The Standard Model

What is our world made of?

If we take a small object (like a rain drop) and divide it into even smaller pieces, we find that it is made up of atoms!
It turns out that even atoms can be further divided into electrons and a nucleus consisting of nucleons (i.e. protons and neutrons), which are finally built up of quarks. According to our current knowledge, quarks and electrons are fundamental  — they can be divided no more and have no internal structure and no spatial extension.

According to our theoretical understanding all forces are transmitted by so called force carriers:

  • Gravity: lets apples fall from trees, the carrier of which has not been observed yet. It is referred to as<stronggraviton.
  • Electromagnetic interaction: makes lightning in a thunderstorm and is the basis of all electricity and magnetism. It is mediated by the photon.
  • Weak interaction: is responsible for the energy production in the sun and for radioactivity. Its force carriers are called W, Z bosons.
  • Strong interaction: binds protons and neutrons to nuclei and quarks to nucleons very strongly via the gluons.

The Standard Model of Particle Physics

All known fundamental particles in the Universe can be classified as matter constituents, force carriers and particles responsible for the creation of mass. Their behaviour is  well described by the Standard Model of Particles Physics, the component of which are summarised in the following picture.

Quarks and leptons are the matter constituents. These are all particles with half-integer spin, known as fermions. To a good approximation the proton is made of two up quarks and one down quark. There are also heavier copies of these two quarks: the charm, strange, bottom and top quark. The electron is a lepton and it has also heavier copies: the muon μ and the tau τ, as well as neutral partners: the neutrinos ν. All known fundamental forces are transmitted via force carriers, called gauge bosons, as described above.
Having particles with a non-vanishing mass (as we observe in nature) leads to mathematical inconsistencies of our theroy. A possible solution was the postulation of a new, unknown particle that is able to elegantly solve this problem: the
Higgs Boson.

Mathematically all properties of the fundamental particles and interactions can be encrypted in this four line formula, called Lagrangian:

  1. the first line of the formula describes the force carriers
  2. the second line describes quarks and leptons as well as their interactions
  3. the third line describes the Higgs particle
  4. the last line makes quarks and leptons massive
Where is gravity?

Gravity is not included because we do not have a quantum version of it and its effects are also negligible in the microworld.
It is well described by the Einsten’s  General Theory of Relativity and physicists are trying to find a model which can embed both the Standard Model and the General Relativity.

How do we know all this?

When we see an object, our eyes are working as detector!
Light is emitted by the Sun and travels to Earth before bouncing off objects and being recorded in  our eyes. With a normal microscope we can only resolve objects that are as large as the wavelength of light, which is about the size of small bacteria.
Our microscopes for looking into the sub-atomic world are particle accelerator and for smaller objects we need shorter wavelengths – which for physicists is  equivalent to higher energies.

The highest possible energies in thelaboratory can currently be created with the Large Hadron Collider (LHC) in CERN, making it our biggest microscope.
In every second at the LHC, we can have 600 million collisions of a proton with  another proton.
The energy of the proton beam in the LHC corresponds to the energy of a 200 ton train with a velocity of more than 100 mph! With the LHC we can see structures that are more than 100 billion times smaller than bacteria! With other experiments, we are also able to observe particles produced in the Earth’s atmospehere, in the Sun, or in the Universe billions of years ago.