Classification of the Elementary Particles

Hyperons

Classification of the Elementary Particles

• Elementary particles are classified into groups according to their mass and spin properties. These are, referring to their masses:

• (1) the photon with zero rest mass and spin 1. It is a massless boson. • (2) the leptons or light particles. These are the electrons, muons and

neutrinos and their antiparticles, all with masses less than the pions and with spin 1/2. For reasons connected with statistical mechanics they are also called fermions. Leptons interact weakly with other particles.

• (3) the mesons or intermediate particles, so called because their masses are between those of the muons and the nucleons. They are the pions and the kaons and have zero or integral spin.

• (4) the baryons. These are the heavy particles of nucleon mass and above. 392 Hyperons have masses greater than the nucleons. The baryons are therefore the nucleons and the hyperons

• All baryons have half-integral spins. Mesons and baryons are strongly reacting particles , and collectively they are called hadrons. An important concept in all nuclear reactions is the conservation of spin angular momentum and, from a study of this applied to individual events, it impossible to assign a quantum number to each particle in terms of the unit h/2p ( ħ ). Baryons and leptons with half integral spins are called fermions while mesons with zero or integral spins are called bosons. Thus the muon (m -meson) is really a lepton with spin 1/2 and therefore a fermion , whereas the photon is a fundamental boson with spin 1. Based on these definitions it is possible to classify some 32 of these particles according to Table 26.1. Some particles are shown with their antiparticles which are distinguished by a bar over the symbol. This table is reproduced diagrammatically in Fig. 26.12, except that the muon neutrinos are omitted.

Mesic Atoms: The Muonium Atom• We have seen that a pion can be regarded as a nuclear

photon for nuclear structure calculations. Similarly, kaons can be regarded as photons associated with shorter-range forces than pions. This leads to the concept of a mesic cloud in the nucleus analogous to the electron cloud of the atom. These mesic clouds are converted to real particles when the proton is struck by a particle of sufficiently high energy to sweep away the meson cloud and cause a rearrangement of the residuals. This often requires baryon or meson collisions since these particles interact strongly within the range of the kaon and pion forces whilst leptons have only weak interaction.

• The interaction of negative muons with matter arises from their relatively long Lifetimes (1 ms). During this time they are rapidly slowed down to rest and are able to replace orbital electrons to produce electrically neutral atoms with m.- orbits.

• These are mesic atoms, and since mm > > me the mesic orbit has a very small radius compared with the electron orbit, as can be seen from the formula for the first Bohr radius

• In this case the p and d particles are held closer together by the comparatively small meson orbit and they eventually overcome their Coulomb repulsion to form 3He by

• • This reaction can only be explained quantum

mechanically since classically the

• Coulomb potential barrier is too high for the reaction to proceed from rest particles. The whole reaction can be written

• , • in which the muon is unchanged and may take the whole of the 5·49

MeV energy. Its role is therefore that of a catalyst. Any particular muon could repeat this reaction to give a catalyzed chain reaction. This has only a finite length partly due to the loss of energy to the g-radiation, but largely due to the muon decay. The above description refers to muons in orbit. What about the opposite picture, i.e., a muon acting as a nucleus to an electron? The muon would be m +, of course, and the electron would revolve around it. This 'atom', the muonium atom, has actually been found. Muonium is a lighter atom than the hydrogen atom, and the two particles do not annihilate, since e- and m +, are not antiparticles to each other. However, Muonium is unstable with the same lifetime as the muon, viz. 2·2 m s. This is the lightest 'atom‘ we know (the positronium 'atom' has no central core) and it is thought that a study of muonium will lead to a further understanding of the muon-electron problem.