Subatomic particle

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Subatomic particles
)
A composite particle proton is made of two up quarks and one down quark, which are elementary particles.

In

fermions
.

Experiments show that light could behave like a

stream of particles (called photons) as well as exhibiting wave-like properties. This led to the concept of wave–particle duality to reflect that quantum-scale particles behave both like particles and like waves; they are sometimes called wavicles to reflect this.[4]

Another concept, the

position and momentum, cannot be measured exactly.[5] The wave–particle duality has been shown to apply not only to photons but to more massive particles as well.[6]

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.

Even among particle physicists, the exact definition of a particle has diverse descriptions. These professional attempts at the definition of a particle include:[7]

Particles in the atom
Subatomic particle Symbol Type Location in atom Charge
(e)
Mass
(u)
Proton p+ Composite Nucleus +1 ≈1
Neutron n0 Composite Nucleus 0 ≈1
Electron e Elementary Shells −1 12000

Classification

By composition

Subatomic particles are either "elementary", i.e. not made of multiple other particles, or "composite" and made of more than one elementary particle bound together.

The elementary particles of the Standard Model are:[8]

The Standard Model classification of particles

All of these have now been discovered through experiments, with the latest being the top quark (1995), tau neutrino (2000), and Higgs boson (2012).

Various

many other elementary particles
, but none have been discovered as of 2021.

Hadrons

The word hadron comes from Greek and was introduced in 1962 by Lev Okun.[9] Nearly all composite particles contain multiple quarks (and/or antiquarks) bound together by gluons (with a few exceptions with no quarks, such as positronium and muonium). Those containing few (≤ 5) quarks (including antiquarks) are called hadrons. Due to a property known as

nucleons) are by far the best known; and the mesons containing an even number of quarks (almost always 2, one quark and one antiquark), of which the pions and kaons
are the best known.

Except for the proton and neutron, all other hadrons are unstable and decay into other particles in microseconds or less. A proton is made of two up quarks and one down quark, while the neutron is made of two down quarks and one up quark. These commonly bind together into an atomic nucleus, e.g. a helium-4 nucleus is composed of two protons and two neutrons. Most hadrons do not live long enough to bind into nucleus-like composites; those that do (other than the proton and neutron) form

exotic nuclei
.

Overlap between Bosons, Hadrons, and Fermions

By statistics

Any 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 elementary fermions have spin 1/2, and are divided into the

leptons
which do not. The elementary bosons comprise the
gauge bosons (photon, W and Z, gluons) with spin 1, while the Higgs boson
is the only elementary 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 elementary particles with spin 3/2, but none have been discovered as of 2021.

Due to the laws for spin of composite particles, the baryons (3 quarks) have spin either 1/2 or 3/2 and are therefore fermions; the mesons (2 quarks) have integer spin of either 0 or 1 and are therefore bosons.

By mass

In

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 elementary and are defined as the elementary 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 decay

Most subatomic particles are not stable. All leptons, as well as baryons decay by either the strong force or weak force (except for the proton). Protons are not known to decay, although whether they are "truly" stable is unknown, as some very important Grand Unified Theories (GUTs) actually require it. The μ and τ muons, as well as their antiparticles, decay by the weak force. Neutrinos (and antineutrinos) do not decay, but a related phenomenon of neutrino oscillations is thought to exist even in vacuums. The electron and its antiparticle, the positron, are theoretically stable due to charge conservation unless a lighter particle having magnitude of electric charge  e exists (which is unlikely). Its charge is not shown yet.

Other properties

All observable subatomic particles have their electric charge an integer multiple of the elementary charge. The Standard Model's quarks have "non-integer" electric charges, namely, multiple of 1/3 e, but quarks (and other combinations with non-integer electric charge) cannot be isolated due to color confinement. For baryons, mesons, and their antiparticles the constituent quarks' charges sum up to an integer multiple of e.

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.[10] 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.[11]

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

Philosophiae Naturalis Principia Mathematica
, originally published in 1687.

Dividing an atom

The negatively charged electron has a mass of about 1/1836 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).

Particle phenomenology systematizes the knowledge about subatomic particles obtained from these experiments.[13]

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:

