arXiv subjects

Werner Rodejohann

Publications and source records attributed to Werner Rodejohann.

At least 91 records · Page 5Linked to original sources

arXiv2014

Signatures of Extra Dimensional Sterile Neutrinos

We study a large extra dimension model with active and sterile Dirac neutrinos. The sterile neutrino masses stem from compactification of an extra dimension with radius $R$ and are chosen to have masses around eV or keV, in order to explain short-baseline anomalies or act as warm dark matter candidates. We study the effect of the sterile neutrino Kaluza-Klein tower in short-baseline oscillation experiments and in the beta spectrum as measurable by KATRIN-like experiments.

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arXiv2014

Sterile neutrinos and right-handed currents in KATRIN

Kurie-plot experiments allow for neutrino-mass measurements based on kinematics in an almost model-independent manner. A future tritium-based KATRIN-like experiment can be sensitive to light sterile neutrinos with masses below 18 keV, which are among the prime candidates for warm dark matter. Here we consider such keV neutrinos in left--right symmetric extensions, i.e. coupled to right-handed currents, which allow for an enhanced contribution to beta decay even for small active--sterile mixing, without violating astrophysical X-ray constraints. The modified spectral shape is in principle distinguishable from the standard contribution---especially for sterile neutrino masses below 9 keV, which can lead to a distinct peak. We compare the sensitivity to constraints from the LHC and neutrinoless double beta decay.

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arXiv2014

Reducing θ_13 to 9 degrees

We propose to consider the possibility that the observed value of $θ_{13}$ is not the result of a correction from an initially vanishing value, but rather the result of a correction from an initially larger value. As an explicit example of this approach, we consider analytically and numerically well-known CKM-like charged lepton corrections to a neutrino diagonalization matrix that corresponds to a certain mixing scheme. Usually this results in generating $θ_{13} = 9^\circ$ from zero. We note here, however, that 9 is not only given by $0 + 9$, but also by $18 - 9$. Hence, the extreme case of an initial value of 18 degrees, reduced by charged lepton corrections to 9 degrees, is possible. For some cases under study new sum rules for the mixing parameters, and correlations with CP phases are found.

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arXiv2013

Lepton number and flavour violation in TeV-scale left-right symmetric theories with large left-right mixing

The various diagrams leading to neutrinoless double beta decay in the left-right symmetric model have different relative magnitudes, depending on the scale of new physics. Neutrinos acquire mass from both type I and/or type II seesaw terms, making an unambiguous analysis difficult. We study the half-life for double beta decay in the case of type II and type I dominance, in the former case including interference terms. If the heavy neutrinos of the type I seesaw model are at the TeV scale, certain processes can be enhanced. In particular, there are regions of parameter space in which the so-called lambda- and eta-diagrams can give sizable contributions to the half-life for the decay. We perform a detailed study of one such scenario, paying careful attention to constraints from lepton flavour violation.

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arXiv2013

Fits to SO(10) Grand Unified Models

We perform numerical fits of Grand Unified Models based on SO(10), using various combinations of 10-, 120- and 126-dimensional Higgs representations. Both the supersymmetric and non-supersymmetric versions are fitted, as well as both possible neutrino mass orderings. In contrast to most previous works, we perform the fits at the weak scale, i.e. we use RG evolution from the GUT scale, at which the GUT-relations between the various Yukawa coupling matrices hold, down to the weak scale. In addition, the right-handed neutrinos of the seesaw mechanism are integrated out one by one in the RG running. Other new features are the inclusion of recent results on the reactor neutrino mixing angle and the Higgs mass (in the non-SUSY case). As expected from vacuum stability considerations, the low Higgs mass and the large top-quark Yukawa coupling cause some pressure on the fits. A lower top-quark mass, as sometimes argued to be the result of a more consistent extraction from experimental results, can relieve this pressure and improve the fits. We give predictions for neutrino masses, including the effective one for neutrinoless double beta decay, as well as the atmospheric neutrino mixing angle and the leptonic CP phase for neutrino oscillations.

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arXiv2013

Neutrinoless Quadruple Beta Decay

We point out that lepton number violation is possible even if neutrinos are Dirac particles. We illustrate this by constructing a simple model that allows for lepton number violation by four units only. As a consequence, neutrinoless double beta decay is forbidden, but neutrinoless quadruple beta decay is possible: $(A,Z) \to (A,Z+4) + 4 e^-$. We identify three candidate isotopes for this decay, the most promising one being Nd-150 due to its high $Q_{0ν4β}$-value of 2 MeV. Analogous processes, such as neutrinoless quadruple electron capture, are also possible. The expected lifetimes are extremely long, and experimental searches are challenging.

