Download Classification of the Elementary Particles

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 muonelectron problem.