Walter Grimus

Ao. Univ. Prof. Dr. Walter Grimus

Particle Physics Group, Faculty of Physics
University of Vienna
Boltzmanngasse 5, A-1090 Wien



Lightest neutrino mass. The blue and red colours indicate the allowed range for the normal and inverted ordering of the neutrino mass spectrum, respectively.

The lepton mixing matrix has a very specific form, radically different from the CKM matrix. This has given a new impetus to the study of the flavour problem. In this context possible structures of the lepton mass matrices are studied. Of particular interest are those structures which can be enforced by finite flavour symmetry groups. Such flavour models inevitably lead to an extended scalar sector whose possible signatures are another area of research.

Models for trimaximal and tri-bimaximal lepton mixing have been investigated in the framework of the type I seesaw mechanism and it has been stressed that using more than three right-handed neutrino singlets together with a suitable flavour symmetry group can lead to successful models of trimaximal and tri-bimaximal lepton mixing. However, in view of the recent measurements of a non-vanishing mixing angle θ13, it is appropriate to seek for alternatives to the tri-bimaximal scheme.

More general investigations comprise numerical studies of the neutrino mass matrix, for instance the study of hybrid textures. Work has also been done on the type~II seesaw mechanism for small vacuum expectation values of Higgs doublets, and its various applications have been pointed out. A particular interesting case is the generation of “naturally” small Dirac neutrino masses, an alternative to the conventional type I seesaw mechanism leading to Majorana neutrinos.

Type I seesaw models have been studied where the scalar doublet which couples to the right-handed neutrino singlets has a small vacuum expectation value, for instance of the order of the electron mass. In that case the seesaw scale could be in the TeV range and lepton number violating production of charged scalars, e.g. e- e- to S- S-, could be possible.

Since models for neutrino masses and lepton mixing necessitate an extended scalar sector which might be restricted by electroweak precision tests, the oblique parameters in the multi-Higgs doublet Standard Model, with possible additional scalar gauge singlets, have been computed.