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arXiv2013

Reproducing lepton mixing in a texture zero model

We note that the emerging features of lepton mixing can be reproduced if, with inverted neutrino mass ordering, both the smallest neutrino mass and the $ττ$ element of the neutrino mass matrix vanish. Then, the atmospheric neutrino mixing angle is less than maximal and the Dirac phase $δ$ is close to $π$. We derive the correlations among the mixing parameters and show that there is a large cancellation in the effective mass responsible for neutrinoless $ββ$ decay. Three simple seesaw models leading to our scenario are provided.

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arXiv2013

Constraining Neutrino Mass from Neutrinoless Double Beta Decay

We re-analyze the compatibility of the claimed observation of neutrinoless double beta decay ($0νββ$) in $^{76}$Ge with the new limits on the half-life of $^{136}$Xe from EXO-200 and KamLAND-Zen. Including recent calculations of the nuclear matrix elements (NMEs), we show that while the claim in $^{76}$Ge is still compatible with the individual limits from $^{136}$Xe, it is inconsistent with the KamLAND-Zen+EXO-200 combined limit for all but one NME calculations. After imposing the most stringent upper limit on the sum of light neutrino masses from Planck, we find that the canonical light neutrino contribution cannot satisfy the claimed $0νββ$ signature or saturate the current limit, irrespective of the NME uncertainties. However, inclusion of the heavy neutrino contributions, arising naturally in TeV-scale Left-Right symmetric models, can saturate the current limit of $0νββ$. In a type-II seesaw framework, this imposes a lower limit on the lightest neutrino mass. Depending on the mass hierarchy, we obtain this limit to be in the range of 0.07 - 4 meV for a typical choice of the right-handed (RH) gauge boson and RH neutrino masses relevant for their collider searches. Using the $0νββ$ bounds, we also derive correlated constraints in the RH sector, complimentary to those from the LHC.

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arXiv2013

A(4) Flavor Symmetry Model for Dirac-Neutrinos and Sizable U(e3)

Models based on flavor symmetries are the most often studied approaches to explain the unexpected structure of lepton mixing. In many flavor symmetry groups a product of two triplet representations contains a symmetric and an anti-symmetric term. If this product of two triplets corresponds to a Majorana mass term, then the anti-symmetric part vanishes, and in economic models tri-bimaximal mixing is achieved. If neutrinos are Dirac particles, the anti-symmetric part is however present and leads to deviations from tri-bimaximal mixing, in particular non-zero U(e3). Thus, the non-vanishing value of U(e3) and the nature of the neutrino are connected. We illustrate this with a model based on A(4) within the framework of a neutrinophilic 2 Higgs doublet scenario.

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arXiv2013

Sterile Neutrino Anarchy

Lepton mixing, which requires physics beyond the Standard Model, is surprisingly compatible with a minimal, symmetryless and unbiased approach, called anarchy. This contrasts with highly involved flavor symmetry models. On the other hand, hints for light sterile neutrinos have emerged from a variety of independent experiments and observations. If confirmed, their existence would represent a groundbreaking discovery, calling for a theoretical interpretation. We discuss anarchy in the two-neutrino eV-scale seesaw framework. The distributions of mixing angles and masses according to anarchy are in agreement with global fits for the active and sterile neutrino parameters. Our minimal and economical scenario predicts the absence of neutrinoless double beta decay and one vanishing neutrino mass, and can therefore be tested in future experiments.

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arXiv2012

Lower Limits on $μ\to e γ$ from new Measurements on $U_{e3}$

New data on the lepton mixing angle $θ_{13}$ imply that the $eμ$ element of the matrix $m_νm_ν^\dagger$, where $m_ν$ is the neutrino Majorana mass matrix, cannot vanish. This implies a lower limit on lepton flavor violating processes in the $eμ$ sector in a variety of frameworks, including Higgs triplet models or the concept of minimal flavor violation in the lepton sector. We illustrate this for the branching ratio of $μ\to e γ$ in the type II seesaw mechanism, in which a Higgs triplet is responsible for neutrino mass and also mediates lepton flavor violation. We also discuss processes like $μ\to e\bar{e}e$ and $μ\to e$ conversion in nuclei. Since these processes have sensitivity on the individual entries of $m_ν$, their rates can still be vanishingly small.

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arXiv2012

On the interpretation of IceCube cascade events in terms of the Glashow resonance

The IceCube experiment (IC) has recently observed 2 cascade events with energies between 1 and 10 PeV. This energy combined with the fact that no muon-track events are observed may be interpreted as a cosmogenic $\bar ν_e$ interacting in IC via the Glashow resonance (GR) $\bar ν_e e \to W^-\to$ (hadrons or $\bar ν_e e$). We point out a unique, background-free signature of the GR, a single isolated muon unaccompanied by any shower activity from the interaction $\bar ν_e e \to W^-\to \bar ν_μμ^-$, and propose it as a test of this interpretation. We calculate the event numbers and find that a single such event is expected over about a three-year period in IC. We also show that, if event rates remain at their current levels then, even with the GR, standard cosmogenic fluxes cannot easily explain the observations. Moreover, if muon-tracks remain conspicuous by their absence, then new physics needs to be invoked. As example scenarios in conformity with the observations, we calculate event rates for neutrino decay and Lorentz-invariance violation.

