Mesons

pion

Particle	Symbol	Anti- particle	Makeup	Rest mass MeV/c2	S	C	B	Lifetime	Decay Modes
Pion	π+	π-	ud	139.6	0	0	0	2.60 x10-8	μ+νμ
Pion	π0	Self		135.0	0	0	0	0.83 x10-16	2γ

The neutral pion decays to an electron, positron, and gamma ray by the electromagnetic interaction on a time scale of about 10^-16 seconds. The positive and negative pions have longer lifetimes of about 2.6 x 10^-8 s.

The negative pion decays into a muon and a muon antineutrino as illustrated below. This decay is puzzling upon first examination because the decay into an electron plus an electron antineutrino yields much more energy. Usually the pathway with the greatest energy yield is the preferred pathway. This suggests that some symmetry is acting to inhibit the electron decay pathway.

The symmetry which suppresses the electron pathway is that of angular momentum, as described by Griffiths. Since the negative pion has spin zero, the electron and antineutrino must be emitted with opposite spins to preserve net zero spin. But the antineutrino is always right-handed, so this implies that the electron must be emitted with spin in the direction of its linear momentum (i.e., also right-handed). But if the electron were massless, it would (like the neutrino) only exist as a left-handed particle, and the electron pathway would be completely prohibited. So the suppression of the electron pathway is attributed to the fact that the electron's small mass greatly favors the left-handed symmetry, thus inhibiting the decay. Weak interaction theory predicts that the fraction of muons decaying into electrons should be 1.28 x 10^-4 and the measured branching ratio is 1.23 +/- 0.02 x 10^-4.

The pion, being the lightest meson, can be used to predict the maximum range of the strong interaction. The strong interaction properties of the three pions are identical. The connection between pions and the strong force was proposed by Hideki Yukawa. 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.

Being composed of an up and an antidown quark, the positive pion would be expected to have a mass about 2/3 that of a proton, yet it's mass is only about 1/6 of that of the proton! This is an example of how hadron masses depend upon the dynamics inside the particle, and not just upon the quarks contained.

The pion is a meson. The π+ is considered to be made up of an up and an anti-down quark. The neutral pion is considered to be a combination

Pions interact with nuclei and transform a neutron to a proton or vice versa:

The pions π⁺ and π^- have spin zero and negative intrinsic parity (Rohlf Sec 17-2).

The psi/J Particle

The psi/J particle is a meson which was discovered in 1974 by experimenters at Stanford (Richter) and Brookhaven National Laboratory (Ting). Slightly more than three times as massive as the proton, this particle decayed slowly and didn't fit into the framework of the up, down, and strange quarks. It is considered to be a charm-anticharm quark pair and was the first firm experimental evidence for the fourth quark. Richter and Ting shared the 1976 Nobel Prize for their discovery.

The Upsilon Particle

The upsilon particle is a meson which was discovered at Fermilab in 1977. It appeared as another long-lived particle which didn't fit into the framework of the first four quarks, the up,down, strange, and charm quarks. It is taken as a botton-antibottom quark pair and was the first experimental evidence of the fifth quark.

Hadrons

Particles that interact by the strong interaction are called hadrons. This general classification includes mesons and baryons but specifically excludes leptons, which do not interact by the strong force. The weak interaction acts on both hadrons and leptons.

Hadrons are viewed as being composed of quarks, either as quark-antiquark pairs (mesons) or as three quarks (baryons). There is much more to the picture than this, however, because the constituent quarks are surrounded by a cloud of gluons, the exchange particles for the color force.

Recent experimental evidence shows the existence of five-quark combinations which are being called pentaquarks.

Baryons

Baryons are massive particles which are made up of three quarks in the standard model. This class of particles includes the proton and neutron. Other baryons are the lambda, sigma, xi, and omega particles. Baryons are distinct from mesons in that mesons are composed of only two quarks. Baryons and mesons are included in the overall class known as hadrons, the particles which interact by the strong force. Baryons are fermions, while the mesons are bosons. Besides charge and spin (1/2 for the baryons), two other quantum numbers are assigned to these particles: baryon number (B=1) and strangeness (S), which in the chart can be seen to be equal to -1 times the number of strange quarks included.

The conservation of baryon number is an important rule for interactions and decays of baryons. No known interactions violate conservation of baryon number.

Recent experimental evidence shows the existence of five-quark combinations which are being called pentaquarks. The pentaquark would be included in the classification of baryons, albeit an "exotic" one. The pentaquark is composed of four quarks and an antiquark, like a combination of an ordinary baryon plus a meson.

Hideki Yukawa and the Pion

Once quantum electrodyamics had produced the picture of the electromagnetic force as a process of exchanging photons, the question of whether or not the other forces were also exchange forces was a natural one. In 1935, Hideki Yukawa reasoned that the electromagnetic force was infinite in range because the exchange particle was massless. He proposed that the short range strong force came about from the exchange of a massive particle which he called a meson. By observing that the effective range of the nuclear force was on the order of a fermi, a mass for the exchange particle could be predicted using the uncertainty principle. The predicted particle mass was about 100 MeV. It did not receive immediate attention since no one knew of a particle which fit that description.

In 1937 a particle of mass close to Yukawa's prediction was discovered in cosmic rays by Anderson & Neddermeyer and by Street & Stevenson in independent experiments. This particle, the muon, turned out not to interact by the strong interaction. Hans Bethe and Robert Marshak predicted that the muon could be a decay product of the particle sought. In 1947, Lattes, Muirhead, Occhialini and Powell conducted a high altitude experiment, flying photographic emulsions at 3000 meters. These emulsions revealed the pion, which met all the requirements of the Yukawa particle.

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.