Sub-Atomic Particles
An Introduction to the particles that make up matter
Elementary Particles
As described in the Quantum Mechanics essay, matter is made up of molecules and these, in turn are made up of atoms. A typical atom consists of a nucleus composed of positively charged protons and neutral neutrons surrounded by a cloud of orbiting negatively charged electrons.
This model was first postulated by Ernest Rutherford in 1913. At the time, it was thought that all matter consisted of these three particles. They were referred to as elementary particles. These particles are tabulated below.
Property
Electron
Proton
Neutron
Symbol e- p+ n
Mass (kg) 9.109 × 10-31 1.673 × 10-27 1.675 × 10-27
Mass (MeV) 0.51 938.2 939.6
Electric Charge -1 +1 0
In 1928 Paul Dirac predicted that all particles should have opposites called anti-particles. The first of these was discovered in 1932 by Carl Anderson. This was an electron with a positive electric charge (+1). This particle is the anti-electron (also called a positron). It is identical in every respect to the electron apart from its electric charge. When an electron and positron come into contact, they mutually annihilate each other producing a flood of energy in accordance with Einstein's famous equation,
E = mc2.
Both the proton and the neutron have anti-particles. These also destroy each other if they meet with their particle. Ordinary matter is made up from particles. It appears that the Universe is made up of ordinary matter.
Matter composed of anti-particles is known as anti-matter. Anti-matter can be created in the laboratory but does not last long as it quickly comes into contact with ordinary matter and is destroyed.
It is now known that there are many more elementary particles than the six described so far. These have been created using modern high-technology equipment. These have been divided into a number of groups depending on their properties. Most of these newly discovered particles have their anti-particles.
Leptons
The electron (e) is the simplest of the leptons. There are two heavier leptons called the muon (m) and the tau (t). Both are unstable and decay to simpler, more stable particles. Both have anti-particles. Muons are found in the air as cosmic rays enter the Earth's atmosphere and smash into atoms and molecules.
Another type of lepton is the enigmatic neutrino (n). This was postulated in 1934 by Enrico Fermi to explain certain aspects of radioactive decay. There are three types of neutrino, each one associated with one of the three lepton described above (e, m, t). They are called the electron neutrino (ne), muon neutrino (nm), and tau neutrino (nt).
Neutrinos hardly react with other types of matter. They can easily pass through the Earth. They have no electric charge. Each one has its anti-particle version so there are six types of neutrinos. Neutrinos have a very low mass and one type can change into one of the other two types.
Leptons are never found in the nucleus of atoms. They are not subject to the Strong Nuclear Force which keeps the nucleus from flying apart. They are sometimes produced in the nucleus but are quickly expelled. Some radioactive atoms break down by a method called beta decay. During beta decay a neutron in the nucleus breaks down to give a proton (which remains in the nucleus), an electron (which flies out and causes the radioactivity of the atom) and an electron neutrino (which departs at the speed of light and is not usually detected). The atom changes to a new one since the number of protons (the Atomic Number) increases by one. Atomic Number is explained in The Elements. The reaction is shown below.
n ---> p+ + e- + ne
The six leptons are tabulated below.
Name of Lepton Symbol Mass (MeV)
Electron e 0.511
Electron Neutrino ne ~ 0
Muon m 106
Muon Neutrino nm ~ 0
Tau t 1,777
Tau Neutrino nt ~ 0
Baryons
The two most common baryons are the proton and neutron. They are both of similar mass but the proton has a single positive charge. They are collectively known as nucleons. Both are found in the nuclei of atoms, being kept there by the Strong Nuclear Force that binds them together.
In recent years it has been suggested that baryons are made up of even more elementary particles called quarks. Quarks are found in six types (called flavours). In 1989 it was shown that only three pairs of quarks can exist. These correspond with the three leptons and the three neutrinos.
Quarks are unusual in having fractional electric charges.
Name of Quark
Symbol
Charge
Mass (MeV)
Up u +(2/3) 2 - 8
Down d -(1/3) 5 - 15
Strangeness s -(1/3) 100 - 300
Charm c +(2/3) 1,000 - 1,600
Bottom (or Beauty) b -(1/3) 4,100 - 4,500
Top (or Truth) t +(2/3) 180,000
Baryons are made up of quark triplets. The proton is composed of two u quarks and a d quark. These quark charges of
+(2/3) +(2/3) -(1/3)
add up to the proton's charge of +1.
