arXiv subjects

Werner Rodejohann

Publications and source records attributed to Werner Rodejohann.

At least 109 records · Page 6Linked to original sources

arXiv2012

"Hidden" O(2) and SO(2) Symmetry in Lepton Mixing

To generate the minimal neutrino Majorana mass matrix that has a free solar mixing angle and Delta m_sol^2 = 0 it suffices to implement an O(2) symmetry, or one of its subgroups SO(2), Z_N (N>2), or D_N (N>2). This O(2) generalizes the hidden Z_2^s of lepton mixing and leads in addition automatically to mu-tau symmetry. Flavor-democratic perturbations, as expected e.g. from the Planck scale, then result in tri-bimaximal mixing. We present a minimal model with three Higgs doublets implementing a type-I seesaw mechanism with a spontaneous breakdown of the symmetry, leading to large theta_13 and small Delta m_sol^2 = 0 due to the particular decomposition of the perturbations under mu-tau symmetry.

hep-ph↗

arXiv2012

Sterile Neutrinos for Warm Dark Matter and the Reactor Anomaly in Flavor Symmetry Models

We construct a flavor symmetry model based on the tetrahedral group A_4 in which the right-handed neutrinos from the seesaw mechanism can be both keV warm dark matter particles and eV-scale sterile neutrinos. This is achieved by giving the right-handed neutrinos appropriate charges under the same Froggatt-Nielsen symmetry responsible for the hierarchy of the charged lepton masses. We discuss the effect of next-to-leading order corrections to deviate the zeroth order tri-bimaximal mixing. Those corrections have two sources: (i) higher order seesaw terms, which are important when the seesaw particles are eV-scale, and (ii) higher-dimensional effective operators suppressed by additional powers of the cut-off scale of the theory. Whereas the mixing angles of the active neutrinos typically receive corrections of the same order, the mixing of the sterile neutrinos with the active ones is rather stable as it is connected with a hierarchy of mass scales. We also modify an effective A_4 model to incorporate keV-scale sterile neutrinos.

hep-ph↗

arXiv2012

Four-jet final state in same-sign lepton colliders and neutrinoless double beta decay mechanisms

If neutrinoless double beta decay is observed, it will be important to understand the mechanism(s) behind this process. Using a minimal supersymmetric extension to the Standard Model in association with a lepton number violating coupling as an example, we show that if neutrinoless double beta decay is mediated by new TeV scale particles, looking for four-jet final states in future linear colliders operating in the same-sign electron mode could provide important information on the underlying mechanisms. We study the prospects for observing this process at the proposed ILC and CLIC energies, and discuss the complementarity between such a four-jet signal and other collider signatures at the LHC.

hep-ph↗

arXiv2012

Proceedings of the first workshop on Flavor Symmetries and consequences in Accelerators and Cosmology (FLASY2011)

The main goals of the first "Workshop on FLAvor SYmmetries and consequences in accelerators and cosmology" (FLASY) was to summarize the theoretical status of flavor symmetries, bringing together young researchers in the field to stimulate discussions and new collaborations, with the aim of investigating possible new physics scenarios to be tested at the LHC, as well as in future neutrino, cosmology experiments and dark matter searches.

hep-ph↗

arXiv2012

Relating large $U_{e3}$ to the ratio of neutrino mass-squared differences

The non-zero and sizable value of $U_{e3}$ puts pressure on flavor symmetry models which predict an initially vanishing value. Hence, the tradition of relating fermion mixing matrix elements with fermion mass ratios might need to be resurrected. We note that the recently observed non-vanishing value of $U_{e3}$ can be related numerically to the ratio of solar and atmospheric mass-squared differences. The most straightforward realization of this can be achieved with a combination of texture zeros and a vanishing neutrino mass. We analyze the implications of some of these possibilities and construct explicit flavor symmetry models that predict these features.

hep-ph↗

arXiv2011

Kinetic and mass mixing with three abelian groups

We present the possible mixing effects associated with the low-energy limit of a Standard-Model extension by two abelian gauge groups U(1)_1 x U(1)_2. We derive general formulae and approximate expressions that connect the gauge eigenstates to the mass eigenstates. Applications using the well-studied groups U(1)_B, U(1)_(B-L), U(1)_(L_alpha - L_beta) (L_alpha being lepton flavor numbers), and U(1)_DM (a symmetry acting only on the dark matter sector) are discussed briefly.

hep-ph↗

arXiv2011

Systematic search for successful lepton mixing patterns with nonzero theta_13

We perform a systematic search for simple but viable lepton mixing patterns. Our main criterion is that the mixing matrix can be parameterized by three rotation angles, which are simple fractions of pi. These simple rotation angles possess exact expressions for their sines and cosines, and often arise in the flavor symmetry models. All possible parameterizations of the mixing matrix are taken into account. In total, twenty successful mixing patterns are found to be consistent with the latest neutrino oscillation data (including the recent T2K results) in the CP conserving case, whereas fifteen mixing patterns are allowed in the maximal CP violating case. Potential radiative corrections to the constant mixing patterns are also calculated by solving the renormalization group equations.

