The discovery of sub-atomic particles
The discovery of anti-matter
Particle Accelerators
Hadrons
Baryons
Mesons
The Standard Model
Fermions
Quarks
Leptons
Fundamental Forces and Force Carrying Particles
Photons (y) – The Electromagnetic Force
Gluons (g) – The Strong Force
Z and W Bosons - The Weak Force
Gravity and the Higgs Boson
The discovery of sub-atomic Particles
By the Late 19th and Early 20th Centaury scientist became aware that although all matter was made from atoms, atoms themselves were made up of sub atomic particles and the first of these to be discovered was the electron by J.J. Thomson in 1897. Later Ernest Rutherford performed a series of experiments between 1909 and 1911 which suggested that atoms also contained small, dense and positively charged nuclei. By 1930 it had been widely accepted that matter was actually made up of three different sub-atomic particles that could be arranged in various ways to produce the elements of the periodic table. Further more the pattern found in the periodic table could now be explained. As well as the known three sub atomic particles scientist had also become aware of three fundamental forces that governed all matter. Gravity and its affects were widely known, Electromagnetic force was another and a third known as the strong force. This force was what held the nucleus together and had to be strong to be strong enough to overcome the electrical force that respells the positively charged protons in a nucleus. Three sub-atomic particles and three fundamental forces seemed elegant and sat very well with scientists of the day. But this model didn’t last for long.
In explaining the “photoelectric effect” (for which Einstein received his first Nobel Prize) Einstein suggested that light travels in little packets or Quanta (from which quantum physics was born) called photons. In 1925 the French Physicist Louis de Broglie showed that this would require particles of matter to travel in waves and when this was applied to electrons by Physicist Paul Dirac he found that it could only work with the existence of a positively charged electron. Dirac’s equation (that earned him the Nobel Prize in 1933) could only work if there was a particle identical to the electron in mass but instead of having a negative charge it must have an opposite positive charge. Einstein’s special theory of relativity, which showed that mass and energy were interchangeable and therefore mass had an energy equivalent and vice versa, combined with quantum theory, meant that all the sub-atomic particles must have opposites. The existence of antimatter was discovered and the elegant model of matter was lost.
When matter and anti matter meet they annihilate one another and produce high energy photons in the form of Gamma Rays. It is therefore very difficult to create antimatter for as soon as it is created it is annihilated.
Another area of science that found many more types of particles was the discovery of cosmic rays. Many scientists used balloons to experiment on this new phenomenon and in 1937 a heavy electron was discovered. It had the mass of more than 200 electrons but was also negatively charged and is now known as a ‘muon’. In 1947 Cecil Powell discovered ‘Pions’.
Rather than waiting for their detectors to find exotic particles in cosmic rays, scientist would make them themselves.
They began creating machines that would accelerate charged particles and make them collide at colossal speeds. These machines are known as particle accelerators and the most famous of these is the Large Hadron Collider (LHC) which accelerates particle around a 27km ringed chamber making them reach just 3 meters per second slower than the speed of light. The LHC is the largest of these particle accelerators but earlier models had given scientist many new particles to consider. They were quite often referred to as a particle zoo because of their number and variety. It was not as simple as finding these particles, scientist wanted to find a way to understand and categorize them.
When two baryons such as protons collide with enough force a new particle such as a pion was produced. This was because the kinetic energy of the first particle before it hit another was greater than the kinetic energy of the struck particle. This energy ‘loss’ was returned as matter (keeping with Einstein’s E=mc2) and this new matter would sometimes be a new particle. In fact the more force given to the first particle would create a larger ‘loss’ of energy and create more exotic particles.
The first thing they noticed about these new particles was that there were two distinct types Hadrons and leptons.
Hadrons are sub-atomic particles that are affected by the ‘strong force’. This includes particles such as protons (p+), neutrons (n0) or pions (p+, p0, p-). The only stable baryon is the proton. Even neutrons that are not bound within a molecule will decay (free neutrons have a half life of 15 minutes). Protons are the least massive of all baryons and this means it has the lowest rest mass/energy making it the most stable of all baryons.
Hadrons are either Baryons or Mesons.
In collision reactions with Baryons it was found that the electrical charge is conserved and also the amount of baryons before the collision is the same after. For example in a collision with two protons colliding (as below) an extra meson may be created but no extra protons
p + p à p + p + p0
In some cases in a collision between two baryons can cause an additional baryon but it will always be accompanied by its anti-baryon which annihilates the additional copy (as below)
p + p à p + p + p + p-
So in Baryon collision reactions the electrical charge is conserved and the number of baryons (or Baryon Number) is also conserved.
Examples of Baryons
Name
Symbol
Proton
p+
Neutron
n0
Delta-plus
D+
Delta-zero
D0
Delta-minus
D-
Sigma-plus
S+
Sigma-zero
S0
Sigma-minus
S-
Lambda
L
The other Leptons and these particles were not affected by the strong force and they included electrons, neutrinos and muons.
In collision reactions the number of messons is not conserved.
