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