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subatomic particle
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{{short description|Particle whose size or mass is less than that of the atom, or of which the atom is composed; small quantum particle}}{{pp-move-indef}}{{Standard model of particle physics}}In the physical sciences, subatomic particles are particles much smaller than atoms.WEB, Subatomic particles,weblink NTD, 5 June 2012, The two types of subatomic particles are: elementary particles, which according to current theories are not made of other particles; and composite particles.BOOK, Bolonkin, Alexander, Universe, Human Immortality and Future Human Evaluation, 2011, Elsevier, 9780124158016, 25, Particle physics and nuclear physics study these particles and how they interact.BOOK
, Fritzsch, Harald
, 2005
, Elementary Particles
,weblink
, 11–20
, World Scientific
, 978-981-256-141-1
, The idea of a particle underwent serious rethinking when experiments showed that light could behave like a stream of particles (called photons) as well as exhibiting wave-like properties. This led to the new concept of wave–particle duality to reflect that quantum-scale "particles" behave like both particles and waves (they are sometimes described as wavicles to reflect this). Another new concept, the uncertainty principle, states that some of their properties taken together, such as their simultaneous position and momentum, cannot be measured exactly.{{Citation |first=W. |last=Heisenberg |title=Über den anschaulichen Inhalt der quantentheoretischen Kinematik und Mechanik |language=de|journal=Zeitschrift für Physik |volume=43 |issue=3–4 |date=1927 |pages=172–198 |doi=10.1007/BF01397280 |postscript=. |bibcode = 1927ZPhy...43..172H }} In more recent times, wave–particle duality has been shown to apply not only to photons but to increasingly massive particles as well.JOURNAL
, 2000
, Wave-particle duality of C60 molecules
, Anton
, Zeilinger
, Gerbrand
, Van Der Zouw
, Claudia
, Keller
, Julian
, Nature (journal), Nature
, Vos-Andreae
, 401
, Olaf
, 10.1038/44348
, Naziz
, 1999Natur.401..680A, 6754, 680–682
, Arndt
, Markus, 18494170
, Interactions of particles in the framework of quantum field theory are understood as creation and annihilation of quanta of corresponding fundamental interactions. This blends particle physics with field theory.

Classification

By statistics

File:Standard Model of Elementary Particles.svg|thumb|392x392px|The Standard ModelStandard ModelAny subatomic particle, like any particle in the three-dimensional space that obeys the laws of quantum mechanics, can be either a boson (with integer spin) or a fermion (with odd half-integer spin). In the Standard Model, all the fundamental fermions have spin 1/2, and are divided into the quarks which carry color charge and therefore feel the strong interaction, and the leptons which do not. The bosons comprise the gauge bosons (photon, W and Z, gluons) with spin 1, and the Higgs boson with spin zero, the only standard model particle with spin zero. The hypothetical graviton is required theoretically to have spin 2, but is not part of the Standard Model. Some extensions such as supersymmetry predict additional fundamental particles with spin 3/2, but none have been discovered as of 2019.

By composition

The elementary particles of the Standard Model include:BOOK
, Cottingham, W.N.
, Greenwood, D.A.
, 2007
, An introduction to the standard model of particle physics
,weblink
, Cambridge University Press
, 1
, 978-0-521-85249-4
, Various extensions of the Standard Model predict the existence of an elementary graviton particle and many other elementary particles.Composite subatomic particles (such as protons or atomic nuclei) are bound states of two or more elementary particles. For example, a proton is made of two up quarks and one down quark, while the atomic nucleus of helium-4 is composed of two protons and two neutrons. The neutron is made of two down quarks and one up quark. Composite particles include all hadrons: these include baryons (such as protons and neutrons) and mesons (such as pions and kaons).

