The Mainz Neutrino Mass Experiment          

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Introduction

Up to recently the neutrinos, the 3 neutral out of the 12 fundamental fermions, out of which all matter is made, were assumed to be massless particles. But recent results from the atmospheric and solar neutrino experiments [1] indicate the existance of neutrino oscillations and therefore require non-zero neutrino masses. Although we now know that neutrinos have masses, we still do not know, which ones. The reason for this is, that these oscillation experiments are only sensitive to differences between squared masses (and mixing parameters) of different neutrinos, but not to its absolute mass values. On the other hand not only for particle phyiscs but also for cosmology it is very important to know the value of the neutrino masses:
  • Particle physics
    We have a very precise understanding of the elementary particles and their interactions by the so-called Standard Model. The neutrinos are assumed to be massless in this model. But this Standard Model neither explains the pattern of the fermion masses nor the mixing between the quarks (6 out of the 12 fundamental fermions). The parameters describing masses and mixing have to be determined by experiments. This unsatisfactory situation calls for an extended theory beyond the Standard Model. In most of such new theories the neutrinos get masses naturally in agreement with the experimental findings mentioned above. Since neutrinos are much lighter than the other fermions their masses are of special interest for these theories. Knowing the neutrino mass values and their mixing might help us to find the right theoretical description out of all possible theories. Concerning the neutrino mass generation in such models it is of special interest whether the neutrino masses are hierarchical



    or nearly degenerated


    In the latter case residual small differences between the various nearly degenerated neutrino masses is the reason for the neutrino oscillation signals observed with solar and atmospheric neutrinos. From the observed in these experiments the two cases cannot be distinguished. To solve this problem a sensitivity on the neutrino mass itself of 1 eV/c²  is required.

  • Cosmology
    According to the big bang theory there exists a huge amount of neutrinos in the universe left over from the big bang, like the photons of the so-called cosmic microwave background radiation, which were discovered more than 30 years ago. The ratio of theses relic neutrinos to atoms is about one billion to one, therefore even very small neutrino masses a very important. From different observations we know, that a large amount of non-visible, so-called Dark Matter, contributes more to the total matter in the universe, than all stars, even more than all the atoms in the universe together. It is very interesting to know, especially for the still open questions of structure formation and the evolution of the universe, whether neutrinos contribute to this dark matter in a significant way or not. Also the answer to this requires a sensitivity on the neutrino mass of 1 eV/c² .
The only way to determine neutrino masses without the requirement of further assumptions are the so-called direct mass measurements, of which the investigation of the endpoint region of the tritium decay spectrum is the most sensitive one: Tritium undergoes a decay into a Helim ion emitting an electron antineutrino and an electron. The electron energy spectrum, the so-called spectrum, is senstive to the value of the electron(anti-)neutrino mass at its upper end at E0 = 18.6 keV (s. figure 1). The measurement of the neutrino mass is therefore nothing else but the extremely precise determination of the shape of the spectrum in the region just below the endpoint E0.