Transregio 27 - Neutrinos and Beyond

Research program


Research program

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 of neutrinos".

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:
  • 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 and m3?
  • 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?
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.