Subatomic Particle

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A subatomic particle is a particle that is smaller (but not necessarily lighter) than an atom . Particle physics and nuclear physics are primarily concerned with subatomic particles . The subatomic particles can be divided into elementary particles and composite particles .

The subatomic particles were extensively studied in the 20th century . Due to the variety of discovered particles, the so-called particle zoo was sometimes referred to . It was only through the concept of quarks that it was possible to understand the internal structure of the hadrons. The development culminated in the standard model that has existed for almost 50 years .

Due to their quantum nature , subatomic particles cannot be thought of as classical particles. Rather, quantum phenomena such as wave-particle dualism , uncertainty relations , vacuum fluctuations and virtual particles appear in the physical description of the behavior and reactions of subatomic particles .

Types of subatomic particles

Elementary particles

Elementary particles of the Standard Model

A distinction is made between the confirmed elementary particles of the Standard Model:

Compound particles

With the composite particles the situation is more complicated. The composite particles that have subatomic dimensions are all made up of combinations of quark elementary particles. The quarks themselves cannot be observed or measured alone, only their decay products. One differentiates:

At the atomic level, i.e. above the subatomic level, there are not only ordinary atoms and molecules, but also so-called exotic atoms , which are created by combining subatomic hadrons and other elementary particles. An example of an exotic atom is muonic hydrogen .

Well-known examples

Fermions and bosons

Occupation number as a function of the difference between energy and chemical potential for bosons ( Bose-Einstein statistics, upper curve ) or fermions (Fermi-Dirac statistics, lower curve), in each case in the special case of freedom from interaction and at constant temperature .


Another important distinction between subatomic particles is that between fermions and bosons . These classes differ in two basic properties:

  • In every quantum state of a system, e.g. B. of an atom, there is only at most one fermion of a given type (see Pauli principle ); this restriction does not apply to bosons. This difference is described by the fact that different probability distributions apply for fermions and bosons, the Fermi-Dirac statistics and the Bose-Einstein statistics . The spin of a particle is linked to the statistics via the spin statistics theorem . Thus, fermions and bosons also differ in their half- integer and integer spin quantum numbers.
  • The elementary fermions, i.e. leptons and quarks, can only be created or destroyed together with an antiparticle. This observation, which explains the stability of matter, is described by the law of conservation of the number of particles ( baryon number , lepton number ). Elementary bosons, on the other hand, can arise and perish individually.

All components of the atom, proton, neutron and electron, are fermions. Only the Pauli principle makes the structure of atomic nuclei and the electron shells understandable. The elementary particles are also mostly fermions. Only the gauge bosons (including the photon) and the Higgs particle are bosons. Among the composite particles, the mesons belong to the bosons.

Important phenomena at the subatomic level

Major discoveries of subatomic particles

Subatomic Particle composition Theoretical concept Discovered experimentally Comments
electron elemental ( lepton ) 1874: G. Johnstone Stoney 1897: JJ Thomson Minimum unit for the electrical charge, which is why Stoney proposed this name in 1891.
Alpha particles composite (atomic nucleus) - 1899: Ernest Rutherford In 1907 it was confirmed by Rutherford and Thomas Royds that they were helium nuclei
photon elementary ( gauge boson ) 1900: Max Planck 1905: Albert Einstein
or Ernest Rutherford (1899) as gamma radiation 		Necessary to understand the black body problem of thermodynamics .
proton compound ( baryon ) - 1919: Ernest Rutherford The nucleus of the hydrogen atom and the first nucleon of the atomic nuclei
neutron compound ( baryon ) 1918, possibly as early as 1917: Ernest Rutherford 1932: James Chadwick The second nucleon of the atomic nucleus.
positron elemental ( antilepton ) 1928: Paul Dirac 1932: Carl D. Anderson Antiparticle of the electron, first evidence of antimatter
Pion compound ( meson ) 1935: Hideki Yukawa 1947: César Lattes , Giuseppe Occhialini and Cecil Powell Pion exchange model describes forces in the atomic nucleus
Muon elemental (lepton) - 1936: Carl D. Anderson -
Kaon compound (meson) - 1947 Discovered in cosmic rays . The first particle with a strange quark .
Lambda baryon compound (baryon) - 1950, possibly even 1947: University of Melbourne The first one discovered Hyperon
neutrino elemental (lepton) 1930: Wolfgang Pauli , named by Enrico Fermi 1956: Clyde Cowan , Frederick Reines Necessary to understand the energy spectrum during beta decay.
Quarks
(up, down, strange) 		elementary 1964: Murray Gell-Mann , George Zweig - Confirmed indirectly because this model explains the particle zoo
Charm quark elementary (quark) 1970 1974: Both by Burton Richter et al. at the Stanford Linear Accelerator Center , as well as Samuel Chao Chung Ting et al. at Brookhaven National Laboratory . (1974) Part of the J / ψ meson
Bottom curd elementary (quark) 1973 1977: Fermilab , group led by Leon Max Lederman Part of the Υ meson
W bosons and Z boson elementary (gauge boson) 1968: Glass show , Weinberg , Salam 1983: CERN Properties confirmed in the 1990s
Top curd elementary (quark) 1973 1995 Lifetime is too short to be able to be detected directly in a hadron
Higgs boson elementary 1964: Peter Higgs et al. 2012: CERN Confirmed since 2014 at the latest

See also

literature

  • RP Feynman and S. Weinberg : Elementary Particles and the Laws of Physics: The 1986 Dirac Memorial Lectures . Cambridge University Press, 1987.
  • Brian Greene: The Elegant Universe . WW Norton & Company, 1999, ISBN 0-393-05858-1 .
  • Robert Oerter: The Theory of Almost Everything: The Standard Model, the Unsung Triumph of Modern Physics . Plume, 2006.
  • Bruce A. Schumm: Deep Down Things: The Breathtaking Beauty of Particle Physics . Johns Hopkins University Press, 2004, ISBN 0-8018-7971-X . .
  • Martinus Veltman: Facts and Mysteries in Elementary Particle Physics . World Scientific, 2003, ISBN 981-238-149-X .
  • GD Coughlan, JE Dodd and BM Gripaios: The Ideas of Particle Physics: An Introduction for Scientists. 3rd edition, Cambridge University Press, 2006. An undergraduate text for those not majoring in physics.
  • David J. Griffiths: Introduction to Elementary Particles . Wiley, John & Sons, Inc., 1987, ISBN 0-471-60386-4 .
  • Gordon L. Kane: Modern Elementary Particle Physics . Perseus Books, 1987, ISBN 0-201-11749-5 .

Individual evidence

  1. ^ Otto Klemperer: Electron Physics: The Physics of the Free Electron . Academic Press , 1959.
  2. ^ The Strange Quark
  3. ↑ SLAC-SP-017 Collaboration (JE Augustin et al.): Discovery of a Narrow Resonance in e + e - Annihilation. In: Physical Review Letters. Volume 33, 1974, pp. 1406-1408 ( online )
  4. ↑ E598 Collaboration (JJ Aubert et al.): Experimental Observation Of A Heavy Particle J. In: Physical Review Letters. Volume 33, 1974, pp. 1404-1406 ( online )
  5. ↑ CERN experiments report new Higgs boson measurements . cern.ch (June 23, 2014)