Particle Composition Theorized Discovered Comments
electron
e
elementary (lepton)
G. Johnstone Stoney (1874)[14]
J. J. Thomson (1897)[15] Minimum unit of electrical charge, for which Stoney suggested the name in 1891.[16] First subatomic particle to be identified.[17]
alpha particle
α
composite (atomic nucleus) never Ernest Rutherford (1899)[18] Proven by Rutherford and Thomas Royds in 1907 to be helium nuclei. Rutherford won the Nobel Prize for Chemistry in 1908 for this discovery.[19]
photon
γ
elementary (quantum) Max Planck (1900)[20] Albert Einstein (1905)[21] Necessary to solve the thermodynamic problem of black-body radiation.
proton
p
composite (baryon) William Prout (1815)[22] Ernest Rutherford (1919, named 1920)[23][24] The nucleus of
1
H
.
neutron
n
composite (baryon) Ernest Rutherford (c.1920[25]) James Chadwick (1932) [26] The second nucleon.
antiparticles   Paul Dirac (1928)[27]
Carl D. Anderson (
e+
, 1932)
Revised explanation uses CPT symmetry.
pions
π
composite (mesons) Hideki Yukawa (1935)
Cecil Powell
(1947)
Explains the nuclear force between nucleons. The first meson (by modern definition) to be discovered.
muon
μ
elementary (lepton) never Carl D. Anderson (1936)[28] Called a "meson" at first; but today classed as a lepton.
kaons
K
composite (mesons) never G. D. Rochester, C. C. Butler (1947)[29] Discovered in cosmic rays. The first strange particle.
lambda baryons
Λ
composite (baryons) never University of Melbourne (
Λ0
, 1950)[30]
The first hyperon discovered.
neutrino
ν
elementary (lepton) Wolfgang Pauli (1930), named by Enrico Fermi Clyde Cowan, Frederick Reines (
ν
e
, 1956)
Solved the problem of energy spectrum of beta decay.
quarks
(
u
,
d
,
s
)
elementary Murray Gell-Mann, George Zweig (1964) No particular confirmation event for the quark model.
charm quark
c
elementary (quark) Sheldon Glashow, John Iliopoulos, Luciano Maiani (1970) B. Richter, S. C. C. Ting (
J/ψ
, 1974)
bottom quark
b
elementary (quark)
Makoto Kobayashi, Toshihide Maskawa
(1973)
Leon M. Lederman (
ϒ
, 1977)
gluons elementary (quantum) Harald Fritzsch, Murray Gell-Mann (1972)[31] DESY (1979)
weak gauge bosons
W±
,
Z0
elementary (quantum) Glashow, Weinberg, Salam (1968) CERN (1983) Properties verified through the 1990s.
top quark
t
elementary (quark)
Makoto Kobayashi, Toshihide Maskawa (1973)[32]
Fermilab (1995)[33] Does not hadronize, but is necessary to complete the Standard Model.
Higgs boson elementary (quantum) Peter Higgs (1964)[34][35] CERN (2012)[36] Thought to be confirmed in 2013. More evidence found in 2014.[37]
tetraquark composite ? Zc(3900), 2013, yet to be confirmed as a tetraquark A new class of hadrons.
pentaquark composite ? Yet another class of hadrons. As of 2019 several are thought to exist.
graviton elementary (quantum) Albert Einstein (1916) Interpretation of a gravitational wave as particles is controversial.[38]
magnetic monopole elementary (unclassified) Paul Dirac (1931)[39] undiscovered

See also

References

  1. ^ "Subatomic particles". NTD. Archived from the original on 16 February 2014. Retrieved 5 June 2012.
  2. .
  3. ^ Fritzsch, Harald (2005). Elementary Particles. .
  4. – via Springer Link. The finite-field model of the photon is both a particle and a wave, and hence we refer to it by Eddington's name "wavicle".
  5. .
  6. ^ Arndt, Markus; Nairz, Olaf; Vos-Andreae, Julian; Keller, Claudia; Van Der Zouw, Gerbrand; Zeilinger, Anton (2000). "Wave-particle duality of C60 molecules".
    S2CID 4424892
    .
  7. ^ "What is a Particle?". 12 November 2020.
  8. ^ Cottingham, W.N.; Greenwood, D.A. (2007). An introduction to the standard model of particle physics. .
  9. .
  10. ^ Greiner, Walter (2001). Quantum Mechanics: An Introduction. .
  11. ^ Eisberg, R. & Resnick, R. (1985). Quantum Physics of Atoms, Molecules, Solids, Nuclei, and Particles (2nd ed.). . 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.
  12. Philosophiae Naturalis Principia Mathematica
    )
  13. .
  14. ^ Stoney, G. Johnstone (1881). "LII. On the physical units of nature". The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science. 11 (69): 381–390.
    ISSN 1941-5982
    .
  15. ^ Thomson, J.J. (1897). "Cathode Rays". The Electrician. 39: 104.
  16. ^ Klemperer, Otto (1959). "Electron physics: The physics of the free electron". Physics Today. 13 (6): 64–66. .
  17. ^ Alfred, Randy. "April 30, 1897: J.J. Thomson Announces the Electron ... Sort Of". Wired.
    ISSN 1059-1028
    . Retrieved 2022-08-22.
  18. ^ Rutherford, E. (1899). "VIII. Uranium radiation and the electrical conduction produced by it". The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science. 47 (284): 109–163.
    ISSN 1941-5982
    .
  19. ^ "The Nobel Prize in Chemistry 1908". NobelPrize.org. Retrieved 2022-08-22.
  20. ^ Klein, Martin J. (1961). "Max Planck and the beginnings of the quantum theory". Archive for History of Exact Sciences. 1 (5): 459–479.
    S2CID 121189755
    .
  21. ^ Einstein, A. (1905). "Über einen die Erzeugung und Verwandlung des Lichtes betreffenden heuristischen Gesichtspunkt". Annalen der Physik (in German). 322 (6): 132–148. .
  22. .
  23. .
  24. ^ "There was early debate on what to name the proton as seen in the follow commentary articles by Soddy 1920 and Lodge 1920.
  25. ISSN 0950-1207
    .
  26. .
  27. .
  28. .
  29. .
  30. ^ Some sources such as "The Strange Quark". indicate 1947.
  31. ^ Fritzsch, Harald; Gell-Mann, Murray (1972). "Current algebra: Quarks and what else?". EConf. C720906V2: 135–165. .
  32. .
  33. .
  34. ^ "Letters from the Past - A PRL Retrospective". Physical Review Letters. 2014-02-12. Retrieved 2022-08-22.
  35. ISSN 0031-9007
    .
  36. .
  37. ^ "CERN experiments report new Higgs boson measurements". cern.ch. 23 June 2014.
  38. ^ Moskowitz, Clara. "Multiverse Controversy Heats Up over Gravitational Waves". Scientific American. Retrieved 2022-08-22.
  39. ISSN 0950-1207
    .

Further reading

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