arXiv subjects

W. Rodejohann

Publications and source records attributed to W. Rodejohann.

At least 37 records · Page 2Linked to original sources

arXiv2005

Neutrinoless Double Beta Decay and Future Neutrino Oscillation Precision Experiments

We discuss to what extent future precision measurements of neutrino mixing observables will influence the information we can draw from a measurement of (or an improved limit on) neutrinoless double beta decay. Whereas the Delta m^2 corresponding to solar and atmospheric neutrino oscillations are expected to be known with good precision, the parameter theta_{12} will govern large part of the uncertainty. We focus in particular on the possibility of distinguishing the neutrino mass hierarchies and on setting a limit on the neutrino mass. We give the largest allowed values of the neutrino masses which allow to distinguish the normal from the inverted hierarchy. All aspects are discussed as a function of the uncertainty stemming from the involved nuclear matrix elements. The implications of a vanishing, or extremely small, effective mass are also investigated. By giving a large list of possible neutrino mass matrices and their predictions for the observables, we finally explore how a measurement of (or an improved limit on) neutrinoless double beta decay can help to identify the neutrino mass matrix if more precise values of the relevant parameters are known.

hep-ph↗

arXiv2005

Minimal Mass Matrices for Dirac Neutrinos

We consider the possibility of neutrinos being Dirac particles and study minimal mass matrices with as much zero entries as possible. We find that up to 5 zero entries are allowed. Those matrices predict one vanishing mass state, CP conservation and U_{e3} either zero or proportional to R, where R is the ratio of the solar and atmospheric Δm^2. Matrices containing 4 zeros can be classified in categories predicting U_{e3} = 0, U_{e3} \neq 0 but no CP violation or |U_{e3}| \neq 0 and possible CP violation. Some cases allow to set constraints on the neutrino masses. The characteristic value of U_{e3} capable of distinguishing some of the cases with non-trivial phenomenological consequences is about R/2 \sin 2 θ_{12}. Matrices containing 3 and less zero entries imply (with a few exceptions) no correlation for the observables. We outline models leading to the textures based on the Froggatt-Nielsen mechanism or the non-Abelian discrete symmetry D_4 \times Z_2.

hep-ph↗

arXiv2005

Flavor Symmetry L_e - L_mu - L_tau, Atmospheric Neutrino Mixing and CP Violation in the Lepton Sector

The PMNS neutrino mixing matrix is given, in general, by the product of two unitary matrices associated with the diagonalization of the charged lepton and neutrino mass matrices, respectively. Assuming that the active flavor neutrinos possess a Majorana mass matrix which is diagonalized by a bimaximal mixing matrix, we give the allowed forms of the charged lepton mixing matrix and the corresponding implied forms of the charged lepton mass matrix. We further assume that the origin of bimaximal mixing is a weakly broken flavor symmetry of the neutrino Majorana mass matrix, corresponding to the conservation of the non-standard lepton charge L' = L_e - L_mu - L_tau. The latter does not predict, in general, the atmospheric neutrino mixing to be maximal. We study the impact of this fact on the allowed forms of the charged lepton mixing matrix and on the neutrino mixing observables, analyzing the case of CP nonconservation in detail. We find, in particular, that the allowed ranges of U_{e3} and of the atmospheric neutrino mixing angle are typically larger when one assumes L' conservation than in the case of exact initial bimaximal mixing. The atmospheric neutrino mixing can take any value inside its currently allowed range, while |U_{e3}|^2 can be as small as 0.007. We discuss under which conditions the atmospheric neutrino mixing angle can be larger or smaller than pi/4. We present also a simple see-saw realization of the implied light neutrino Majorana mass matrix and consider leptogenesis in this scenario.

hep-ph↗

arXiv2004

Theory of Neutrinos

After a brief overview of the present knowledge of neutrino masses and mixing, we summarize what can be learned about physics beyond the standard model from the various proposed neutrino experiments. We also comment on the impact of the experiments on our understanding of the origin of the matter-antimatter asymmetry of the Universe as well as what can be learned from some experiments outside the domain of neutrinos.

hep-ph↗

arXiv2004

Type II See-Saw Mechanism, Deviations from Bimaximal Neutrino Mixing and Leptogenesis

