Standard Model

The Standard Model is the name given to the current theory of fundamental particles and how they interact.

The concept of a particle is a natural idealization of our everyday observation of matter. Footballs or dust particles, under ordinary conditions, are stable objects that move as a whole and obey simple laws of motion. However, neither of these is actually a structureless object. That is, if sufficiently large forces are applied to them, they can be broken apart into smaller pieces. The idea that there must be some set of smallest constituent parts, which are the building blocks of all matter, goes back to ancient history.

In the 1930s protons, neutrons, and electrons were assumed to be the smallest objects into which matter could be divided. It is now known that protons and neutrons are made up of the much smaller quarks. In addition to protons and neutrons, over 100 other particles made from quarks and/or anti-quarks, the so-called hadrons, were discovered between 1930 and the present time. It is interesting to note, that an anti-proton made of anti-quarks completely annihilates into energy when in contact with a proton made of quarks. Electrons, and their heavier partners the muons and taus, as well as their nearly massless companion neutrinos, are called leptons.

All we know today is, that quarks and leptons are smaller than 10-19 meters in radius. It would require more powerful particle accelerators to look that deep into matter. It is possible that future evidence will again reveal a substructure for these particles.

According to the Standard Model, fundamental matter particles divide into two classes: quarks and leptons. There are six particles of each class and six corresponding anti-particles. The strong force binds (glues) quarks together in hadrons; the particles that mediate this force are called gluons. The electromagnetic force based on the electrical charge of particles is communicated by photons. The W and Z-bosons carry the weak force, which is for example responsible for the radioactive decay of nuclei. The theory does not include the effects of gravitational interactions - this has not yet been achieved.

The Standard Model was the triumph of particle physics in the 1970s. It incorporated all that was known at that time and has since then successfully predicted the outcome of a large variety of experiments.

But we do not know what causes the fundamental particles to have masses. The simplest idea involves one additional particle, called the Higgs boson, and one more force type mediated by exchanges of this boson. The Higgs particle has not yet been observed, and from previous experiments is expected to be heavier than an atom of silver. The Large Hadron Collider (LHC) at the European Laboratory CERN is intended to search for the Higgs particle and to look deeper into matter than ever before.

 

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