Examples of Messons
Name
Symbol
Phi
F
Pi-plus
p+
Pi-zero
p0
Pi-minus
p-
Kay or Kappa-plus
K+
Kay or Kappa-zero
K0
Kay or Kappa-minus
K-
The three sub-atomic particles that make up an atom comprising of Protons and neutrons at the centre with electrons orbiting around the outside is well known and adequate to explain much of chemistry but the standard model goes deeper to categorize what these particles are, the forces that act on particles and what they themselves are made of. All matter is made up of 12 fundamental particles and governed by four fundamental forces. For example Protons and Neutrons are a type of particle known as a Baryon and these are made up of three smaller particles called quarks.
Fermions is the name given to a groups of these sub-atomic particles which make up matter and include Quarks and Leptons.
In 1964 American physicist Murray Gell-Man suggested that the observations from hadron reactions was because hadrons were not fundamental particles but were themselves made up of smaller particles which he named quarks.
These particles are affected by the strong force and include neutrons, protons and pions.
There are six deferent types of quarks and these are known as “flavours” (because they were originally named after flavours of ice-cream). The flavours are Up (u), Down (d), Charm (c), Strange (s), Top (t) and Bottom (b). The quarks flavours are paired into “generations” as shown in the diagram below.
Generation One
Up and down quarks are in the first generation and are the lightest of the quarks. These quarks make up all stable matter in the universe such as protons and neutrons.
Generation Two
Particles made from these quarks only last for a short time and are made up of heavier elements not normally found in nature. Once created they will decay quickly into first generation particles.
Generation Three
Particles made of top and bottom quarks are the heaviest of all and will decay even quicker, first into second generation and then into the stable first generation particles.
To make baryons three quarks from a single generation are combined, but to make mesons only two quarks from different flavours or even quarks and anti-quarks are used.
Here are some more examples of how quarks are combined to make particles.
Example of baryons
Baryon
Symbol
Quarks
Proton
p+
u, u, d (up, up ,down)
Neutron
n0
u, d, d (up, down, down)
Delta-plus
D+
u, u, d (up, up ,down)
Delta-zero
D0
u, d, d (up, down, down)
Delta-minus
D-
d, d, d (down, down, down)
Sigma-plus
S+
u, u, s (up, up, strange)
Sigma-zero
S0
u, d, s (up, down, strange)
Sigma-minus
S-
d, d, s (down, down, strange)
Lambda
L
u, d, s, (up, down strange)
Examples of mesons
Meson
Symbol
Quarks
Phi
F
s, s¬ (strange, anti-strange)
Pi-plus
p+
u, d¬ (up, anti-down)
Pi-zero
p0
u, u¬ or d, d¬ (up, anti-up or down, anti-down)
Pi-minus
p-
d, u¬ (down, anti-up)
Kay or Kappa-plus
K+
u, s¬ (up, anti-strange)
Kay or Kappa-zero
K0
d, s¬ (down, anti-strange)
Kay or Kappa-minus
K-
s, u¬ (strange, anti-up)
Leptons are another type of matter particle with the most commonly known being the electron. Leptons are not affected by the strong force but are affected by the electromagnetic and weak forces. Examples of these include electrons, neutrinos and muons.
Leptons are setup similarly to quarks in that they also contain generations.
Generation 1
This pair consists of the electron and electron neutrino and these are the lightest of the generations.
Generation 2
The muons are about 200 times more massive than the electrons
Generation 3
The tauon particles are 3,500 times more massive than the electron.
Also the electron, muon and tauon have a charge of -1 while their neutrino pair have no charge at all.
Fundamental Forces and Force Carrying Particles
Photons (y) – The Electromagnetic Force
This particle is responsible for the electromagnetic which affects charged particles such as protons and electrons. It is the attractive and repulsive force as found in magnetism and electrical charges. Electromagnetic force also determines the atomic structure which allows certain atoms to bind together and create molecules.
Gluons are mass less particles responsible for the strong force which binds quarks together to form neutrons and protons. It also binds neutrons and protons together in the nucleus of atoms. The strong force is the strongest of all the fundamental forces and is 102 times stronger than the electromagnetic force. But the strong force has a very limited range extending only as far as the nucleus on an atom which is 10-15m.
Z and W Bosons - The Weak Force
The weak force is about 1010 times weaker than the electromagnetic force and has a shorter range than the strong force. Sometimes known as the weak nuclear force because it is affects are most prominent in nuclear reactions. A neutron in an unstable isotope is made more stable by the weak force which changes one of the quarks (neutron = d, d, u and proton = u, u, d) making the neutron into a stable proton (as previously stated a proton has the least mass and is the most stable of all baryons. Free neutrons have a half life of 15 minutes but are made stable when part of stable nuclei). As the mass of a neutron is greater than a proton the ‘lost’ mass produces a high energy, negatively charged, electron in a process called beta decay.
The force of gravity is probably the most well known of the fundamental forces but it has not yet been explained in the standard model. It is the weakest of all the forces at about 1040 times weaker than the strong force, but it has got an infinite range.
The Higgs Boson was the theoretical particle (proposed in 1964) responsible for the mass of matter and was discovered on the 4th of July 2012 at the Large Hadron Collider. Newton stated that gravity is equal to the mass of the two bodies divided by the distance between their centres and this works with matter that is not moving at relatively low speeds but Einstein showed that this is different when the two bodies are travelling at speeds closer to the speed of light (or the cosmic speed limit which light travels at).
It may be possible that superstring theory will be able to unify gravity with the other fundamental forces, completing the standard model and giving us a consistent quantum theory of gravity.