A possible interplay of both terms in the type II see-saw formula is illustrated by presenting a novel way to generate deviations from exact bimaximal neutrino mixing. In type II see-saw mechanism with dominance of the non-canonical SU(2)_L triplet term, the conventional see-saw term can give a small contribution to the neutrino mass matrix. If the triplet term corresponds to the bimaximal mixing scheme in the normal hierarchy, the small contribution of the conventional see-saw term naturally generates non-maximal solar neutrino mixing. Atmospheric neutrino mixing is also reduced from maximal, corresponding to 1 - \sin^2 2 θ_{23} of order 0.01. Also, small but non-vanishing U_{e3} of order 0.001 is obtained. It is also possible that the Δm^2 responsible for solar neutrino oscillations is induced by the small conventional see-saw term. Larger deviations from zero U_{e3} and from maximal atmospheric neutrino mixing are then expected. This scenario links the small ratio of the solar and atmospheric Δm^2 with the deviation from maximal solar neutrino mixing. We comment on leptogenesis in this scenario and compare the contributions to the decay asymmetry of the heavy Majorana neutrinos as induced by themselves and by the triplet.

hep-ph↗

arXiv2004

On Deviations from Bimaximal Neutrino Mixing

The PMNS neutrino mixing matrix U_{PMNS} is in general a product of two unitary matrices U_{lep} and U_ν arising from the diagonalization of the charged lepton and neutrino mass matrices, U_{PMNS} = U^{\dagger}_{lep} U_ν. Assuming that U_ν is a bimaximal mixing matrix, we investigate the possible forms of U_{lep}. We identify three possible generic structures of U_{lep}, which are compatible with the existing data on neutrino mixing. One corresponds to a hierarchical ``CKM-like'' matrix. In this case relatively large values of the solar neutrino mixing angle θ_{sol}, and of |U_{e3}|^2, are typically predicted, \tan^2θ_{sol} \gtap 0.42, |U_{e3}|^2 \gtap 0.02, while the atmospheric neutrino mixing angle θ_{atm} can deviate noticeably from π/4, \sin^2 2θ_{atm} \gtap 0.95. The second corresponds to one of the mixing angles in U_{lep} being equal to π/2, and predicts practically maximal atmospheric neutrino mixing \sin^2 2θ_{atm} \simeq 1. Large atmospheric neutrino mixing, \sin^2 2θ_{atm} \gtap 0.95, is naturally predicted by the third possible generic structure of U_{lep}, which corresponds to all three mixing angles in U_{lep} being large. We focus especially on the case of CP-nonconservation, analyzing it in detail. We show how the CP-violating phases, arising from the diagonalization of the neutrino and charged lepton mass matrices, contribute to the measured neutrino mixing observables.

hep-ph↗

arXiv2003

A Parametrization for the Neutrino Mixing Matrix

We propose a flexible and model independent parametrization of the neutrino mixing matrix, which takes advantage of the fact that there are up to three small quantities in neutrino mixing phenomenology: (i) the deviation from maximal mixing of solar neutrinos, (ii) the mixing matrix element U_{e3} and (iii) the deviation from maximal mixing of atmospheric neutrinos. It is possible to quantify those three observations with a parameter λ\sim 0.2, which appears at least linearly in all elements of the mixing matrix. The limit λ-> 0 corresponds to exact bimaximal mixing. Present and future experiments can be used to pin down the power of λrequired to usefully describe the observed phenomenology. Observing that the ratio of the two measured mass squared differences is roughly λ^2 allows to further study the structure of the Majorana mass matrix. We comment on the implications of this parametrization for neutrinoless double beta decay and on the oscillation probabilities in long-baseline experiments.

hep-ph↗

arXiv2003

Neutrino mass matrices leaving no trace

We point attention to the fact that in SO(10) models with non-canonical (type II) see-saw mechanism and exact b-tau unification the trace of the neutrino mass matrix is very small, in fact practically zero. This has the advantage of being a basis independent feature. Taking a vanishing trace as input, immediate phenomenological consequences for the values of the neutrino masses, the CP phases or the predictions for neutrinoless double beta decay arise. We analyze the impact of the zero trace condition for the normal and inverted mass ordering and in case of CP conservation and violation. Simple candidate mass matrices with (close to) vanishing trace and non-zero U_{e3} are proposed. We also compare the results with the other most simple basis independent property, namely a vanishing determinant.

hep-ph↗

arXiv2003

On the Connection of Leptogenesis with Low Energy CP Violation and LFV Charged Lepton Decays