By mass

In special relativity, the energy of a particle at rest equals its mass times the speed of light squared, {{nowrap begin}}E = mc2{{nowrap end}}. That is, mass can be expressed in terms of energy and vice versa. If a particle has a frame of reference in which it lies at rest, then it has a positive rest mass and is referred to as massive.All composite particles are massive. Baryons (meaning "heavy") tend to have greater mass than mesons (meaning "intermediate"), which in turn tend to be heavier than leptons (meaning "lightweight"), but the heaviest lepton (the tau particle) is heavier than the two lightest flavours of baryons (nucleons). It is also certain that any particle with an electric charge is massive. When originally defined in the 1950s, the terms baryons, mesons and leptons referred to masses; however, after the quark model became accepted in the 1970s, it was recognised that baryons are composites of three quarks, mesons are composites of one quark and one antiquark, while leptons are fundamental and are defined as the fundamental fermions with no color charge. All massless particles (particles whose invariant mass is zero) are elementary. These include the photon and gluon, although the latter cannot be isolated.

By generation

The standard model fermions (quarks and leptons) come in three generations; each generation has two quarks with electric charges +2/3 and -1/3 respectively, one lepton with charge -1, one neutrino (uncharged), and their antiparticles. All everyday matter is made of particles from the first generation; second and third generation particles have similar interactions to their first-generation cousins, but larger masses. Apart from the neutrinos, all second and third generation particles are unstable and rapidly decay to first-generation particles via the weak interaction, so they only occur in particle accelerators or products of cosmic-rays hitting Earth's atmosphere.
The first generation contains the up quark, down quark, electron, electron neutrino (and their antiparticles).
The second generation is the charm quark, strange quark, muon, muon neutrino, and their antiparticles.
The third generation is the top quark, bottom quark, tau, tau neutrino, and their antiparticles.
There has been speculation about the existence of fourth or higher generations, but current evidence does not favour this; the LEP collider measurements showed that there are only three types of neutrino with the standard interaction with the Z boson.
Unlike the fermions, the Standard Model bosons do not have multiple generations.

By electric charge

The fundamental particles divide by electric charge as follows:

The up-family quarks (up, charm and top) have charge +2/3.
The down-family quarks (down, strange and bottom) have charge -1/3.
The charged leptons (electron, muon and tau) have charge -1.
The neutrinos have zero charge (the name means "little neutral one").
All the bosons have zero charge, except for the W+ and W- which are +1 and -1.
Antiparticles have the opposite sign charge to the corresponding particle, and the W+ and W- are antiparticles of each other. So for example the only particles of charge -2/3 are anti-up-family quarks, while the fundamental particles of charge +1 are the antielectron (positron), antimuon, antitau and the W+. Thus the proton (two up and one down quark) has charge +1, while the neutron (one up and two down quarks) has zero charge.

Other properties

Through the work of Albert Einstein, Satyendra Nath Bose, Louis de Broglie, and many others, current scientific theory holds that all particles also have a wave nature.BOOK
, Walter Greiner
, 2001
, Quantum Mechanics: An Introduction
,weblink
, 29
, Springer (publisher), Springer
, 978-3-540-67458-0
, This has been verified not only for elementary particles but also for compound particles like atoms and even molecules. In fact, according to traditional formulations of non-relativistic quantum mechanics, wave–particle duality applies to all objects, even macroscopic ones; although the wave properties of macroscopic objects cannot be detected due to their small wavelengths.BOOK
, Eisberg, R.
, Resnick, R.
, yes
, 1985
, Quantum Physics of Atoms, Molecules, Solids, Nuclei, and Particles
, John Wiley & Sons
, 2nd, 59–60
, 978-0-471-87373-0
, For both large and small wavelengths, both matter and radiation have both particle and wave aspects. [...] But the wave aspects of their motion become more difficult to observe as their wavelengths become shorter. [...] For ordinary macroscopic particles the mass is so large that the momentum is always sufficiently large to make the de Broglie wavelength small enough to be beyond the range of experimental detection, and classical mechanics reigns supreme.
, Interactions between particles have been scrutinized for many centuries, and a few simple laws underpin how particles behave in collisions and interactions. The most fundamental of these are the laws of conservation of energy and conservation of momentum, which let us make calculations of particle interactions on scales of magnitude that range from stars to quarks.Isaac Newton (1687). Newton's Laws of Motion (Philosophiae Naturalis Principia Mathematica) These are the prerequisite basics of Newtonian mechanics, a series of statements and equations in Philosophiae Naturalis Principia Mathematica, originally published in 1687.

