Pions
A pion (or pi-meson) is one of the particles that carry the strong nuclear force between protons and neutrons (and other hadrons), which binds an atomic nucleus together. Pions are denoted + , - , 0 , depending on charge.
Pions were predicted to exist in 1935 by the Japanese theoretical physicist Hideki Yukawa when, in the first paper he ever published, Hideki proposed there was a new force involved in holding the nucleus together. This new force was overcoming the positive charge on each proton that was trying to blow the nucleus apart. Eventually this force came to be known as the strong nuclear force. Yukawa predicted that this new force was carried by a massive particle with only a very short range (about 10 cm). Hideki predicted the mass of this new particle would be about 300 times that of the electron, or about 150 MeV. To account for all the possible interactions between nucleons (protons and neutrons), the particle would have to exist in three charge states: positive, neutral, and negative.
The charged pions should have been observable in cloud chambers. In 1936, American physicist Carl Anderson, discoverer of the positron, proving the existence of antimatter) utilized the cloud chamber to discover a new charged particle with mass between that of the electron and the proton. Anderson called this particle the mesotron, from the Greek, meaning middle. Yukawa claimed this particle as his strong force carrier. As it turned out, the particle was actually a muon.
Pions are actually the products of primary cosmic rays colliding with Earth's atmosphere where vast numbers of positive, negative, and neutral pions are produced. The charged pions decay within 10 seconds to positive and negative muons, which then decay into positrons or electrons. English physicist Cecil Powell and his group finally discovered the charged pions on a mountaintop in 1947. By gaining altitude they had reached a level where the pion had not yet decayed.
Pions create a web of force which holds the nucleons together. They are not classified with the W and Z particles or the photon, however, because they are not elementary particles. Pions are made up of quarks and transmit the strong force at the larger nuclear level, whereas at the deeper level of quarks the strong force is transmitted by gluons.
In 1949, the neutral pion was the first subatomic particle to be discovered with the aid of an accelerator. A neutral pion decays into two energetic photons gamma rays, each of which then decays into a positron and an electron. The neutral pion and gamma rays, however, left no tracks in the cloud chamber. Using new synchrocyclotron and sophisticated electronic methods, the existence of the neutral pion was later confirmed, and the pion family was complete.
According to contemporary models the pion has three varieties. The pi-zero (zero electric charge) has a mass of 135 MeV, and the pi-plus and pi-minus (with one unit of positive and negative charge respectively) have a mass of 140 MeV. The extra mass and angular momentum of the pi-plus and pi-minus is accounted for by the giving off of neutrinos during their decay.
Pions are bosons with zero spin. The pi-zero is made up of a quark doublet of an up/antiup quark pair, or a down/antidown quark pair (it can be made up of either combination). The pi-minus is a down/antiup pair, and the pi-plus is an up/antidown quark pair.
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