The Hadrons and Quarks


Mesons are a class of particle that are particle that have a spin of zero and that are subject to the strong force within the atom. The title was originally meant for the Muon but, because it was found to be a Lepton, the title now means a particle that is similar to Baryons but have varying masses. These particle do not have a conservation of Meson number and can therefore can spontaneously appear and disappear.

The first Mesons are the pi mesons, known as the pion, of which there are three types. There is the positive pion with the same positive charge as the proton or positron, the antipion with the same charge as the electron and the antiproton, and the neutral pion. The pion breaks down into a muon and muon antineutrino while the antimuon breaks down into an antimuon and muon neutrino. This gives a nice net of zero in terms of the muon number. The neutral pion in interesting in that has on 29/30 the mass of the charged pions and an average lifetime of only a millionth of a billionth of a second. When it decays it produces two gamma rays whose energy are proportional to its mass using E=mc˛.

The Baryons and Mesons are combined by physicists into a category called the Hadrons. There were originally seven; the proton, neutron, their antiparticles, and the pions, but as time passed more and more were discovered. In 1944, the K Meson or Kaon was discovered with a mass approximately half that of a proton. Then the heavier Hadrons, the Hyperons, were discovered. Over time it seemed there would be hundreds and hundreds of Hadrons, which was playing havoc with the overall simplicity that these physicists were searching for in the universe. The first hint of order came in the form of an idea that many of the particles could be considered the same but in a different spin, called isospin. This was vital because it gave order to the zoo of Hadrons and it was conserved. The next breakthrough was how strange a particle was. When doing the computations of the Kaons and Hyperons they found that these particles lasted thousands of times longer than they should. Therefore, these were given a strangeness number while the protons, neutrons, and pions had strangeness numbers of zero. Next came the property of parity but that was not always conserved and the model began to have trouble. Then, in 1963, Gell-Mann, the physicist who had proposed strangeness published the theory that all the Hadrons were composed of even smaller particles, similar to the relationship of protons to atoms. He made a number of major statements with his theory. He assigned the Mesons two sub-particles and the Baryons three. He said there were three main types of these sub-particles, quarks, and each had an opposite for antiparticles. He named these sub-particles up, down and strange. Finally, for the first time in the history of physics, he gave particles fractional charges.

Particle Charge
Up +2/3
Down -1/3
Strange -1/3

These quarks are combined to form the basic sub-atomic particle and the various Baryons and Mesons.

Particle Combination Sum of charges
Proton 2 Up Quarks
1 Down Quark		 +2/3+2/3-1/3= +1
Antiproton 2 Up Antiquarks
1 Down Antiquark		 -2/3-2/3+1/3= -1
Neutron 1 Up Quark
2 Down Quarks		 +2/3-1/3+1/3= 0
Antineutron 1 Up Antiquark
2 Down Antiquarks		 -2/3+1/3+1/3= 0
Positive Pion 1 Up Quark
1 Down Antiquarks		 +2/3+1/3= +1
Negative Pion 1 Up Antiquark
1 Down Quark		 -2/3-1/3= -1

Some physicist about thirty years ago thought that if the Quarks were to be the other set of fundamental particles (with the Leptons) then there should be symmetry between the Leptons and the Quarks in terms of their number. With this in mind physicists in 1974 found the Charmed Quark, the counterpart to the Strange Quark. In 1978 the Bottom Quark was discovered, (Asimov 284) and in 1994 the Top Quark was found at the Fermilab Collider Detector. (Fermilab)

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