Neutrinos are special among elementary particles in that they
interact only by the extremely feeble "weak force", their masses
are very much smaller than those of the other particles, they may
well be identical with their own antiparticles, and so many of them
were produced in the Big Bang that they are the most abundant
particles in the universe besides photons. While neutrino
experiments are truly challenging, these elusive particles offer
unique windows of opportunity to unravel fundamental questions such
as why particles have masses at all or why there is far more matter
than antimatter in the universe. Moreover, neutrinos can reach us
from places otherwise hidden from view so that one can study the
interior of the Earth, the Sun, or collapsing stars in the "light
The past decade has witnessed a series of major breakthroughs in our
understanding of neutrinos, notably the discovery that they have
non-vanishing masses, the first solid evidence for the long-sought
"physics beyond the Standard Model". Moreover, the different
neutrino types or "flavours" νe, νμ and ντ are
quantum-mechanical superpositions of individual mass states, i.e.
each of them represents a different mixture of masses m1, m2
and m3. At present we know only the differences of these
masses from "flavor oscillation experiments" that exploit a
quantum interference effect over distances of up to thousands of
kilometers. While their absolute mass scale remains one of the
unresolved key questions to be addressed in this SFB/TR 27, their
masses definitely are so small that they provide only a sub-dominant
fraction of the cosmological dark matter that shapes the dynamics
and evolution of the universe. Well-motivated extensions of the
Standard Model suggest that new "weakly interacting massive
particles" with properties closely related to those of neutrinos
likely play the dominant role. The search for weakly interacting
dark-matter particles often requires similar techniques as neutrino
experiments, naturally forming a complementary research activity.
The recent discoveries have spawned several unresolved fundamental
questions at the intersection of particle physics, astrophysics and
cosmology, providing the main scientific motivation for this SFB/TR:
The experimental, theoretical and astrophysical groups in Germany
forming the consortium of this SFB/TR, together with their
international partners, play a leading role in this cutting-edge
effort. As the pace of new and fundamental discoveries is expected
to remain fast, strategic positioning and enhanced networking
efforts are required in an environment of strong and well-structured
international competition, notably in Japan and the US.
To advance our fundamental understanding of weakly interacting
particles in a coherent and unified approach, this SFB/TR centers on
several key issues. The first is a model-independent
experimental investigation of neutrino masses in the sub-eV range
(KATRIN), the search for neutrino-less double beta decay with
extremely low levels of background (GERDA) to identify neutrinos as
their own antiparticles and the investigation of 3-flavor-mixing
effects in neutrino oscillations (Double Chooz). The investigation
of cold dark matter is performed by searching for scatterings of
weakly interacting massive particles (CRESST, EDELWEISS) and their
annihilation signal in the galaxy (AMS). Another
experimental direction will focus on R&D efforts for the
development of large volume scintillators (LENA) for low-energy
neutrinos as well as cryogenic detector development and the
characterization of a large-scale cryogenic dark matter experiment
(EURECA), including the development of analysis tools. Theoretical
efforts focus on the phenomenology of neutrinos and weakly
interacting particles in theories extending the Standard Model, the
calculation of reliable nuclear matrix elements for double beta
decay, the development of unified theories for neutrinos, dark
matter and cosmology, and on the modeling of core-collapse
supernovae and gamma-ray bursts as neutrino sources.
This combined effort will have far-reaching consequences for our
understanding of the "inner space" of the particle world and the
"outer space" of the universe. It will serve as a major benchmark
for a unified model of elementary particles and fix the roles of hot
and cold dark matter in the universe. The unified approach and
synergy within this SFB/TR adds significant value to the activities
of the individual groups. The interdisciplinary exchange and the
expected strong international visibility will provide ideal
opportunities to promote the careers of young scientists and in
particular of young female researchers.
- What is the neutrino mass scale and why is it so small?
- Are neutrinos identical with their antiparticles, i.e. are they "Majorana particles"?
- What is the complete structure and origin of the mixing matrix that relates the flavor states νe,
νμ and ντ to the states with masses m1, m2
- What is the nature of the cosmological dark matter?
- What can neutrinos tell us about hidden regions of the universe?
- Are neutrinos responsible for the matter-antimatter asymmetry in the universe and hence our very existence?
- Which is the correct unified theory for neutrinos and weakly interacting particles?