Dividing an atom

The negatively charged electron has a mass equal to {{frac|1837 or 1836}} of that of a hydrogen atom. The remainder of the hydrogen atom's mass comes from the positively charged proton. The atomic number of an element is the number of protons in its nucleus. Neutrons are neutral particles having a mass slightly greater than that of the proton. Different isotopes of the same element contain the same number of protons but differing numbers of neutrons. The mass number of an isotope is the total number of nucleons (neutrons and protons collectively).Chemistry concerns itself with how electron sharing binds atoms into structures such as crystals and molecules. Nuclear physics deals with how protons and neutrons arrange themselves in nuclei. The study of subatomic particles, atoms and molecules, and their structure and interactions, requires quantum mechanics. Analyzing processes that change the numbers and types of particles requires quantum field theory. The study of subatomic particles per se is called particle physics. The term high-energy physics is nearly synonymous to "particle physics" since creation of particles requires high energies: it occurs only as a result of cosmic rays, or in particle accelerators. Particle phenomenology systematizes the knowledge about subatomic particles obtained from these experiments.Taiebyzadeh, Payam (2017). String Theory; A unified theory and inner dimension of elementary particles (BazDahm). Riverside, Iran: Shamloo Publications Center. {{ISBN|978-600-116-684-6}}.

History

The term "subatomic particle" is largely a retronym of the 1960s, used to distinguish a large number of baryons and mesons (which comprise hadrons) from particles that are now thought to be truly elementary. Before that hadrons were usually classified as "elementary" because their composition was unknown.A list of important discoveries follows:{| class="wikitable"
!Particle
!Composition
!Theorized
!Discovered
!Comments
|-
|Electron {{subatomic particle|electron}}
|elementary (lepton)
|G. Johnstone Stoney (1874)
|J. J. Thomson (1897)
|Minimum unit of electrical charge, for which Stoney suggested the name in 1891.
JOURNAL, Klemperer, Otto, 1959, Electron physics: The physics of the free electron, Physics Today, 13, 6, 64–66, 1960PhT....13R..64K, 10.1063/1.3057011,
|-
|alpha particle {{subatomic particle|alpha}}
|composite (atomic nucleus)
|never
|Ernest Rutherford (1899)
|Proven by Rutherford and Thomas Royds in 1907 to be helium nuclei.
|-
|Photon {{subatomic particle|photon}}
|elementary (quantum)
|Max Planck (1900) {{nobreak|Albert Einstein}} (1905)
|Ernest Rutherford (1899) as γ rays
|Necessary to solve the thermodynamic problem of black-body radiation.
|-
|Proton {{subatomic particle|proton}}
|composite (baryon)
|long ago
|Ernest Rutherford (1919, named 1920)
|The nucleus of {{SimpleNuclide2|hydrogen|1|link=yes}}.
|-
|Neutron {{subatomic particle|neutron}}
|composite (baryon)
|Ernest Rutherford ({{circa}}1918)
|James Chadwick (1932)
|The second nucleon.
|-
|Antiparticles