hep-ph↗

arXiv2011

The Glashow resonance at IceCube: signatures, event rates and $pp$ vs. $pγ$ interactions

We revisit the signatures of the Glashow resonance process $\barν_e e \to W$ in the high-energy astrophysical neutrino observatory IceCube. We note that in addition to the standard hadronic and electromagnetic showers produced by an incoming neutrino at the resonance energy of $E_ν\approx 6.3$ PeV, there are two clear signals of the process: the "pure muon" from $\barν_e e \to \barν_μμ$ and the "contained lollipop" from $\barν_e e \to \barν_ττ$. The event rate and the signal-to-background ratio (the ratio of the resonant to concurrent non-resonant processes) are calculated for each type of interaction, based on current flux limits on the diffuse neutrino flux. Because of the low background in the neighborhood of the resonance, the observation of only one pure muon or contained lollipop event essentially signals discovery of the resonance, even if the expected event numbers are small. We also evaluate the total event rates of the Glashow resonance from the extra-galactic diffuse neutrino flux and emphasize its utility as a discovery tool to enable first observations of such a flux. We find that one can expect 3.6 (0.65) events per year for a pure $pp$ ($pγ$) source, along with an added contribution of 0.51 (0.21) from non-resonant events. We also give results as a function of the ratio of $pp$ vs $pγ$ sources.

astro-ph.HE↗

arXiv2011

Neutrino-less Double Beta Decay and Particle Physics

We review the particle physics aspects of neutrino-less double beta decay. This process can be mediated by light massive Majorana neutrinos (standard interpretation) or by something else (non-standard interpretations). The physics potential of both interpretations is summarized and the consequences of future measurements or improved limits on the half-life of neutrino-less double beta decay are discussed. We try to cover all proposed alternative realizations of the decay, including light sterile neutrinos, supersymmetric or left-right symmetric theories, Majorons, and other exotic possibilities. Ways to distinguish the mechanisms from one another are discussed. Experimental and nuclear physics aspects are also briefly touched, alternative processes to double beta decay are discussed, and an extensive list of references is provided.

hep-ph↗

arXiv2011

Gauged L_mu - L_tau Symmetry at the Electroweak Scale

The extension of the Standard Model by a spontaneously broken abelian gauge group based on the L(mu)-L(tau) lepton number can resolve the longstanding discrepancy between experimental and theoretical values for the magnetic moment of the muon. It furthermore naturally generates mu-tau symmetric lepton mixing, introduces neutrino non-standard interactions, and the associated gauge boson Z' serves as a mediator to the right-handed neutrino sector. A detailed fit to electroweak data is performed to identify the allowed values for the mass of Z' and its mixing with the Standard Model Z. An economical new scalar sector is constructed that spontaneously breaks the new symmetry and leads to experimental consequences such as lepton flavor violation and collider signatures. Furthermore we discuss the non-abelian extension to an SU(2)', particularly the neutrino sector.

hep-ph↗

arXiv2011

Light Sterile Neutrinos: Models and Phenomenology

Motivated by recent hints in particle physics and cosmology, we study the realization of eV-scale sterile neutrinos within both the seesaw mechanism and flavor symmetry theories. We show that light sterile neutrinos can rather easily be accommodated in the popular A_4 flavor symmetry models. The exact tri-bimaximal mixing pattern is perturbed due to active-sterile mixing, which we discuss in detail for one example. In addition, we find an interesting extension of the type I seesaw, which can provide a natural origin for eV-scale sterile neutrinos as well as visible admixtures between sterile and active neutrinos. We also show that the presence of sterile neutrinos would significantly change the observables in neutrino experiments, specifically the oscillation probabilities in short-baseline experiments and the effective mass in neutrino-less double beta decay. The latter can prove particularly helpful to strengthen the case for eV-scale sterile neutrinos.

hep-ph↗

arXiv2011

Testing the Bimodal/Schizophrenic Neutrino Hypothesis in Neutrinoless Double Beta Decay and Neutrino Telescopes

The standard assumption is that all three neutrino mass states are either Dirac or Majorana. However, it was recently suggested by Allaverdi, Dutta and one of the authors (R.N.M.) that mixed, or bimodal, flavor neutrino scenarios are conceivable and are consistent with all known observations (these were called "schizophrenic" in the ADM paper). In that case each individual mass eigenstate can be either Dirac or Majorana, so that the flavor eigenstates are "large" admixtures of both. An example of this "bimodal" situation is to consider one mass state as a Dirac particle (with a sterile partner), while the other two are of Majorana type. Since only Majorana particles contribute to neutrinoless double beta decay, the usual dependence of this observable on the neutrino mass is modified within this scenario. We study this in detail and, in particular, generalize the idea for all possible bimodal combinations. Inevitably, radiative corrections will induce a pseudo-Dirac nature to the Dirac states at the one-loop level, and the effects of the pseudo-Dirac mass splitting will show up in the flavor ratios of neutrinos from distant cosmological sources. Comparison of the effective mass in neutrinoless double beta decay as well as flavor ratios at neutrino telescopes, for different pseudo-Dirac cases and with their usual phenomenology, can distinguish the different bimodal possibilities.