Assuming only a hierarchical structure of the heavy Majorana neutrino masses and of the neutrino Dirac mass matrix m_D of the see-saw mechanism, we find that in order to produce the observed baryon asymmetry of the Universe via leptogenesis, the scale of m_D should be given by the up-quark masses. Lepton flavor violating decays μ\to e + γ, τ\to μ+ γand τ\to e + γare considered and a characteristic relation between their decay rates is predicted. The effective Majorana mass in neutrinoless double beta decay depends on the CP violating phase controlling the leptogenesis if one of the heavy Majorana neutrinos is much heavier than the other two. Successful leptogenesis requires a rather mild mass hierarchy between the latter. The compatibility of this hierarchical see-saw model with the low energy neutrino mixing phenomenology requires that the mixing angle limited by the CHOOZ and Palo Verde experiments should be relatively large, \sin^2θ_3 \gtap 0.01. The CP violation effects in neutrino oscillations can be observable. In general, there is no direct connection between the latter and the CP violation in leptogenesis. If the CP violating phases of the see-saw model satisfy certain relations, the baryon asymmetry of the Universe and the rephasing invariant J_{CP} which determines the magnitude of the CP violation effects in neutrino oscillations, depend on the same CP violating phase and their signs are correlated.

hep-ph↗

arXiv2002

On the Neutrino Mass Spectrum and Neutrinoless Double-Beta Decay

Assuming 3-nu mixing, neutrino oscillation explanation of the solar and atmospheric neutrino data and of the first KamLAND results, massive Majorana neutrinos and neutrinoless double-beta decay generated only by the (V-A) charged current weak interaction via the exchange of the three Majorana neutrinos, we analyze in detail the possibility of determining the type of the neutrino mass spectrum by measuring of the effective Majorana mass | | in neutrinoless double-beta decay. The three possible types of neutrino mass spectrum are considered: i) normal hierarchical (NH), m1 << m2 << m3, ii) inverted hierarchical (IH), m1 << m2 \simeq m3, and iii) quasi-degenerate (QD), m1 \simeq m2 \simeq m3 \geq 0.20 eV. The uncertainty in the measured value of | | due to the imprecise knowledge of the relevant nuclear matrix elements is taken into account in the analysis. We derive the ranges of values of tan^2 theta_\odot, theta_\odot being the mixing angle which controls the solar neutrino oscillations, and of the nuclear matrix element uncertainty factor, for which the measurement of | | would allow one to discriminate between the NH and IH, NH and QD and IH and QD spectra.

hep-ph↗

arXiv2002

On the CP Violation Associated with Majorana Neutrinos and Neutrinoless Double-Beta Decay

Assuming 3-neutrino mixing and massive Majorana neutrinos, we analyze the possibility of establishing the existence of CP-violation associated with Majorana neutrinos in the lepton sector i) by measuring of the effective Majorana mass | | in neutrinoless double beta-decay with a sufficient precision and ii) by measuring of, or obtaining a stringent upper limit on, the lightest neutrino mass m1. Information on m1 can be obtained in the tritium beta decay experiment KATRIN and from astrophysical and cosmological observations. Proving that the indicated CP-violation takes place requires, in particular, a relative experimental error on the measured value of | | not bigger than 20 %, a ``theoretical uncertainty'' in the value of | | due to an imprecise knowledge of the corresponding nuclear matrix elements smaller than a factor of 2, a value of tan^2 theta_(solar) larger than 0.55, and values of the relevant Majorana CP-violating phases typically within the intervals of (Pi/2 - 3 Pi/4) and (5Pi/4 - 3Pi/2).

hep-ph↗

arXiv2002

Measuring leptonic CP Violation in Neutrinoless Double Beta Decay

We investigate under which circumstances one can show the existence of leptonic CP violation with the help of a positive or negative signal in neutrinoless double beta decay. The possibilities of cancellations are investigated for special mass hierarchies and the different solar solutions. The possibility that the mixing angle connected with the solar neutrino problem is smaller or larger than pi/4 is taken into account. The non-maximality of that angle in case of the LMA solution allows to make several useful statements. The four different CP conserving possibilities are analyzed. It is implemented how precisely the oscillation parameters will be known after current and future experiments have taken data. The area in parameter space, in which CP violation has to take place, is largest for the LOW solution and in general larger for the inverse mass scheme.