|Paul Dirac (1928)
|Carl D. Anderson ({{subatomic particle|positron|link=yes}}, 1932)
|Revised explanation uses CPT symmetry.
|-
|Pions {{subatomic particle|pion}}
|composite (mesons)
|Hideki Yukawa (1935)
|César Lattes, Giuseppe Occhialini (1947) and Cecil Powell
|Explains the nuclear force between nucleons. The first meson (by modern definition) to be discovered.
|-
|Muon {{subatomic particle|muon}}
|elementary (lepton)
|never
|Carl D. Anderson (1936)
|Called a "meson" at first; but today classed as a lepton.
|-
|Kaons {{subatomic particle|kaon}}
|composite (mesons)
|never
|1947
|Discovered in cosmic rays. The first strange particle.
|-
|Lambda baryons {{subatomic particle|Lambda}}
|composite (baryons)
|never
|University of Melbourne ({{subatomic particle|Lambda0}}, 1950)Some sources such as WEB,weblink The Strange Quark, indicate 1947.
|The first hyperon discovered.
|-
|Neutrino {{subatomic particle|neutrino}}
|elementary (lepton)
|Wolfgang Pauli (1930), named by Enrico Fermi
|Clyde Cowan, Frederick Reines ({{subatomic particle|electron neutrino|link=yes}}, 1956)
|Solved the problem of energy spectrum of beta decay.
|-
|Quarks({{subatomic particle|up quark}}, {{subatomic particle|down quark}}, {{subatomic particle|strange quark}})
|elementary
|Murray Gell-Mann, George Zweig (1964)
| colspan=2 |No particular confirmation event for the quark model.
|-
|charm quark {{subatomic particle|charm quark}}
|elementary (quark)
|1970
|1974
|
|-
|bottom quark {{subatomic particle|bottom quark}}
|elementary (quark)
|1973
|1977
|
|-
|Weak gauge bosons
|elementary (quantum)
|Glashow, Weinberg, Salam (1968)
|CERN (1983)
|Properties verified through the 1990s.
|-
|top quark {{subatomic particle|top quark}}
|elementary (quark)
|1973
|1995
|Does not hadronize, but is necessary to complete the Standard Model.
|-
|Higgs boson
|elementary (quantum)
|Peter Higgs et al. (1964)
|CERN (2012)
|Thought to be confirmed in 2013. More evidence found in 2014.WEB,weblink CERN experiments report new Higgs boson measurements, cern.ch, 23 June 2014,
|-
|Tetraquark
|composite
| ?
|Zc(3900), 2013, yet to be confirmed as a tetraquark
|A new class of hadrons.
|-
|Graviton
|elementary (quantum)
|Albert Einstein (1916)
|undiscovered
|Interpretation of a gravitational wave as a particle is controversial.
|-
|Magnetic monopole
|elementary (unclassified)
|Paul Dirac (1931)
|undiscovered
|
|}

See also

{{cmn|colwidth=20em| }}

References

{{Reflist}}

Further reading

General readers
  • Feynman, R.P. & Weinberg, S. (1987). Elementary Particles and the Laws of Physics: The 1986 Dirac Memorial Lectures. Cambridge Univ. Press.
  • BOOK, Brian Greene, Brian Greene, The Elegant Universe, W.W. Norton & Company, 1999, 978-0-393-05858-1, The Elegant Universe,
  • Oerter, Robert (2006). The Theory of Almost Everything: The Standard Model, the Unsung Triumph of Modern Physics. Plume.
  • Schumm, Bruce A. (2004). Deep Down Things: The Breathtaking Beauty of Particle Physics. Johns Hopkins University Press. {{ISBN|0-8018-7971-X}}.
  • BOOK, Martinus Veltman, Martinus Veltman, Facts and Mysteries in Elementary Particle Physics, World Scientific, 2003, 978-981-238-149-1,


Textbooks
  • Coughlan, G.D., J.E. Dodd, and B.M. Gripaios (2006). The Ideas of Particle Physics: An Introduction for Scientists, 3rd ed. Cambridge Univ. Press. An undergraduate text for those not majoring in physics.
  • BOOK, Griffiths, David J., Introduction to Elementary Particles, John Wiley & Sons, 1987, 978-0-471-60386-3,
  • BOOK, Kane, Gordon L., Modern Elementary Particle Physics, Perseus Books, 1987, 978-0-201-11749-3,

External links

{{Particles}}{{Composition}}

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