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4.5 Fundamental Interactions
4.5.1 Introduction
Symmetries play an important and crucial role in physics.
Global symmetries give rise to conservation laws
and local symmetries yield forces. Four fundamental
interactions are known to date, i.e. gravitation, the weak
interaction, electromagnetism, and the strong interaction.
The Standard Model (SM) provides a theoretical
framework in which electromagnetism and the weak
interaction (which are unified into the electroweak interaction)
and many aspects of the strong interactions can
be described to astounding precision in a single coherent
picture. It has three generations of fundamental fermions
which fall into two groups, leptons and quarks. The latter
are the building blocks of hadrons and in particular of
baryons, e.g. protons and neutrons, which contain three
quarks. Forces are mediated by bosons: the photon, the
W- and Z 0 -bosons, and eight gluons.
The SM is found to describe observations hitherto very
well, and as far as we know, the Standard Model is valid
up to the Grand Unification Theory (GUT) energy scale,
∼10 16 GeV. Still, it has some theoretical difficulties. The
electroweak and colour interactions are deduced from
local U(1)⊗SU(2)⊗SU(3) gauge invariance requirements,
and in order to make them work one has to introduce a
spontaneously symmetry-breaking Higgs-field. Gravity
does not fit into this picture. We do not know why there
are exactly three fermion families, nor what causes the
mixing of neutrino types. The presence of dark matter
and the prevalence of matter against antimatter in the
Universe, which might be related to CP violation, are
also unexplained. There are extensions of the Standard
Model trying to explain these effects, but so far we have
no experimental evidence supporting any of them in spite
of great efforts in particle and astroparticle physics.
One of the great actual challenges in physics therefore
is the search for new phenomena, beyond the SM,
pointing to a more general unified quantum field theory
which provides a description of all four fundamental
forces. The existence of phenomena such as neutrino
oscillations, dark matter and the matter–antimatter
asymmetry are three striking manifestations of physics
beyond the SM.
Accurate calculations within the SM now provide a
basis to searches for deviations from SM predictions.
Such differences would reveal clear and undisputed
signs of still other types of new physics and hints for the
validity of speculative extensions to the SM. The variety
of often speculative models beyond the present SM, that
by no means can be discussed here, include e.g. left–
right symmetry, fundamental fermion compositeness,
new particles, leptoquarks, supersymmetry, supergravity
and many more. Further, above the Planck energy scale
we may expect to have new physical laws which also
allow for Lorentz and CPT violation. Interesting candidates
for an all encompassing quantum field theory are
string or membrane theories which in their low energy
limit may include supersymmetry.
Experiments at nuclear physics facilities at low and
intermediate energies offer in this respect a variety of
possibilities which are complementary to approaches
in high energy physics and in some cases exceed those
significantly in their potential to steer physical model
building.
To address open issues of the SM and search for
physics beyond, theoretical and experimental activities
in the field of fundamental interactions will concentrate
in the next decade on the following key topics:
1. Fundamental symmetries
2. Neutrinos
3. Electroweak interactions
In doing so the following key questions will be
addressed:
1. Which fundamental symmetries are conserved in
nature
2. What is the origin of the matter dominance in the
universe
3. Are there new sources of CP violation
4. What are the properties of antimatter
5. What are the properties of the neutrino
6. Are there other than the four known fundamental
forces
7. Are there new particles and what is their role in the
universe
8. What are the precise values of the fundamental constants
In order to investigate these key questions, the following
key issues will be addressed:
• Fundamental fermions
– Neutrino oscillations and the neutrino mixing
matrix
– Neutrino masses (direct measurements and double
β decay experiments)
– Quark mixing matrix and unitarity
– New (time reversal invariant) interactions in nuclear
β decays and neutron decay
• Discrete symmetries
– Parity violation
– Time reversal and CP violation in the quark sector
(e.g. electric dipole moments)
– CPT and Lorentz invariance
• Properties of known basic interactions
– QED and fundamental constants (e.g. g-2, fine
structure constant, H-like ions, anti-hydrogen …)
– QCD (exotic atoms)
– Gravity (e.g. matter versus antimatter behaviour)
152 | Perspectives of Nuclear Physics in Europe – NuPECC Long Range Plan 2010