Search

We revisit various contributions to neutrinoless double beta decay (0νββ) in a TeV-scale left-right symmetric model (LRSM) for type-I seesaw dominance. We show that the momentum-dependent effects due to WL − WR exchange (λ diagram) and WL − WR mixing (η diagram) could give dominant contributions to the 0νββ amplitude in a wide range of the LRSM parameter space with large left-right neutrino mixing. In particular, for a relatively large WL−WR mixing, the η contribution by itself could saturate the current experimental limit on the 0νββ half-life, thereby providing stringent constraints on the relevant LRSM parameters, complementary to the indirect constraints derived from lepton-flavor-violating observables. In a simplified scenario parametrized by a single light-heavy neutrino mixing, the inclusion of the λ and η contributions leads to significantly improved 0νββ constraints on the light-heavy neutrino mixing as well as on the WL − WR mixing parameters. We also present a concrete TeV-scale LRSM setup, where the mixing effects are manifestly enhanced, and discuss the interplay between 0νββ, lepton flavor violation, and electric dipole moment constraints. [ABSTRACT FROM AUTHOR]

We study the production of heavy neutrinos at the Large Hadron Collider through the dominant s-channel production mode as well as the vector boson fusion process. We consider the TeV scale minimal linear seesaw model containing two heavy singlets with the opposite lepton number. This model is fully reconstructible from oscillation data apart from an overall normalization constant which can be constrained from the metastability of the electroweak vacuum and bounds coming from lepton flavor violation searches. The Dirac nature of heavy neutrinos in this model implies suppression of the conventional same-sign-dilepton signal at the Large Hadron Collider. We analyze the collider signatures with the trilepton final state and missing transverse energy as well as vector boson fusion type signals which are characterized by two additional forward tagged jets. Our investigation reveals that due to stringent constraints on light-heavy mixing coming from lepton flavor violation and metastability bounds, the model can be explored only for a light to moderate mass range of heavy neutrinos. We also note that in case of a positive signal, flavor counting of the final trilepton channel can give information about the mass hierarchy of the light neutrinos. [ABSTRACT FROM AUTHOR]

We consider the minimal seesaw model in which two gauge singlet right-handed neutrinos with opposite lepton numbers are added to the Standard Model. In this model, the smallness of the neutrino mass is explained by the tiny lepton number violating coupling between one of the singlets with the standard left-handed neutrinos. This allows one to have the right-handed neutrino mass at the TeV scale as well as appreciable mixing between the light and heavy states. This model is fully reconstructible in terms of the neutrino oscillation parameters apart from the overall coupling strengths. We show that the overall coupling strength y„ for the Dirac-type coupling between the left-handed neutrino and one of the singlets can be restricted by consideration of the (meta)stability bounds on the electroweak vacuum. In this scenario the lepton flavor violating decays of charged leptons can be appreciable, which can put further constraint on yv for right-handed neutrinos at TeV scale. We discuss the combined constraints on yv for this scenario from the process μ eγ and from the consideration of vacuum (meta)stability constraints on the Higgs self-coupling. We also briefly discuss the implications for neutrinoless double beta decay and possible signatures of the model that can be expected at colliders. [ABSTRACT FROM AUTHOR]

We study the implications of the recent results on neutrinoless double beta decay (0νββ) from GERDA-I (76Ge) and KamLAND-Zen+EXO-200 (136Xe) and the upper limit on the sum of light neutrino masses from Planck. We show that the upper limits on the effective neutrino mass from Xe136 are stronger than those from 76Ge for most of the recent calculations of the nuclear matrix elements (NMEs). We also analyze the compatibility of these limits with the claimed observation in 76Ge and show that while the updated claim value is still compatible with the recent GERDA limit as well as the individual Xe136 limits for a few NME calculations, it is inconsistent with the combined Xe136 limit for all but one NME. Imposing the most stringent limit from Planck, we find that the canonical light neutrino contribution cannot saturate the current limit, irrespective of the NME uncertainties. Saturation can be reached by inclusion of the right-handed (RH) neutrino contributions in TeV-scale left-right symmetric models with type-II seesaw. This imposes a lower limit on the lightest neutrino mass. Using the 0νββ bounds, we also derive correlated constraints in the RH sector, complimentary to those from direct searches at the LHC. [ABSTRACT FROM AUTHOR]

