Karlsruhe Tritium
←Home
Motivation
Neutrino Experiment
Evidence for massive neutrinos:
In modern particle physics, one of the most intriguing and most challenging tasks is to discover the rest mass of neutrinos, bearing fundamental implications to particle physics, astrophysics and cosmology. Until recently, according to the Standard Model (SM) of particle physics, neutrinos were assumed to be massless. However, actual investigations of neutrinos from the sun and of neutrinos created in the atmosphere by cosmic rays, in particular the recent results of the Super-Kamiokande and SNO experiments, have given strong evidence for massive neutrinos indicated by neutrino oscillations.
Neutrino oscillations imply that a neutrino from one specific weak interaction flavor, e.g.a muon neutrino νμ , transforms into another weak flavor eigenstate, i.e. an electron neutrino νe or a tau neutrino ντ , while travelling from the source to the detector. The existence of neutrino oscillations, on one hand, requires a non-trivial mixing between these weak interaction eigenstates (νe , νμ , ντ) and the corresponding 3 neutrino mass states (νi , νii , νiii ) and, moreover, that these masses (m1, m2, m3) are not degenerate, i.e. the mass eigenvalues differ from each other. Consequently, the experimental evidence for neutrino oscillation proves that neutrinos have non-zero masses. (Fig.1; click figure to enlarge and to get detailed caption)
Implications of neutrino masses:
The existence of neutrino oscillations and therefore of neutrino mixing and masses has far-reaching implications to numerous fields of particle physics, astrophysics and cosmology:
The SM of particle physics, which very precisely describes the present experimental data up to the electroweak scale, offers no explanation for the observed pattern of the fermion masses or the mixing among the fermion generations. In particular, it offers no explanation for neutrino masses and neutrino mixing. Accordingly, the recent experimental evidence for neutrino masses and mixing is the first indication for physics beyond the Standard Model.
There are many theories beyond the Standard Model, which explore the origins of neutrino masses and mixings. In these theories, which often work within the framework of Supersymmetry, neutrinos naturally acquire mass. A large group of models makes use of the so-called see-saw effect to generate neutrino masses. Other classes of theories are based on completely different possible origins of neutrino masses, such as radiative corrections arising from an extended Higgs sector. As neutrino masses are much smaller than the masses of the other fermions, the knowledge of the absolute values of neutrino masses is crucial for our understanding of the fermion masses in general. Recently it has been pointed out that the absolute mass scale of neutrinos may be even more significant and straightforward for the fundamental theory of fermion masses than the determination of the neutrino mixing angles and CP-violating phases. It will most probably be the absolute mass scale of neutrinos which will determine the scale of new physics.
All these theories extended beyond the SM can be grouped into two different classes, leading either to a hierarchical pattern for the neutrino mass eigenvalues mi
or resulting in a nearly degenerate pattern of neutrino masses
As neutrino oscillation experiments are only sensitive to the differences of the squared masses Δm2 (e.g. Δm212=|m12-m22|), they cannot measure absolute values of nu masses. While they do not distinguish between the two classes of models, oscillation experiments allow to set a lower bound on the absolute nu-mass, as at least one of the neutrino mass eigenvalues should satisfy the inequality:
Analysis of the actual results of Super-Kamiokande in terms of oscillations of atmospheric neutrinos thus gives a lower bound on m3:
However, the fundamental mass scale of neutrinos can be located many orders of magnitude above this lower bound (e.g. at around 1 eV/c2), as suggested by mass models with degeneracy. Discrimination between hierarchical and degenerate mass models thus requires a sensitivity on the absolute nu-mass scale in the sub-eV/c2 range.
Finally, theoretical models come to different conclusions of whether neutrino masses are of the Dirac- or of the Majorana type. A massive neutrino which is identical to its own antiparticle is called a Majorana particle, while for Dirac-type neutrinos the lepton number distinguishes neutrinos from antineutrinos. This requires the development of experimental techniques for ν-masses in the sub-eV/c2 range, which do not depend on assumptions about the Dirac or Majorana character of the neutrino mass, i.e. KATRIN.