The neutron is made from two d quarks and a u quark. These quark charges of
-(1/3) -(1/3) +(2/3)
add up to the neutron's charge of 0.
The proton and neutron are stable particles in the most nuclei. Outside the nucleus or in certain unstable nuclei, neutrons decay as shown above.
There exist other baryons, produced in high energy experiments, that are less stable. These too are made up of quark triplets. Hundreds of these particles are known. Some of them are tabulated below.
Baryon Particle Quark Triplet Charge
p (proton) uud +(2/3)+(2/3)-(1/3) = +1
n (neutron) udd +(2/3)-(1/3)-(1/3) = 0
D- ddd -(1/3)-(1/3)-(1/3) = -1
L0 uds +(2/3)-(1/3)-(1/3) = 0
S+ uus +(2/3)+(2/3)-(1/3) = +1
W- sss -(1/3)-(1/3)-(1/3) = -1
C1++ cuu +(2/3)+(2/3)+(2/3) = +2
All six quarks have their anti-quarks with charges opposite in value to their quark counterparts. The (u) anti-quark has a charge of -(2/3) while the (d) anti-quark has a charge of +(1/3). The anti-proton is made up of (u)(u)(d) and has a charge of -1.
Mesons
Mesons are particles only discovered when the forces binding nucleons together were investigated. In a nucleus, the protons and neutrons are not really separate entities, each with its own distinct identity. They change into each other by rapidly passing particles called pions (p) between themselves. Pions are the most common of the mesons.
Mesons are composed of quarks. Mesons are composed of a quark / anti-quark pair. The positive pion (p+) is made from a u quark and and a (d) anti quark. The negative pion (p-) is made from a d quark and a (u) anti quark.
Some of the many known mesons are tabulated below.
Meson Particle
Quark Pair
Charge
p+ (positive pion) u(d) +(2/3)+(1/3) = +1
p- (negative pion) (u)d -(2/3)-(1/3) = -1
K0 (neutral kaon) d(s) -(1/3)+(1/3) = 0
f s(s) -(1/3)+(1/3) = 0
D- d(c) -(1/3)-(2/3) = -1
J (or j) c(c) +(2/3)-(2/3) = 0
Kaons are short lived mesons that decay into simpler particles. Normally, particles and anti-particles decay in a similar way. The example below shows the decay of the neutron and the anti-neutron.
n ---> p+ + e- + ne
(n) ---> p- + e+ + (ne)
The decays are mirror images of each other. Kaons are unique in that the matter and anti-matter forms occasionally decay in slightly different modes. This is referred to as a breakdown of a property called parity.
This breakdown of parity conservation may account for the fact that the Universe is mainly matter rather than a 50-50 mixture of matter and anti-matter. A mixed matter Universe would not last long as the matter and anti-matter would destroy each other.
Forces
All of the above particles are referred to as Fermions. Particles have a property called spin. The spin of Fermions has half-integer values (1/2, 3/2, etc). Because of this type of spin, Fermions obey the Pauli Exclusion Principle. This means that two fermions cannot occupy the same energy states. With electrons this gives rise to atoms whos electrons are distributed in shells. These shells give atoms their differing chemical properties.
There is another type of particle. These are called Bosons. Bosons have integer spin (0, 1, 2). Bosons do not obey the Pauli Exclusion Principle. The best known Boson is the massless photon, a quantum of light.
Bosons are known as the force carriers. When two particles interact they exchange a Boson. The photon is the force carrier for the Electromagnetic Force. Three bosons (W+, W- and Z0) carry the Weak Nuclear Force. This is the force responsible for beta decay.
Various gluons carry the Strong Nuclear Force. Some people suggest the existence of a graviton to carry the Gravitational Force.
New theories called Superstrings, Twisters and M Theory are attempting to link relativity (especially gravity) and predict the properties of all the sub-atomic particles and the forces of nature.
Note: 1MeV = 1 million electron volts = 1.6 × 10-13J = 1.8 × 10-30kg