hep-ph↗

arXiv2002

Leptogenesis and low energy observables in left-right symmetric models

In the context of left-right symmetric models we study the connection of leptogenesis and low energy parameters such as neutrinoless double beta decay and leptonic CP violation. Upon imposition of a unitarity constraint, the neutrino parameters are significantly restricted and the Majorana phases are determined within a narrow range, depending on the kind of solar solution. One of the Majorana phases gets determined to a good accuracy and thereby the second phase can be probed from the results of neutrinoless double beta decay experiments. We examine the contributions of the solar and atmospheric mass squared differences to the asymmetry and find that in general the solar scale dominates. In order to let the atmospheric scale dominate, some finetuning between one of the Majorana phases and the Dirac CP phase is required. In this case, one of the Majorana phases is determined by the amount of CP violation in oscillation experiments.

hep-ph↗

arXiv2001

Leptogenesis, neutrinoless double beta decay and terrestrial CP violation

Leptogenesis in left-right symmetric theories is studied. The usual see-saw mechanism is modified by the presence of a left-handed Higgs triplet. A simple connection between the properties of the light left-handed and heavy right-handed neutrinos is found. Predictions of this scenario for neutrinoless double beta decay and terrestrial CP violation in long-baseline experiments are given. These observables can in principle distinguish different realizations of the model.

hep-ph↗

arXiv2001

A simple connection between neutrino oscillation and leptogenesis

The usual see-saw formula is modified by the presence of two Higgs triplets in left-right symmetric theories. The contribution from the left-handed Higgs triplet to the see-saw formula can dominate over the conventional one when the neutrino Dirac mass matrix is identified with the charged lepton or down quark mass matrix. In this case an analytic calculation of the lepton asymmetry, generated by the decay of the lightest right-handed Majorana neutrino, is possible. For typical parameters, the out-of-equilibrium condition for the decay is automatically fulfilled. The baryon asymmetry has the correct order of magnitude, as long as the lightest mass eigenstate is not much lighter then 10^{-6} to 10^{-8} eV, depending on the solution of the solar neutrino problem. A sizable signal in neutrinoless double beta decay can be expected, as long as the smallest mass eigenstate is not much lighter than 10^{-3} eV and the Dirac mass matrix is identified with the charged lepton mass matrix.

hep-ph↗

arXiv2001

Leptogenesis in left-right symmetric theories

The masses and mixing of the light left-handed neutrinos can be related to those of the heavy right-handed neutrinos in left-right symmetric theories. Properties of the light neutrinos are measured in terrestrial experiments and the CP-violating decays of their heavy counterparts produce a baryon asymmetry via the well-known leptogenesis mechanism. The left-handed Higgs triplet, present in left-right symmetric theories, modifies the usual see-saw formula. It is possible to relate the lepton asymmetry to the light neutrino parameters when the triplet and the top quark through the usual see-saw mechanism give dominant contribution to the neutrino mass matrix. We find that in this situation the small angle MSW and vacuum solutions produce reasonable asymmetry, whereas the large angle MSW case requires extreme fine-tuning of the three phases in the mixing matrix.

hep-ph↗

arXiv2001

"Neutrinoless Double Beta Decay" at a Neutrino Factory

We examine the prospects of detecting an analogous process of neutrinoless double beta decay at a neutrino factory from a high energy muon storage ring. Limits from LEP experiments, neutrinoless double beta decay as well as from global fits have to be incorporated and severely restrict the results. We investigate what limits on light and heavy effective Majorana neutrino masses can be obtained and compare them with existing ones. Discussed are also contributions from right-handed neutrinos and purely right-handed interactions. Other ``new physics'' contributions to the same final state might produce large event numbers.

hep-ph↗

arXiv2000

Cancellations in Neutrinoless Double Beta Decay and the Neutrino Mass Matrix

In a degenerate scheme with mass m_0 a complete analysis of the allowed range of the effective electron neutrino Majorana mass is performed. Special attention is paid to effects of cancellations caused either by intrinsic CP parities of the eigenstates (CP invariance) or by complex mixing matrix elements (CP violation). We investigate all possibilities and give in each case constraints on the phases, the relative CP parities or the neutrino mass scale. A solar mixing angle \sin^2 2 θsmaller than 0.7 jeopardizes the degenerate mass scheme. A key value of /m_0 is identified, which is independent on the solar solution and would rule out certain schemes. Also it would answer the question regarding the presence of CP violation. Even if a total neutrino mass scale and an effective mass is measured, the value of the phases or parities is not fixed, unless in some special cases. The resulting uncertainty in the other mass matrix elements is at least of the same order than the one stemming from nuclear matrix elements calculations.

hep-ph↗