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## Types of particles

The Universe can be divided into three types of particles: matter, anti-matter and radiation (in the modern Quantum-mechanical view of Nature, radiation can also be treated as particles). Anti-matter is not just a science fiction idea, it was first proposed by Paul Dirac in the 1920s and is made every day in particle accelerators as well as in Nature. Today we can even make anti-hydrogen atoms. Clearly what we see in the Universe is composed of matter, not anti-matter. When matter and anti-matter come together they annihilate each other, producing lots of radiation in the form of high energy gamma rays. In a future blog I will discuss the ideas physicists have as to why our Universe seems to have more matter than anti-matter (if the amounts were exactly balanced all the matter and anti-matter would have mutually annihilated and there would be no matter left in the Universe, and hence no “us”).

The Universe is divided into matter, anti-matter and radiation.

## The discovery of atoms

The word “atom” comes from the Greek word “atomos” which means “indivisible”. The idea of atoms thus dates back a couple of thousand years, but it was only in the 19th Century that evidence for their existence was really found. Through the work of John Dalton and others in the field of Chemistry, strong evidence was established that matter was composed of elementary building blocks, with each element being a different building block with different chemical properties. The Periodic Table of the elements was drawn up in the mid 1800s, and by the end of the 19th Century scientists had measurements of the masses of different elements, noting that e.g. Carbon was more massive than Hydrogen.

The first sub-atomic particle to be discovered was the electron, by J.J. Thomson in 1897. Then, in a series of experiments in 1909-10 the atomic nucleus was discovered by Ernest Rutherford and co-workes. Thus the modern picture of the atom emerged, negatively charged electrons in orbit around a positively charged nucleus. This is the so called “solar system model” because of its similarity to our Solar System. By the early 1930s it was known that the nucleus consisted of positively charged protons and of neutrons, which have no electrical charge.

The “solar system” model of the atom has the electrons orbiting the nucleus.

## The particle zoo

In the 1950s particle accelerators were used to probe the structure of matter. Initially electrons were accelerated to close to the speed of light, and smashed into stationary targets. As accelerators got more powerful physicists started accelerating protons, which are nearly 2,000 times more massive than electrons and hence much harder to accelerate. Physicists found a plethora of particles emerging from these particle accelerator collisions. Below is a picture of particle tracks in a typical bubble chamber, the device used for detecting these sub-atomic particles.

In the 1950s hundreds of new particles were being created in particle accelerators.

Physicists gave names to these new particles, sigma particles, pions, rho particles, D particles, kaons etc. So many new particles were being created in these experiments that physicists started running out of names for them. Some patterns started emerging. One was that particles could be divided into either hadrons (from the Greek word “hadròs” meaning “stout, thick”) or leptons (from the Greek word “lepton” meaning “fine, small, thin”).

Matter can be divided into hadrons (heavy particles) and leptons (light particles)

## Three quarks for Muster Mark

In the 1960s theoreticians tried to find a model which could be used to explain these hundreds of particles and the division into hadrons and leptons. It was Murrray Gell-Mann of Caltech who came up with the idea that the hadrons were composed of more fundamental particles which he called quarks. The word comes from a line in Finnegans Wake, a book written by James Joyce.

Three quarks for Muster Mark!
Sure he has not got much of a bark
And sure any he has it’s all beside the mark.

Initially Gell-Mann proposed three quarks as sufficient to explain all the observed hadrons, these three he called up, down and strange. However, we now believe we need an additional 3, making 6 quarks in all, to explain all hadrons. The names of the other 3 are charm, top and bottom.

The 6 quarks believed to constitute all hadrons
Name Generation Year proposed Year discovered
up 1st 1964 1968
down 1st 1964 1968
strange 2nd 1964 1968
charm 2nd 1970 1974
bottom 3rd 1973 1977
top 3rd 1973 1995

All hadrons are composed of quarks in this model. Protons and neutrons, the most well known examples of hadrons, are composed of 3 quarks. Any hadron which is composed of 3 quarks and which can decay into a proton is called a baryon. It may surprise you to know that a neutron, if it is not in a nucleus, will decay into a proton, with a half-life of about 14 minutes.

The other type of hadron is called a meson. Mesons are made up of just 2 quarks, and always in a quark-antiquark pair. Mesons cannot decay into a proton, as they have too few quarks.

Hadrons can be further divided into baryons and mesons.

## The standard model

The standard model of particle physics is shown in the figure below.

The standard model of particle physics.

You will notice in each box a number of figures. For example, for the up quark it has $2.4 MeV/c^{2}$ along the top, and 2/3 and 1/2 along the left hand side. The top figure refers to the rest mass of the particle expressed in energy (matter and energy are related via Einstein’s famous equation $E=mc^{2}$). This is the energy required to create this particle in an accelerator. The next figure, 2/3 in the case of the up quark, is the electric charge. For a proton, the 3 quarks which make it up are u,u and d, giving a charge of 2/3 + 2/3 – 1/3 = 1. For a neutron, the 3 quarks which make it up are u,d and d, giving a charge of 2/3 – 1/3 – 1/3 = 0.

The final figure, 1/2 for the up quark, is the quantum-mechanical spin of the particle. I will explain what this means in a separate blog. All quarks have a spin of 1/2, as do all leptons. Bosons have an integer spin.

The quarks and leptons fall into 3 generations. The first generation is normal matter. The 2nd and 3rd generations of matter seem to be heavier (more massive) versions of the 1st generation, and (apart from the 3 generations of neutrinos) will decay into particles in the 1st generation. We have no idea at the present time as to why Nature has 3 copies of matter, 3 generations. We currently believe that quarks are fundamental particles, and cannot be split up into anything simpler.

The best known example of a lepton is the electron, but another example many people have heard of is the neutrino. The electron and the neutrino are both 1st generation leptons, but there are 2nd and 3rd generation leptons just as there are 2nd and 3rd generation quarks making up the hadrons. We currently believe that leptons are, like quarks, fundamental particles.

The right hand column of the figure are bosons. In the modern quantum mechanical view of Nature, forces are carried (mediated) by particles called bosons. The photon is an example of a boson. It is a “particle of light”, but also the particle responsible for the electro-magnetic force. The weak nuclear force (responsible for radioctive decay) is mediated by the W and Z bosons, and the strong nuclear force (responsible for holding the nucleus together) is mediated by gluons.

You will notice that this figure does not include the famous Higgs boson. I will post a separate blog in the near future about the Higgs boson, why it was proposed, and whether CERN has actually discovered it with the Large Hadron Collider.