Since the negative pion has spin zero, the electron and antineutrino must be emitted with opposite spins to preserve net zero spin. The symmetry which suppresses the electron pathway is that of angular momentum, as described by Griffiths. This suggests that some symmetry is acting to inhibit the electron decay pathway. Usually the pathway with the greatest energy yield is the preferred pathway.
Quark and lepton plus#
This decay is puzzling upon first examination because the decay into an electron plus an electron antineutrino yields much more energy. The negative pion decays into a muon and a muon antineutrino as illustrated below. The positive and negative pions have longer lifetimes of about 2.6 x 10 -8 s. The decay is by the electromagnetic interaction on a time scale of about 10 -16 seconds.
The neutral pion decays to two photons (gamma rays) 98.8% of the time. We now know that the pion is a meson, a composite particle, and the current view is that the strong interaction is an interaction between quarks, but the Yukawa theory stimulated a major advance in the understanding of the strong interaction and exchange forces in general. Yukawa worked out a potential for the force and predicted its mass based on the uncertainty principle from measurements of the apparent range of the strong force in nuclei. The connection between pions and the strong force was proposed by Hideki Yukawa. The strong interaction properties of the three pions are identical. The pion, being the lightest meson, can be used to predict the maximum range of the strong interaction. There was a recent claim of observation of particles with five quarks ( pentaquark), but further experimentation has not borne it out. Mesons are bosons, while the baryons are fermions. Three quark combinations are called baryons. Mesons are intermediate mass particles which are made up of a quark-antiquark pair.
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