hep-ph↗

arXiv2011

Phenomenological Consequences of sub-leading Terms in See-Saw Formulas

Several aspects of next-to-leading (NLO) order corrections to see-saw formulas are discussed and phenomenologically relevant situations are identified. We generalize the formalism to calculate the NLO terms developed for the type I see-saw to variants like the inverse, double or linear see-saw, i.e., to cases in which more than two mass scales are present. In the standard type I case with very heavy fermion singlets the sub-leading terms are negligible. However, effects in the percent regime are possible when sub-matrices of the complete neutral fermion mass matrix obey a moderate hierarchy, e.g. weak scale and TeV scale. Examples are cancellations of large terms leading to small neutrino masses, or inverse see-saw scenarios. We furthermore identify situations in which no NLO corrections to certain observables arise, namely for mu-tau symmetry and cases with a vanishing neutrino mass. Finally, we emphasize that the unavoidable unitarity violation in see-saw scenarios with extra fermions can be calculated with the formalism in a straightforward manner.

hep-ph↗

arXiv2011

Neutrinoless Double Beta Decay, the Inverted Hierarchy and Precision Determination of theta(12)

Ruling out the inverted neutrino hierarchy with neutrinoless double beta decay experiments is possible if a limit on the effective mass below the minimal theoretically possible value is reached. We stress that this lower limit depends strongly on the value of the solar neutrino mixing angle: it introduces an uncertainty of a factor of 2 within its current 3 sigma range. If an experiment is not background-free, a factor of two in effective mass corresponds to a combined factor of 16 improvement for the experimental parameters running time, detector mass, background level and energy resolution. Therefore, a more precise determination of theta(12) is crucial for the interpretation of experimental results and the evaluation of the potential and requirements for future experiments. We give the required half-lifes to exclude (and touch) the inverted hierarchy regime for all double beta decay isotopes with a Q-value above 2 MeV. The nuclear matrix elements from 6 different groups and, if available, their errors are used and compared. We carefully put the calculations on equal footing in what regards various convention issues. We also use our compilation of matrix elements to give the reachable values of the effective mass for a given half-life value.

hep-ph↗

arXiv2011

Lower Bounds on U_{e3}

We give minimal values for the smallest lepton mixing parameter U_{e3}, applying 2-loop renormalization group equations in an effective theory approach. This is relevant in scenarios that predict an inverted neutrino mass spectrum with the smallest mass and U_{e3} being zero at tree level, a situation known to be preserved at 1-loop order. At 2-loop, U_{e3} is generated at a level of 10^{-12}-10^{-14}. Such small values are of interest in supernova physics. Corresponding limits for the normal mass ordering are several orders of magnitude larger. Our results show that U_{e3} can in general expected be to be non-zero.

hep-ph↗

arXiv2011

Extended Empirical Fermion Mass Relation

It is known that the charged lepton masses obey to high precision an interesting empirical relation (Koide relation). In turn, the light neutrino masses cannot obey such a relation. We note that if neutrinos acquire their mass via the seesaw mechanism, the empirical mass relation could hold for the masses in the Dirac and/or heavy Majorana mass matrix. Examples for the phenomenological consequences are provided. We furthermore modify the mass relation for light neutrino masses including their Majorana phases, and show that it can be fulfilled in this case as well, with interesting predictions for neutrinoless double beta decay. Finally, we remark that while the relation does not hold for the up- and down-quarks, it may be valid for the u, d, s quarks, and for the c, b, t quarks.

hep-ph↗

arXiv2010

Neutrino Phenomenology of gauged L_mu - L_tau: MINOS and beyond

If a Z' gauge boson from a gauged L_mu - L_tau symmetry is very light, it is associated with a long-range leptonic force. In this case the particles in the Sun create via mixing of the Z' with the Standard Model Z a flavor-dependent potential for muon neutrinos in terrestrial long-baseline experiments. The potential changes sign for anti-neutrinos and hence can lead to apparent differences in neutrino and anti-neutrino oscillations without introducing CP or CPT violation. This could for instance explain the recently found discrepancy in the MINOS experiment. We obtain the associated parameters of gauged L_mu - L_tau required to explain this anomaly. The consequences for future long-baseline experiments are also discussed, and we compare the scenario to standard NSIs. When used to explain MINOS, both approaches have severe difficulties with existing limits.

hep-ph↗

arXiv2010

Neutrinoless Double Beta Decay in Particle Physics

Neutrinoless double beta decay is a process of fundamental importance for particle physics. It can be mediated by light massive Majorana neutrinos (standard interpretation) or by something else (non-standard interpretations). We review its dependence on the neutrino parameters, its complementarity to other observables sensitive to neutrino mass, and emphasize its ability to distinguish different neutrino mass models. Then we discuss mechanisms different from light Majorana neutrino exchange, and show what can be learned from those and how they could be tested.

hep-ph↗