We discuss the optimal setup for a low energy neutrino factory in order to achieve a 5σ discovery of a nonzero mixing angle θ13, a nonzero CP phase δCP, and the mass hierarchy. We explore parent muon energies in the range 5-16 GeV, and baselines in the range 500-5000 km. We present the results in terms of the reach in sin²θ13, emphasizing the dependence of the optimal baseline on the true value of δCP. We show that the sensitivity of a given setup typically increases with parent muon energy, reaching saturation for higher energies. The saturation energy is larger for longer baselines; we present an estimate of this dependence. In the light of the recent indications of a large θ13, we also determine how these preferences would change if indeed a large θ13 is confirmed. In such a case, the baselines ~2500 km (~1500 km) may be expected to lead to hierarchy determination (δCP discovery) with the minimum exposure. [ABSTRACT FROM AUTHOR]

In this work we investigate the usefulness of nuSTORM as a probe of two new-physics scenarios that are sterile neutrinos and nonunitarity of the neutrino mixing matrix. For the sterile neutrino we show the importance of the neutral current events when combined with the charged current events to constrain the effective mixing angle, θμμ, and the sterile mixing angles θ14 and θ24. We also study the role nuSTORM will play in the study of neutrino oscillation physics if the three generation neutrino mixing matrix is nonunitary. In this context we elucidate the role of nuSTORM, considering both charged current and neutral current events, in constraining the various nonunitarity parameters such as α11, |α21|, and α22. [ABSTRACT FROM AUTHOR]

We have studied the correlations among the three absolute neutrino mass observables--the effective Majorana mass (mee) which can be obtained from neutrinoless double beta decay, the electron neutrino mass (mβ) which is measured in single beta decay experiments, and the sum of the light neutrino masses (Σ) which is constrained from cosmological observations, in the context of minimal left-right symmetric model. Two phenomenologically interesting cases of type-I seesaw dominance as well as type-II seesaw dominance have been considered. We have taken into account the independent constraints coming from lepton flavor violation, single β decay, cosmology, and neutrinoless double beta decay and have determined the combined allowed parameter space that can be probed in the future experiments. We have also analyzed the correlations and tensions between the different mass variables. In addition, the constraints on the masses of the heavy particles coming from lepton flavor violation and the bounds on three absolute neutrino mass observables are also determined. We show that these constraints can rule out some of the parameter space which are not probed by the collider experiments. [ABSTRACT FROM AUTHOR]

We analyze the effect of the dark large mixing angle (DLMA) solution on the effective Majorana mass (mββ) governing neutrinoless double beta decay (0νββ) in the presence of a sterile neutrino. We consider the 3 + 1 picture, comprised of one additional sterile neutrino. We check that the Mikheyev-Smirnov-Wolfenstein resonance in the Sun can take place in the DLMA parameter space in this scenario. Next, we investigate how the values of the solar mixing angle θ12 corresponding to the DLMA region alter the predictions of mββ by including a sterile neutrino in the analysis. We also compare our results with three-generation cases for both standard large mixing angle (LMA) and DLMA. Additionally, we evaluate the discovery sensitivity of the future 136Xe experiments in this context. [ABSTRACT FROM AUTHOR]

We consider a class of gauged U(1) extensions of the Standard Model (SM), where the light neutrino masses are generated by an inverse seesaw mechanism. In addition to the three right handed neutrinos, we add three singlet fermions and demand an extra Z2 symmetry under which, the third generations of both of the neutral fermions are odd, which in turn gives us a stable dark matter candidate. We express the U(1) charges of all the fermions in terms of the U(1) charges of the standard model Higgs and the new complex scalar. We study the bounds on the parameters of the model from vacuum stability, perturbative unitarity, dark matter relic density, and direct detection constraints. We also obtain the collider constraints on the Z' mass and the U(1)' gauge coupling. Finally, we compare all the bounds on the Z' mass versus the U(1)' gauge coupling plane. [ABSTRACT FROM AUTHOR]

