PlanetPhysics/Elementary Particles 2

This is a contributed topic on elementary particles and their interactions.

Brief History
Elementary particle physics is about a century old as it began with J. J. Thomson's discovery of the electron in 1897; the electron `remains' an elementary particle, whereas a few other particle were found to be composites of other, `elementary' particles as in the case of nucleons (proton and neutron), for example. Neutrons, neutrinos and positrons came about in 1930 though it took many more years to prove the existence of neutrinos; thus, the $$neutrino$$ ($$\nu_e$$) was not detected experimentally until 1953, but a four fermion interaction theory is not renormalizable. Yukawa succeeded in extending the electromagnetic (em) theory of radiation to the strong interactions, introducing a new type of field quantum-- the pion ($$\pi$$). The pion corresponds in nuclear physics to the photon of Electromagnetism, but it has zero spin and also a non--zero mass. Furthermore, the Yukawa theory is found to be renormalizable, although its field carrier took awhile to be discovered experimentally. Yukawa's idea of the nuclear exchange interactions remains valid even if many more nuclear particles have been discovered other than those predicted by his theory. Thus, after some initial confusion about the nature of the new particle discovered, a new fermion is identified, the $$muon^-, \mu^-$$. This initial confusion was that the massive $$muon$$ was thought at first to be Yukawa's predicted pion, but Conversi et al. in Rome succeeded in proving otherwise. The `real' pion was soon afterwards discovered confirming Yukawa's prediction, but somewhat surprisingly experimental evidence also emerged for the existence of strange particles which required the introduction of completely new conservation laws and additive quantum numbers.

Quantum field theory-- the `merging' of Lorentz invariance and quantum mechanics-- allows an adequate description of elementary particles and their interactions, although quantum chromodynamics (QCD) still falls short of many nyclear physicists' expectations.

The physicists who contributed in an essential way early in the last century to the discovery of three elementary particles : the electron $$e$$, the photon $$\gamma$$ and the proton $$p$$ were: J.J. Thomson, E. Rutherford, M. Planck, A. Einstein, Chadwick and W. Mosley; the proton however has lost its `elementary' status some 40 years ago. Furthermore, Heisenberg in his last published book argued against the use of the term `elementary' for any particle, but few have followed his suggestion in either the high-energy or the quantum theoretical physics camp.

Table 1.2: Elementary particles in 2003
Spin J valueSymbols---Generic name--Observed


 * $$0 \, \, \, \, \, $$ $$\, \, \, \, \, H$$Higgs scalar---No
 * $$1/2 \, \, \, \, \, \, \, $$ $$e^-,\mu,\tau,\nu_e,\nu_{\mu},\nu_{\tau}$$leptons--Yes
 * $$1/2 \, \, \, \, \, \, \, $$ $$u, d, c, s, t, b$$---quarks--yes
 * $$1 \, \, \, \, \, \, \, $$ $$\gamma$$--photon-yes
 * $$1 \, \, \, \, \, \, \, $$ $$g^i_j$$gluons (8)--yes
 * $$1 \, \, \, \, \, \, \, $$ $$W^{+,-} \, Z^0$$---vector bosons--yes
 * $$2 \, \, \, \, \, \, \, \, \, $$ $$\Gamma$$---graviton--no

All of the spin $$0, 1/2, 1, 3/2, 2...$$ Hadrons have become $$\overline{q}q$$ or qqq bound states. There are left over a total of: $$1+12+1+8+3=25$$ particles, plus, for fermions, their antiparticles (which in a quantum relativistic theory need not be counted separately).

[Entry in progress]