A Fresh Look at keV Sterile Neutrino Dark Matter from Frozen-In Scalars

Sterile neutrinos with a mass of a few keV can serve as cosmological warm dark matter. We study the production of keV sterile neutrinos in the early universe from the decay of a frozen-in scalar. Previous studies focused on heavy frozen-in scalars with masses above the Higgs mass leading to a hot spectrum for sterile neutrinos with masses below 8-10 keV. Motivated by the recent hints for an X-ray line at 3.55 keV, we extend the analysis to lighter frozen-in scalars, which allow for a cooler spectrum. Below the electroweak phase transition, several qualitatively new channels start contributing. The most important ones are annihilation into electroweak vector bosons, particularly W-bosons as well as Higgs decay into pairs of frozen-in scalars when kinematically allowed.Comment: 19 pages, 4 figures, model section (sec. 2) splits in effective description (sec. 2) and UV completion (sec. 5), minor changes, references added, matches published versio

Confronting Flavour Symmetries and extended Scalar Sectors with Lepton Flavour Violation Bounds

We discuss the tension between discrete flavour symmetries and extended scalar sectors arising from lepton flavour violation experiments. The key point is that extended scalar sectors will generically lead to flavour changing neutral currents, which are strongly constrained by experiments. Due to the large parameter space in the scalar sector such models will, however, usually have no big problems with existing and future bounds (even though the models might be constrained). This changes considerably once a flavour symmetry is imposed in addition: Due to the symmetry, additional relations between the different couplings arise and cancellations become impossible in certain cases. The experimental bounds will then constrain the model severely and can easily exclude it. We consider two examples which show how these considerations are realized. The same logic should apply to a much wider class of models.Comment: 19 pages, 2 figures; Introduction extended, typos corrected, charged lepton sector of model 2 corrected; matches journal versio

A Possible Solution of the Flavor Problem and Radiative Neutrino Masses

In dieser Arbeit behandeln wir zwei wichtige Probleme des Standard Modells der Teilchenphysik: das Familien-Problem sowie den Grund für die Kleinheit von Neutrinomassen. Das erste Problem könnte mit der Herkunft nicht-abelscher diskreter Familien-Symmetrien zusammenhängen. Wir diskutieren die Möglichkeit sie von der spontanen Brechung einer kontinuierlichen Familien-Symmetrie, d.h. SU(2) oder SU(3) zu erhalten. Weiter untersuchen wir ihre mögliche Herkunft von einer Orbifold Kompaktifizierung. Wir diskutieren alle diskreten Symmetrien, die man von einem zwei-dimensionalen Orbifold T2/ZN erhalten kann. Es sind die Gruppen A4, S4, D4, D3 und D6. Wir stellen die Idee vor, die Brechung einer Orbifold GUT mit der von dem Orbifold induzierten Familiensymmetrie zu kombinieren und zeigen die Konstruktion anhand eines sechs-dimensionalen supersymmetrischen SO(10) x S4 orbifold GUT Modells. Zur Erklärung der zweiten Frage schlagen wir ein Ein-Schleifen Neutrino-Massen Modell im Rahmen links-rechts symmetrischer Modelle vor. Wir beobachteten, daß die Hierarchie von den geladenen Lepton-Massen zu den recht-händigen Neutrino-Massen übertragen wird, was wir als "die radiative Übermittlung der Lepton Familien Hierarchy" bezeichnen. Schließlich, haben wir die phänomenologischen Aspekte des Modells untersucht, wie Lepton Familien Verletzung, Familien Zahl Verletzung und Familien ändernde neutrale Ströme

Connections between the Seesaw and Dark Matter Searches

In some dark matter models, the coupling of the dark matter particle to the standard model Higgs determines the dark matter relic density while it is also consistent with dark matter direct detection experiments. On the other hand, the seesaw for generating the neutrino masses probably arises from a spontaneous symmetry breaking of global lepton number. The dark matter particle thus can significantly annihilate into massless Majorons when the lepton number breaking scale and hence the seesaw scale is near the electroweak scale. This leads to an interesting interplay between neutrino physics and dark matter physics and the annihilation mode has an interesting implication on dark matter searches.Comment: 4 pages. Major revision. To appear in PR