We investigate the consequences of μ-τ reflection symmetry in presence of a light sterile neutrino for the 3 + 1 neutrino mixing scheme. We discuss the implications of total μ-τ reflection symmetry as well as partial μ-τ reflection symmetry. For the total μ-τ reflection symmetry we find that values of θ23 and δ remain confined near π/4 and ± π/2, respectively. The current allowed region for θ23 and δ in case of inverted hierarchy lies outside the area preferred by the total μ-τ reflection symmetry. However, interesting predictions on the neutrino mixing angles and Dirac CP violating phases are obtained considering partial μ-τ reflection symmetry. We obtain predictive correlations between the neutrino mixing angle θ23 and Dirac CP phase δ and study the testability of these correlations at the future long baseline experiment DUNE. We find that while the imposition of μ-τ reflection symmetry in the first column admits both normal and inverted neutrino mass hierarchy, demanding μ-τ reflection symmetry for the second column excludes the inverted hierarchy. Interestingly, the sterile mixing angle θ34 gets tightly constrained considering the μ-τ reflection symmetry in the fourth column. We also study the implications of μ-τ reflection symmetry for the Majorana phases and neutrinoless double beta decay in the 3 + 1 scenario. [ABSTRACT FROM AUTHOR]

We study the minimal type-III seesaw model in which we extend the standard model by adding two SU (2)L triplet fermions with zero hypercharge to explain the origin of the nonzero neutrino masses. We show that the naturalness conditions and the limits from lepton flavor violating decays provide very stringent bounds on the model parameters along with the constraints from the stability/metastability of the electroweak vacuum. We perform a detailed analysis of the model parameter space including all the constraints for both normal as well as inverted hierarchies of the light neutrino masses. We find that most of the regions that are allowed by lepton flavor violating decays and naturalness fall in the stable/metastable region depending on the values of the standard model parameters. [ABSTRACT FROM AUTHOR]

Sensitivity of the magnetized Iron Calorimeter (ICAL) detector at the proposed India-based Neutrino Observatory (INO) to invisible decay of the mass eigenstate ν3 using atmospheric neutrinos is explored. A full three-generation analysis including Earth matter effects is performed in a framework with both decay and oscillations. The wide energy range and baselines offered by atmospheric neutrinos are shown to be excellent for constraining the ν3 lifetime. We find that with an exposure of 500 kton-yr the ICAL atmospheric experiment could constrain the ν3 lifetime to τ3/m3>1.51×10-10 s/eV at the 90% C.L. This is 2 orders of magnitude tighter than the bound from MINOS. The effect of invisible decay on the precision measurement of θ23 and |Δm²32| is also studied. [ABSTRACT FROM AUTHOR]

We study the effect of nonstandard interactions (NSIs) on the propagation of neutrinos through the Earth's matter and how it affects the hierarchy sensitivity of the DUNE experiment. We emphasize the special case when the diagonal NSI parameter εee = - 1, nullifying the standard matter effect. We show that if, in addition, CP violation is maximal then this gives rise to an exact intrinsic hierarchy degeneracy in the appearance channel, irrespective of the baseline and energy. Introduction of the off diagonal NSI parameter, εeτ, shifts the position of this degeneracy to a different εee. Moreover the unknown magnitude and phases of the off diagonal NSI parameters can give rise to additional degeneracies. Overall, given the current model independent limits on NSI parameters, the hierarchy sensitivity of DUNE can get seriously impacted. However, a more precise knowledge of the NSI parameters, especially εee, can give rise to an improved sensitivity. Alternatively, if a NSI exists in nature, and still DUNE shows hierarchy sensitivity, certain ranges of the NSI parameters can be excluded. Additionally, we briefly discuss the implications of εee = - 1 (in the Earth) on the Mikheyev-Smirnov-Wolfenstein effect in the Sun. [ABSTRACT FROM AUTHOR]