In astrophysics and cosmology, neutrino masses and mixings play an important role in numerous scenarios, ranging from the formation of light nuclei during the Big Bang nucleosynthesis (BBN), the formation and evolution of large scale structures in the universe, up to stellar evolution and the very end of a heavy star, a supernova explosion. Of special interest are the so-called relic neutrinos: according to the Big Bang theory a huge amount of neutrinos in the universe is left over from the very beginning, equivalent to the photons of the so-called Cosmic Microwave Background Radiation (CMBR). The ratio of the relic neutrinos to baryons (protons and neutrons) is about 109:1, therefore even small neutrino masses are of great importance: they could contribute considerably to the large amount of non-visible, so-called Dark Matter. As cosmological models of structure formation depend on the type of Dark Matter (non-relativistic or Cold Dark Matter CDM and relativistic or Hot Dark Matter HDM) and massive neutrinos are the only known candidates for HDM, the determination of the neutrino masses plays a vital role.
Fig. 3 shows the different contributions to the total matter-energy density Omega of the universe, arising from luminous matter, baryons, CDM and the so-called Dark Energy (which is equivalent to the cosmological constant Λ). While the individual contributions of these components are more or less well determined, the contribution Ων of neutrino HDM can vary in the interval
The lower bound on Ων arises from the results of Super-Kamiokande on the oscillations of atmospheric neutrinos. The upper bound comes from current tritium β decay experiments and, independently, from recent studies of the evolution of large scale structures in the universe making use of the solar and atmospheric oscillation results . The parameter range of Ων for neutrino HDM, which is presently allowed by experiment, thus spans two orders of magnitude. Clearly, the present situation with regard to Ων is not satisfactory. A determination of Ων or a significant constraint on the allowed parameter range of Ων would lead to a much better understanding of the role of neutrino HDM in the evolution of large scale structures.
One means of identifying neutrino HDM and fixing Ων are precise measurements of the temperature fluctuations of the CMBR with balloon or satellite experiments. However, the interpretation of these data crucially depends on model assumptions and the precise knowledge of other cosmological parameters. If, on the other hand, the absolute mass scale of neutrinos could be determined with sub-eV precision by a laboratory experiment, the corresponding Ων parameter would be an important input for the next generation satellite CMBR experiments like MAP and PLANCK.
The investigation of the still open role of neutrino HDM in the evolution of large scale structure is one of the main motivations for the proposed next-generation tritium β decay experiment KATRIN, which is designed to measure the absolute mass of the electron neutrino with sub-eV sensitivity. Correspondingly, KATRIN would be sensitive to a neutrino HDM contribution down to a value of Ων= 0.025, thus significantly constraining the role of neutrino HDM in structure formation.
Motivation in a Nutshell:
Particle Physics:
Experiments on solar and atmospheric neutrinos have demonstrated the existence of neutrino oscillations. Hence, neutrinos must be massive. These "oscillation experiments" determine differences of squared masses, but no absolute mass values. Neutrino masses might be "hierarchical" or "nearly degenerate". Theoretical models allow for both scenarios. Investigating the endpoint of the tritium beta decay spectrum --KATRIN-- allows to check the absolute value of the electron neutrino down to about 0.35 eV/c2 and helps to decide which scenario is chosen by nature.
Astrophysics and Cosmology:
Most of the universe´s matter density is in the form of -unknown- dark matter or dark energy. One candidate for dark matter -the only known so far- are massive neutrinos. Even with masses as small as 3 eV/c2 they could make up about 20% of the universe´s mass. With a sensitivity down to about 0.35 eV/c2, KATRIN will either detect a neutrino mass of cosmological relevance or exclude (in case of a negative result) any significant contribution of neutrinos to the universe´s matter content and therefore reduce the role of neutrinos in the forming of large cosmological structures.
