The Particles

Close to end of the 1920 decade and early the next one, the image of the constitution of matter was given by the atomic theory. This image changed when C. Anderson discovered, in 1932, the electron with positive electric charge or positron. This discovery was a affair, even when the relativistic quantum mechanics -the union of the quantum mechanics and the special theory of relativity- predicted it. It came to start a series of discoveries that changed the image on the constitution of matter. First because the discovery opened the possibility to the physicists could think that in nature antiparticles are produced in general; second for its technological use let the physicists reach higher energies and new purer states of matter. For example, the physicists invented the positron-electron collider and discovered the lepton $\tau$ and other states produced abundantly in the collision $e^-e^+$. We will comment briefly, in the Section 4, about the applications of $e^+$ in medicine.

H. Yukawa, in 1935, predicted the meson $\pi$. This was the first particle that had its own theoretical settings. It was, at that time, the carrier of the strong nuclear forces.

In the cosmic rays the physicists discovered in 1936 the lepton $\mu$. Also they discovered in the cosmic rays, during the decade of 1940, the baryon $\Lambda^0$. In that epoch, the physicists had not the theoretical frame for those particles. No one knew nor understood why those particles pop off from the cosmic rays. For that the name of strange. The groups of strange particles comprehend the $\Lambda^0$ group, the $\Sigma$ group, $K$ group, and the $\Xi$ group. Their production always is associated. For example, always that there appears a $\Lambda^0$ there appears another strange particle, could be a $K^+$. All the strange particles decay via weak interaction; the strange resonances do it through strong interaction. The mean life are of the order of $10^{-8}$ seconds for the particles and of the order of $10^{-23}$ seconds for the resonances. The physicists produced them in the modern accelerators. And they study them in controlled situations. In that situations the physicists study the mechanisms of creation and the physical properties of the particles and resonances. The physicists easily measure the masses, the decay channels, the mean lifes, the spins, the electric charges, etc.

In the decade of 1930 Pauli proposed the existence of neutrinos, basing his arguments purely in the conservation of energy and of angular momentum in nuclear disintegrations. Fermi, in 1933, incorporated the neutrino to the theoretical frame of the beta disintegrations. In that channel, the associated neutrino to the electron was detected by Reins and Cowand in 1954, after of very intensive investigation. Their technique consisted in observing the reaction $\nu p \to ne^+$ (inverse beta decay).

In 1962, the physicists sought, deliberately, and found a second class of neutrino. The neutrino associated to the lepton $\mu$. This, even it is produced in the cosmic rays, the physicists created it using the accelerators of particles. And detected it in the same way that detected the neutrino associated to the electron. The $\mu$ neutrino is an experimental evidence.

The neutrinos are not created alone. They are created always that the charged leptons associated are created. For example, when an electron is created alone in a hadronic reaction, always is created the neutrino associated with the positron. This production follows the conservation of the leptonic number.

In the decade of 1960, the physicists discovered the resonances $\Omega$, $\Xi$, $\Sigma$, etc. These hadronic states are created by strong forces; and decay by action of these. The mean life, for these states, is of the order of $10^{-23}$ seconds, as we have stated above. The physicists have detected many of those resonant states, and excited states of those.

In the decade of 1960 it was discovered that neutrinos -the one associated with the electron and the one associated with the muon are distinguishable-. That is, do not give origin to the same reaction in the final state.

In 1974 S.C.C. Ting and B.D.Richter discovered the resonance $J/\Psi$. And in 1978 L. Lederman and co-workers discovered the resonances $\Upsilon$, $\Upsilon$', and $\Upsilon$''. In that decade the physicists discovered the lepton $\tau$. Was evident that there must exist a third neutrino; the neutrino associated with the lepton $\tau$. The physicists indirectly detected it in 1997, at Fermilab.

Actually the physicists register close to 400 states, that they call particles, resonances, etc. The complete list is given in the Particle Data Book. See References.