2
H. Schopper
indestructible atoms, like Democritus, or the four elements and the regular bodies
of Plato.
Since Newton who introduced infinitely hard smooth balls as constituents of
matter 1 and who described gravitation as the first force acting between them, the
concept of understanding nature in terms of ‘eternal’ building blocks hold together
by forces has not changed during the past 200 years. What has changed was the
nature of the elementary building blocks and new forces were discovered. The
chemists discovered the atoms of the 92 elements which, however, contrary to their
name, were found to be divisible consisting of a nucleus surrounded by an electron
cloud. Then it was found that the atomic nuclei contain protons and neutrons.
Around 1930 the world appeared simple with everything consisting of these three
particles: protons, neutrons and electrons.
Then came the ‘annus mirabilis’ 1931 with the discovery of the positron as the
first representative of antimatter and the mysterious neutrino in nuclear beta-decay
indicating a new force, the weak interaction. In the following decades the ‘particle
zoo’ with all its newly discovered mesons, pions and ‘strange’ particles was leading
to great confusion. Simplicity was restored when all these hundreds of ‘elementary
‘particles could be understood in terms of a new kind of elementary particles, the
quarks and their antiquarks. The systematics of these particles is mainly determined
by the strong nuclear force, well described today by the quantum chromodynamics
QCD. Whether quarks and gluons (the binding particles of the strong interaction)
exist only inside the atomic nuclei or whether a phase transition into a quark-gluon
plasma is possible, is one the intriguing questions which still needs an answer.
Impressive progress was made in another domain, in the understanding of the
weak nuclear force responsible for radioactive beta-decay and the energy production
in the sun. Three kinds of neutrinos (with their associated antiparticles) were
found and recently it could be shown that the neutrinos are not massless as
had been originally assumed. The mechanism of the weak interaction could be
clarified to a large extent by the discovery of its carriers, the W- and Z-particles.
All the experimental results obtained so far will be summarized in this volume
and the beautiful theoretical developments will be presented. The climax is the
establishment of the ‘Standard Model of Particle Physics’ SM which has been shown
to be a renormalizable gauge theory mainly by the LEP precision experiments.
The LEP experiments have also shown that there are only three families of quarks
and leptons (electron, muon, tau-particle and associated neutrinos), a fact not yet
understood.
All the attempts to find experimental deviations from the SM have failed so far.
However, the SM cannot be the final theory for the understanding of the microcosm.
Its main fault is that it has too many arbitrary parameters (e.g. masses of the
particles, values of the coupling constants of the forces, number of quark and lepton
families) which have to be determined empirically by experiment. An underlying
theory based on first principles is still missing and possible ways into the future will
be discussed below.
1 Isaac Newton, Optics, Query 31, London 1718.
H. Schopper
indestructible atoms, like Democritus, or the four elements and the regular bodies
of Plato.
Since Newton who introduced infinitely hard smooth balls as constituents of
matter 1 and who described gravitation as the first force acting between them, the
concept of understanding nature in terms of ‘eternal’ building blocks hold together
by forces has not changed during the past 200 years. What has changed was the
nature of the elementary building blocks and new forces were discovered. The
chemists discovered the atoms of the 92 elements which, however, contrary to their
name, were found to be divisible consisting of a nucleus surrounded by an electron
cloud. Then it was found that the atomic nuclei contain protons and neutrons.
Around 1930 the world appeared simple with everything consisting of these three
particles: protons, neutrons and electrons.
Then came the ‘annus mirabilis’ 1931 with the discovery of the positron as the
first representative of antimatter and the mysterious neutrino in nuclear beta-decay
indicating a new force, the weak interaction. In the following decades the ‘particle
zoo’ with all its newly discovered mesons, pions and ‘strange’ particles was leading
to great confusion. Simplicity was restored when all these hundreds of ‘elementary
‘particles could be understood in terms of a new kind of elementary particles, the
quarks and their antiquarks. The systematics of these particles is mainly determined
by the strong nuclear force, well described today by the quantum chromodynamics
QCD. Whether quarks and gluons (the binding particles of the strong interaction)
exist only inside the atomic nuclei or whether a phase transition into a quark-gluon
plasma is possible, is one the intriguing questions which still needs an answer.
Impressive progress was made in another domain, in the understanding of the
weak nuclear force responsible for radioactive beta-decay and the energy production
in the sun. Three kinds of neutrinos (with their associated antiparticles) were
found and recently it could be shown that the neutrinos are not massless as
had been originally assumed. The mechanism of the weak interaction could be
clarified to a large extent by the discovery of its carriers, the W- and Z-particles.
All the experimental results obtained so far will be summarized in this volume
and the beautiful theoretical developments will be presented. The climax is the
establishment of the ‘Standard Model of Particle Physics’ SM which has been shown
to be a renormalizable gauge theory mainly by the LEP precision experiments.
The LEP experiments have also shown that there are only three families of quarks
and leptons (electron, muon, tau-particle and associated neutrinos), a fact not yet
understood.
All the attempts to find experimental deviations from the SM have failed so far.
However, the SM cannot be the final theory for the understanding of the microcosm.
Its main fault is that it has too many arbitrary parameters (e.g. masses of the
particles, values of the coupling constants of the forces, number of quark and lepton
families) which have to be determined empirically by experiment. An underlying
theory based on first principles is still missing and possible ways into the future will
be discussed below.
1 Isaac Newton, Optics, Query 31, London 1718.