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arXiv2012

Neutrinoless double beta decay and neutrino physics

The connection of neutrino physics with neutrinoless double beta decay is reviewed. After presenting the current status of the PMNS matrix and the theoretical background of neutrino mass and lepton mixing, we will summarize the various implications of neutrino physics for double beta decay. The influence of light sterile neutrinos and other exotic modifications of the three neutrino picture is also discussed.

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arXiv2012

Simple two Parameter Description of Lepton Mixing

We note that a simple two parameter description of lepton mixing is possible which reproduces the features that apparently emerge from global fits at the 1σlevel: if U_{e3} is non-zero it implies that the solar neutrino mixing parameter \sin^2 θ_{12} is less than 1/3 by order |U_{e3}|^2. If the CP phase δis around πit implies that the atmospheric neutrino mixing parameter \sin^2 θ_{23} is less than 1/2 by order |U_{e3}|. The mixing scheme can be described by a 23-rotation appearing to the right of a tri-bimaximal mixing matrix. We quantify the excellent agreement of the scheme with data statistically, and comment on model building aspects.

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arXiv2012

Neutrino Hierarchies from a Gauge Symmetry

We consider the phenomenology of the gauged abelian symmetry B + 3 (L_e - L_mu - L_tau). Right-handed neutrinos necessary to cancel triangle anomalies are used in a type-I seesaw scheme to create active neutrino masses. Breaking the B + 3 (L_e - L_mu - L_tau) symmetry spontaneously below the seesaw scale generates low energy neutrino mass matrices with the approximate symmetries L_e (leading to normal hierarchy) or L_e - L_mu - L_tau (inverted hierarchy). For the latter we need to introduce a Z_2 symmetry which decouples one of the right-handed neutrinos. If exact, this Z_2 leads to a Majorana dark matter candidate that interacts with the Standard Model via the Z' and a scalar s originating from spontaneous breaking of the new symmetry. The measured relic abundance of the dark matter particle can be obtained around the scalar and Z' resonances, while direct detection experiments are mainly sensitive to scalar exchange, which is induced by mass mixing of s with the standard Higgs.

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arXiv2012

Impact of massive neutrinos on the Higgs self-coupling and electroweak vacuum stability

The presence of right-handed neutrinos in the type I seesaw mechanism may lead to significant corrections to the RG evolution of the Higgs self-coupling. Compared to the Standard Model case, the Higgs mass window can become narrower, and the cutoff scale become lower. Naively, these effects decrease with decreasing right-handed neutrino mass. However, we point out that the unknown Dirac Yukawa matrix may impact the vacuum stability constraints even in the low scale seesaw case not far away from the electroweak scale, hence much below the canonical seesaw scale of 10^15 GeV. This includes situations in which production of right-handed neutrinos at colliders is possible. We illustrate this within a particular parametrization of the Dirac Yukawas and with explicit low scale seesaw models. We also note the effect of massive neutrinos on the top quark Yukawa coupling, whose high energy value can be increased with respect to the Standard Model case.

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arXiv2012

Lepton Number and Lepton Flavor Violation through Color Octet States

We discuss neutrinoless double beta decay and lepton flavor violating decays such as $μ-> eγ$ in the colored seesaw scenario. In this mechanism, neutrino masses are generated at one-loop via the exchange of TeV-scale fermionic and scalar color octets. The same particles mediate lepton number and flavor violating processes. We show that within this framework a dominant color octet contribution to neutrinoless double beta decay is possible without being in conflict with constraints from lepton flavor violating processes. We furthermore compare the "direct" color octet contribution to neutrinoless double beta decay with the "indirect" contribution, namely the usual standard light Majorana neutrino exchange. For degenerate color octet fermionic states both contributions are proportional to the usual effective mass, while for non-degenerate octet fermions this feature is not present. Depending on the model parameters, either of the contributions can be dominant.

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arXiv2012

Linear Collider Test of a Neutrinoless Double Beta Decay Mechanism in left-right Symmetric Theories

There are various diagrams leading to neutrinoless double beta decay in left-right symmetric theories based on the gauge group SU(2)_L x SU(2)_R. All can in principle be tested at a linear collider running in electron-electron mode. We argue that the so-called lambda-diagram is the most promising one. Taking the current limit on this diagram from double beta decay experiments, we evaluate the relevant cross section e e to W_L W_R, where W_L is the Standard Model W-boson and W_R the one from SU(2)_R. It is observable if the life-time of double beta decay and the mass of the W_R are close to current limits. Beam polarization effects and the high-energy behaviour of the cross section are also analyzed.

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