Neutrinos in the standard model
Due to their special properties neutrinos are among the most interesting particles of the standard model of particle physics. Neutrinos (as well as antineutrinos) appear in three generations (flavors). Corresponding to their charged lepton partners, they are called electron, muon and tau neutrinos. But unlike their partners, neutrinos carry no electric charge and are considered massless (i.e. their rest mass is zero). They only take part in weak nuclear interactions that are transmitted by the heavy Z0 and W± bosons. As a consequence, these “ghost particles“ interact extremely rarely with normal matter and experiments aiming at their detection require very large volumes and target masses. On the other hand, the elusiveness of neutrinos makes them perfect probes for the observation of astrophysical objects. Electrically neutral and featuring an extraordinary interaction length, they are neither deflected by magnetic fields nor absorbed by while traveling to detectors on Earth. Even more, neutrinos permit to directly observe the fusion processes taking place in the center of our Sun or the core-collapse explosion of a galactic Supernova.
Neutrinos extending the standard model
A first hint of physics beyond the standard model was found by the Homestake solar neutrino experiment in the 1970s. Ray Davis and his collaborators observed a clear deficit in the measured neutrino rate compared to the predictions based on astrophysical models of the Sun. This disappearance of solar electron neutrinos can be explained in the framework of neutrino oscillations: Neutrinos that are produced in a certain flavor (electron) at the source have a non-zero probability to convert into another flavor (muon/tau) during their propagation through space, while for instance the Homestake detector was only sensitive to the electron flavor. This flavor mixing is caused by the difference between the mass eigenstates (solutions of the Dirac equation) that describe neutrino propagation and the flavor eigenstates which take part in weak interaction at production and detection. As the mass eigenstates develop at different phase factors during propagation, interference effects appear in the superposition of these states leading to distance-dependent variations in flavor detection probabilities. The occurence of oscillations is incompatible with the original standard model as it requires mass differences between the eigenstates and therefore non-zero neutrino masses.
Around the turn of the millennium, neutrino flavor oscillations were established beyond doubt by several experiments. Independent confirmation was given by the SNO experiment for solar neutrinos, the Super-Kamiokande detector observing atmospheric neutrinos and the reactor neutrino experiment KamLAND. Precision measurements of the parameters governing the oscillations – the mixing angles θij and the mass squared differences ∆mij2 (i,j =1,2,3) determining the oscillation amplitudes resp. oscillation lengths – are currently on-going. The last missing mixing angle θ13 was determined only in 2012 by the reactor neutrino experiments Daya Bay, Double Chooz and RENO. And there are still further open questions? What ist the neutrino mass hierarchy (cf. Fig. 3), i.e. is the third mass eigenstate heavier (normal hierarchy) or lighter (inverted hierarchy) than the others? Are there differences in the oscillation probabilities of neutrinos and antineutrinos, reflected by a CP-violating complex phase in the neutrino mixing matrix (analogously to the quark sector)? Is the neutrino its own antiparticle and thus of Majorana nature (investigated by neutrino-less double beta decay experiments)? Does the neutrino feature a non-zero magnetic moment?
Precision measurements of the decay width of the Z0 boson at LEP show that there are only the three known light neutrino flavors taking part in weak interaction. However, several neutrino experiments with short oscillation baselines have provided hints that there might be a further light neutrino state not subject to weak interaction. This raises the possibility of the existence of a novel “sterile” neutrino flavor which would stand clearly outside the framework of the standard model.
Fig. 3: Scheme of normal and inverted mass hierarchies