Radiative Transmission of Lepton Flavor Hierarchies

We discuss a one loop model for neutrino masses which leads to a seesaw-like formula with the difference that the charged lepton masses replace the unknown Dirac mass matrix present in the usual seesaw case. This is a considerable reduction of parameters in the neutrino sector and predicts a strong hierarchical pattern in the right handed neutrino mass matrix that is easily derived from a $U(1)_H$ family symmetry. The model is based on the left-right gauge group with an additional $Z_4$ discrete symmetry which gives vanishing neutrino Dirac masses and finite Majorana masses arising at the one loop level. Furthermore, it is one of the few models that naturally allow for large (but not necessarily maximal) mixing angles in the lepton sector. A generalization of the model to the quark sector requires three iso-spin singlet vector-like down type quarks, as in $E_6$. The model predicts an inert doublet type scalar dark matter.Comment: 11 pages, 1 figur

Golden Ratio Prediction for Solar Neutrino Mixing

It has recently been speculated that the solar neutrino mixing angle is connected to the golden ratio phi. Two such proposals have been made, cot theta_{12} = phi and cos theta_{12} = phi/2. We compare these Ansatze and discuss a model leading to cos theta_{12} = phi/2 based on the dihedral group D_{10}. This symmetry is a natural candidate because the angle in the expression cos theta_{12} = phi/2 is simply pi/5, or 36 degrees. This is the exterior angle of a decagon and D_{10} is its rotational symmetry group. We also estimate radiative corrections to the golden ratio predictions.Comment: 15 pages, 1 figure. Matches published versio

Non-Abelian Discrete Dark Matter

We consider the minimal model in which dark matter is stabilized by a non-Abelian discrete symmetry. The symmetry group is taken to be D_3, which is the smallest non-Abelian finite group. The minimal model contains (nontrivial) singlet and doublet scalar representations of D_3 which couple to the Standard Model fields via the Higgs portal. This construction predicts two species of dark matter over much of the parameter space. Nontrivial interactions under D_3 lead to a novel thermal history of dark matter, while the multi-component nature of dark matter can be tested by future direct detection experiments.Comment: 12 pages, 6 figure

A4 Flavor Models in Split Seesaw Mechanism

A seesaw mechanism in an extra-dimension, known as the split seesaw mechanism, provides a natural way to realize a splitting mass spectrum of right-handed neutrinos. It leads to one keV sterile neutrino as a dark matter candidate and two heavy right-handed neutrinos being responsible for leptogenesis to explain the observed baryon asymmetry of the Universe. We study models based on $A_4$ flavor symmetry in the context of the split seesaw mechanism. It is pointed out that most of known $A_4$ flavor models with three right-handed neutrinos being $A_4$ triplet suffer from a degeneracy problem for the bulk mass terms, which disturbs the split mechanism for right-handed neutrino mass spectrum. Then we construct a new $A_4$ flavor model to work in the split seesaw mechanism. In the model, the experimentally observed neutrino masses and mixing angles can be realized from both type I+II seesaw contributions. The model predicts the $\mu-\tau$ symmetry in the neutrino mass matrix at the leading order, resulting in the vanishing $\theta_{13}$ and maximal $\theta_{23}$. The flavor symmetry $A_4$ is broken via the flavon vacuum alignment which can be obtained from the orbifold compactification. The model can be consistent with all data of neutrino oscillation experiments, cosmological discussions of dark matter abundance, leptogenesis, and recent astrophysical data.Comment: 21 pages, 1 figure, version to appear in JHE

Cascade Textures and SUSY SO(10) GUT

We give texture analyses of cascade hierarchical mass matrices in supersymmetric SO(10) grand unified theory. We embed cascade mass textures of the standard model fermion with right-handed neutrinos into the theory, which gives relations among the mass matrices of the fermions. The related phenomenologies, such as the lepton flavor violating processes and leptogenesis, are also investigated in addition to the PMNS mixing angles.Comment: 27 pages, 4 figures, comments and references added, final versio