What is the smallest unit of matter

If you break an everyday object, e.g. a coffee mug, in the middle, then again and again, where would you end up? Could you go on and on Or would you find a set of indivisible building blocks that make up everything? Physicists have found just that - matter is made up of elementary particles, the smallest things in the universe. Particles react with one another according to a theory called the "Standard Model".

The standard model is a strikingly elegant encapsulation of the strange quantum world of indivisible, infinitely small particles. It also encompasses the forces that control how particles move, how they interact, and how they combine to shape the world around us.

So how does it work?

If we enlarge the particles of the cup, we see interconnected molecules made up of atoms. A molecule is the smallest unit of any chemical compound. An atom is the smallest unit of every element in the periodic table.

But the atom is not the smallest unit of matter. Experiments showed that every atom has a tiny, dense nucleus surrounded by a cloud of even smaller electrons. As far as we know, the electron is one of the fundamental, indivisible building blocks of the universe. It was discovered as the first Standard Model particle. Electrons are bound to the nucleus of an atom by electromagnetism. They attract each other by exchanging so-called photons, which are light quanta that transport electromagnetic force, one of the fundamental forces in the Standard Model.

The nucleus has more secrets because it contains protons and neutrons. At first it was thought that these were themselves fundamental particles, but in 1968 physicists discovered that protons and neutrons are actually made up of quarks that are indivisible. A proton contains two up quarks and one down quark. A neutron contains two down quarks and one up quark.

The nucleus is held together by the strong force, another fundamental force in the Standard Model. Just as photons transport electromagnetic force, so-called gluons transport strong force. Electrons as well as up and down quarks are apparently all we need to build atoms and thus to describe normal matter. However, high-energy experiments showed that there are actually six quarks - down and up, strange and charm, and bottom and top quarks - and they come in a wide variety of masses. The same was found for electrons that have heavier siblings, the muon and the tauon. Why are there three (and only three) different types of each of these particles? That remains a mystery.

These heavy particles only arise for a very short moment in high-energy collisions and do not occur in everyday life. This is because they break down very quickly into the lighter particles.

This decay involves the exchange of force-transporting particles, called the W and Z, which - unlike photons - have mass. They transport the weak force, the last force in the standard model. The same force can transform protons and neutrons into one another, an essential part of the fusion processes that drive the sun. In order to observe the W and the Z directly, we needed the high-energy collisions in a particle accelerator.

There is another type of particle in the Standard Model called neutrinos. These interact with other particles through the weak force. Tens of thousands of neutrinos fly through us every second, many of them from the sun. Measurements of weak reactions showed that there are different types of neutrinos associated with the electron, muon and tauon.

All of these particles also have counterparts in antimatter that are oppositely charged but otherwise identical. Matter and antimatter particles are produced in pairs in high-energy collisions and they cancel each other out when they meet.

The last particle of the Standard Model is the Higgs boson, a quantum ripple in the underlying energy field of the universe. By interacting with this field, all fundamental matter particles acquire mass, according to the standard model. The ATLAS experiment in the large hadron storage ring examines the standard model in detail. By making accurate measurements of the particles and forces of the universe, ATLAS physicists can find answers to the puzzles the Standard Model does not explain.

For example: How does gravity fit in here? What is the real connection between force carriers and mass particles? What is "dark matter" that makes up most of the mass in the universe but remains inexplicable? While the Standard Model provides a nice explanation for the world around us, there is still a whole universe full of puzzles to explore.