Introduction to Particle Physics

Why do physicists study particles?

Physicists study particles because everything is made of them, including us!

Since time immemorial people have been trying to understand what the Universe is made of. One of the earliest theories said that everything could be built from just four elements, Earth, Air, Fire and Water. This was a great scientific theory because it was simple. But it had one big drawback: it was wrong.

The Greek philosopher, Leucippus of Miletus and his follower Democritus set the scene for modern physics in 5th century B.C. Athens when they considered what would happen if you chopped up matter into ever smaller pieces. There would be a limit, they said, beyond which you could not go. And they called their fundamental particles atoms.

Testing the theory was not easy with ancient Greek technology and it was not until the 17th century that what we today call elements were found. There were all kinds of them and, for a while, the ideas people had for describing them were far more complex than the Earth, Air, Fire and Water model. But people started to notice patterns in the masses of the elements and eventually Russian scientist, Dmitri Mendeleev, arranged them into his famous periodic table of the elements, first published in 1869.

The patterns Mendeleev documented are called a symmetry and whenever there is symmetry in nature, it means that there's a simpler way of describing things. With the elements, the patterns in the masses were a clue that all the elements are made up of smaller particles and their differing masses are simply determined by how many of those particles they have inside them.

That observation eventually brought the number of fundamental particles down from all the elements to just three: protons, neutrons and electrons. Protons and neutrons are about 2000 times heavier than electrons, so it's the number of them that determines an atom's mass. The number of protons, which are positively electrically charged, matches the number of electrons in an atom and determines its chemical properties.

This century, physicists have seen another explosion of complexity, finding all kinds of particles that couldn't be explained by the proton, neutron, electron model. By looking for symmetries in their properties, they have discovered that there are still smaller particles locked up inside protons and neutrons. These are called quarks and as far as we know today, they are the fundamental particles of matter. To all intents and purposes, they are the "atoms" dreamed up by Leucippus 2500 years ago and they are among the particles studied at CERN.

A proton has three smaller particles called quarks inside it.

Particles of matter are not the end of the story, however. There also has to be something to make them stick together and organize themselves into complex objects. These are nature's forces: gravity, electromagnetism, weak and strong.

Gravity is the most familiar to us, but it is the weakest. At the other end of the scale is the strong force which binds atomic nuclei. The weak force plays an important role in the energy generating processes of stars, and it also causes some kinds of radioactivity. Electromagnetism is what brings light and energy to us from the Sun and holds electrons in orbit around nuclei to form atoms. The electromagnetic force is about 100 times weaker than the strong force, the weak force about 10,000 times weaker, and gravity is some hundred million million million million million million times weaker.

Gravity holds the planets in orbit around the Sun but is the weakest force of them all.

According to current theory, forces are carried by particles that are different from the particles of matter. The carriers of the strong force are called gluons because they "glue" the quarks together into particles like protons and neutrons and in turn glue protons and neutrons together into atomic nuclei. Photons carry the electromagnetic force and particles called W and Z carry the weak force. Gravitons are believed to carry gravity, but they have so far not been found.

Physicists think that all the forces can be explained in a single theoretical framework, and they put this hypothesis to the test at laboratories like CERN. The first step to unifying the forces was made by Scottish mathematician James Clerk Maxwell at the end of the 19th century. He realised that electricity and magnetism had a lot in common and wrote down a theory to describe them both. Then in the 1970s three scientists, the Americans Sheldon Glashow and Stephen Weinberg along with Pakistani physicist Abdus Salam worked out how to unify electromagnetism and the weak force.

Confirming their idea rested on finding the W and Z particles and when this was done at CERN in 1983, it brought the Nobel prize to the laboratory for the first time. CERN then turned its efforts to studying W and Z particles in detail, and studying Z particles is what you will be doing later.

Prof. Carlo Rubbia and Prof. Simon van der Meer celebrating their Nobel Prize

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