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Linné on line arrow Physics and the Cosmos arrow Physics and the Cosmos arrow Quarks and leptons, the smallest building blocks of matter?

Quarks and leptons, the smallest building blocks of matter?

As far as we know today quarks and leptons are the smallest building blocks of matter. In the beginning of the 1960's one had discovered a large amount of different particles, most of them so called hadrons, which interact via the strong force, but also some leptons which are not affected by the strong force. This complicated the simple picture that everything consists of protons, neutrons and electrons. In addition to these "necessary" particles there was a plethora of particles that did not seem to fit the bigger picture and for this reason were not needed.

In 1964 Murray Gell-Mann and Georg Zweig independently presented a new way of getting all the different particles into order. They introduced a new sub-level, the quarks. With the help of three different quarks they could explain all the known hadrons and in addition predict the mass of a new particle which had not yet been observed but later confirmed their model. The three different quarks where called up u, down d, and strange s. There are two different kinds of hadrons, states consisting of three quarks called baryons and states consisting of a quark anti-quark pair called mesons. A proton for example is a uud state and a neutron is a udd state. In the same way as earlier in history one succeeded in explaining a large amount of different particles by introducing a small number of building blocks.  

In the beginning a lot of people doubted that the quarks really existed and instead they maintained that the quarks where only mathematical constructions. One reason for this skepticism was that one had never observed single quarks. In order that the proton and neutron get the correct electric charge the quarks must have charges which are fractional. The u quark has the electric charge +2/3 and the d quark -1/3 in units where the electron has charge -1 and the proton +1. But no one has ever observed a particle with fractional charge. The proof that quarks really exist came in 1969 when one observed the substructure of the proton in an experiment at the Stanford Linear Accelerator Center in California. This discovery was later awarded the 1990 Nobel Prize. By irradiating a fixed target of protons with high energy electrons and studying how the electrons were scattered one could deduce that the proton has a substructure, the quarks.

Even after discovering the quarks there were lots of questions that remained to be answered about the interactions of the quarks. Some of these questions have been partially answered today. For example: What keeps the quarks together in the proton?. However, many questions still remain unanswered. For example whether quarks have substructure, what gives the quarks their masses, how many generations of quarks and leptons are there and how many forces are there in nature.

To answer these questions one is performing a number of different particle physics experiments. These are often based on colliding high energy particles such as electrons and protons with each other and studying the particles that are produced. Some examples are the electron-positron collider LEP at CERN, the Tevatron proton-antiproton collider at Fermilab and the upcoming proton-proton collider LHC at CERN. One is also trying to do experiments to answer these questions at lower energies such as the WASA experiment at the The Svedberg Laboratory in Uppsala. There is also a large number of other experiments which use different techniques, for example to detect neutrinos and study their interactions.

An introduction to particle physics has also been prepared by The Particle Data Group which is called The Particle Adventure.

See also the public page of the Atlas experiment at CERN.