Setup of a Drift Tube Muon Tracker and Calibration of Muon ...

However, it did not yet deliver an answer to the solar neutrino puzzle but ratherconfirmed the lower than expected ν e flux.With the experimental observation of neutrino oscillations (cf. Section 2.2) in1998 by the Super-Kamiokande experiment [12] and the confirmation for solar neutrinosin particular by the SNO experiment in 2001 [13], the solar neutrino puzzlecould finally be resolved. The latter experiment succeeded in measuring both thesolar ν e flux in CC reactions as well as the fluxes from all neutrino types through NCprocesses. Whereas the ν e flux was again only about one third of the expected, thetotal flux was in agreement with the predictions. The answer to the solar neutrinopuzzle was found: some of the originally electron type neutrinos change their flavoron the way from the Sun to Earth and are thus not detected by experiments mereleysensitive to ν e .The evidence of neutrino oscillations also showed that neutrinos have a non vanishingmass after all. However, it does not give an answer to the absolute neutrinosmasses—only the squared mass splittings can be determined, leaving also the questionof the mass hierarchy mostly unanswered. The determination of the absolute(electron type) neutrino mass has been addressed by several experiments, e.g. byinvestigating the endpoint energy of the β decay of tritium. Both the Mainz [14] andthe Troitsk [15] experiments found an upper limit slightly above 2eV. Currently, theKATRIN [16] experiment is being built aiming at a sensitivity of 0.2 eV.Neutrinos are the only leptons with no charge. This leaves the possibility for theneutrino to be its own anti particle. This scenario was first suggested by E. Majoranain 1937 [17] and such a neutrino would be called a Majorana particle. If neutrinos aredifferent from their anti particles they are called Dirac particles. The assumption ofneutrinos being their own anti particles leads to some interesting consequences likethe neutrinoless double-beta decay (0νββ). In an ordinary (two neutrino) doublebetadecay, two electrons and two neutrinos are emitted. If the neutrino is indeed aMajorana particle, the two neutrinos could annihilate, thus only two electrons wouldbe emitted. Several experiments now search for the neutrinoless double-beta decay(e.g. [18–20]).2.2 Neutrino Mixing and OscillationsNeutrino oscillations have been experimentally verified by various experiments duringthe last decade. The existence of neutrino oscillations proofs that neutrinos havea mass, after all. Also, lepton number conservation within each lepton generation isbroken. This section gives a short introduction to neutrino oscillations. First, thegeneral case in vacuum is considered. It is followed by a look at the influence ofmatter to neutrino oscillations.2.2.1 Oscillations in VacuumIn case of a non vanishing neutrino mass, the neutrino’s mass eigenstates |ν i 〉 donot necessarily have to be the same as the weak eigenstates |ν α 〉. A neutrino |ν α 〉 offlavor α, where α = e,µ or τ can then be described by a superposition of the mass7