matter and antiprotons are antimatter, they are in no danger to one another: electrons are destroyed by their antiparticle, the positron, while the antiproton is at risk only from protons or neutrons. ...By 1974 Budker... succeeded in making and cooling antiprotons, but not in sufficient numbers to make an intense beam.

The Mystery of the Missing Antimatter (2010)

[edit]
by Helen Quinn & Yossi Nir
  • It is just like matter except with a reversal of charges. ...We make it and study it in our laboratories, but find little of it in nature. The laws of physics for antimatter are almost an exact mirror of those for matter.
  • For each type of matter particle there is a matching type of antimatter particle. ...[W]e can convert energy from radiation into a matched pair...
  • [T]heories suggest that, at very early times... all possible types of particles and antiparticles, existed equally in a hot, dense, and very uniform plasma. ...[A]s the Universe expanded and cooled... annihilation could still occur whenever a particle met an antiparticle, but the reverse... creation of a particle and an antiparticle, became... rare.
  • [H]igh energy laboratories can produce particles with energies similar to those that prevailed in the [very early] Universe... Nuclear physics allows us to model the primordial production of small nuclei from collisions starting with protons and neutrons, long before stars began to form. Because we know... what energies are required for collisions to take apart... light elements [atomic number less than 11] into... protons and neutrons, we can identify... the time at which the Universe became cold enough that this destruction practically ceased, and... production of elements started in earnest.
  • The fate of antimatter to disappear was sealed by the time the Universe was no older than a millionth of a second.
  • [T]he mystery of the missing antimatter... What laws of nature, not yet manifest in experiments and not part of our current Standard Model, were active in the early Universe, allowing the observed amount of matter to persist while all antimatter disappeared from the Universe?

See also

[edit]
[edit]
Wikipedia
Wikipedia
Wikipedia has an article about:
  • Antimatter Colin Blakemore, Fay Dowker, Graham Thompson & Sir Martin Rees, BBC/OU/Vega video, What is it? Relation to the Structure of the Universe.
  • Antimatter Melvyn Bragg (4 Oct, 2007) In Our Time, BBC Radio.
  • Faq, Centre for Antimatter-Matter Studies, Australian Research Council Centre of Excellence
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