The Standard Model
From the greeks to the Standard Model
Across the ages the humans have tried to understand there surrounding world. One of the key question they tried to answer was ``What is matter made of ?''. From the ancient Greek Democritus to the famous Mendeliev in the XIXth century many different answer have been proposed. At the beginning of XXth century most of the scientists agreed on the fact that matter was made of atoms.
These atoms were considered as ``non breakable'' particles (as suggested by the Greek meaning of their name) but a few years later Rutherford showed that they were in fact made of a nucleus and an electronic cloud. Further studies showed that the nucleus was in fact made of protons and neutrons. Later (in the 60s), it was discovered that these protons and neutrons where themselves made of other particles called ``quarks''. The properties of the quarks have been intensively studied by particle physicists during the last 40 years. These studies have led to the ``Standard Model'' of particle physics.
The Standard Model of particle physics describes the tiniest particles discovered so far and the way they interact with the fundamental forces of the nature.
These particles can be divided into 2 categories: leptons and quarks. These 2 categories contain 3 generations of 2 particles. With these 12 particles (and 12 associated ``anti-particles'') it is possible to build all known object on the earth or anywhere else in the universe.
These particles can interact with each other in different ways. There are 4 known fundamental interactions between these particles:
There have been many experimental verifications of the Standard Model and it is now the scientific model that has been tested with the best precision!
Despite its successes the Standard Model does not answer to all our questions...
- The gravitation which is the weakest of all the interactions but also the only one whose effects are felt at very long distances. The gravitation is responsible for the dynamics of our solar system (ie how the planets move).
- The electromagnetic interaction has also an infinite range, but the existence of positive and negative charges screens long distance effects. The chemical bound between atoms in a molecule are created by this force.
- The weak interaction has been discovered in some nuclear interaction and has a very limited range. The $\beta$ radioactivity is due to the weak interaction.
- The strong interaction is the strongest known interaction but its range is very limited.
Beyond the Standard Model
The main question unanswered by the Standard Model is the origin of the mass of the elementary particles.
An ad-hoc mechanism, called the Higgs mechanism, has been proposed but it relies on a particle (called the Higgs boson) that has not yet been observed.
The Standard Model also does not explain why leptons and quarks can be gathered in 3 generations. How gravitation acts on elementary particles is also not well described by the Standard Model. Other questions remain unanswered as well.
For all these reasons physicists believe that there is another theory ``beyond the Standard Model''. Some candidates theories ``beyond the Standard Model'' have been suggested. The most popular theory is called ``supersymmetry''. According to this theory each particle would have a ``superpartner'' yet to be discovered. Other theories suggest the existence of smaller particles that would compose that current ``elementary particles''. The world we experience every day is made of 3 spatial dimensions, but there might be some extra-dimensions at the scale of the elementary particles...
Many acceptable theories ``beyond the Standard Model'' have been suggested, but only experimental verifications can decide which one is THE right one.
How to find out which theory is the good one?
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Last modified: "Sep 16 2003"
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