We consider singlet extensions of the standard model, both in the fermion and in the scalar sector, to account for the generation of neutrino mass at the TeV scale and the existence of dark matter, respectively. For the neutrino sector we consider models with extra singlet fermions that can generate neutrino mass via the so-called inverse or linear seesaw mechanism, whereas a singlet scalar is introduced as the candidate for dark matter. We show that although these two sectors are disconnected at low energy, the coupling constants of both the sectors get correlated at the high-energy scale by the constraints coming from the perturbativity and stability/metastability of the electroweak vacuum. The singlet fermions try to destabilize the electroweak vacuum while the singlet scalar aids the stability. As an upshot, the electroweak vacuum may attain absolute stability even up to the Planck scale for suitable values of the parameters. We delineate the parameter space for the singlet fermion and the scalar couplings for which the electroweak vacuum remains stable/metastable and at the same time gives the correct relic density and neutrino masses and mixing angles as observed. [ABSTRACT FROM AUTHOR]

The measurement of a nonzero value of the 1-3 mixing angle has paved the way for the determination of leptonic CP violation. However, the current generation long-baseline experiments T2K and NOνA have limited sensitivity to δCP. In this paper, we show, for the first time, the significance of atmospheric neutrino experiments in providing the first hint of CP violation in conjunction with T2K and NOνA. In particular, we find that adding atmospheric neutrino data from a magnetized iron calorimeter detector at the India-Based Neutrino Observatory to T2K and NOνA results in a twofold increase in the range of δCP values for which a 2ɑ hint of CP violation can be obtained. In fact, in the parameter region unfavorable for the latter experiments, the first signature of CP violation may well come from the inclusion of atmospheric neutrino data. [ABSTRACT FROM AUTHOR]

In this paper we consider the generation of naturally small neutrino masses from a dimension-7 operator. Such a term can arise in the presence of a scalar quadruplet and a pair of vectorlike fermion triplets and enables one to obtain small neutrino masses through the TeV scale linear seesaw mechanism. We study the phenomenology of the charged scalars of this model, in particular, the multilepton signatures at the Large Hadron Collider. Of special importance is the presence of the same-sign-tri-lepton signatures originating from the triply charged scalars. The Standard Model background for such processes is small, and hence this is considered as a confirming evidence of new physics. We also looked for events with three, four, five, and six leptons that have negligible contamination from the Standard Model. We further point out the spectacular lepton flavor violating the four-lepton signal that can be the hallmark for these types of models. We also compute the added contributions in the rate for the Standard Model Higgs decaying to two photons via the charged scalars in this model. [ABSTRACT FROM AUTHOR]

We study the implications of the Dark-LMA (DLMA) solution to the solar neutrino problem for neutrinoless double beta decay (0νββ). We show that, while the predictions for the effective mass governing 0νββ remain unchanged for the inverted mass scheme, that for normal ordering becomes higher for the DLMA parameter space and moves into the "desert region" between the two. This sets a new goal for the sensitivity reach for the next-generation experiments if no signal is found for the inverted ordering by the future search programs. [ABSTRACT FROM AUTHOR]

We study the origin, consequences, and testability of a hypothesis of "partial μ-τ" reflection symmetry. This symmetry predicts |Uμi|=|Uτi|(i=1,2,3) for a single column of the leptonic mixing matrix U. Depending on whether this symmetry holds for the first or second column of U, different correlations between θ23 and δCP can be obtained. This symmetry can be obtained using discrete flavor symmetries. In particular, all of the subgroups of SU(3) with three-dimensional irreducible representation, which are classified as class C or D, can lead to partial μ-τ reflection symmetry. We show how the predictions of this symmetry compares with the allowed area in the sin²θ23-δCP plane as obtained from the global analysis of neutrino oscillation data. Furthermore, we study the possibility of testing these symmetries at the proposed DUNE and Hyper-Kamiokande (HK) experiments (T2HK, T2HKK), by incorporating the correlations between θ23 and δCP that are predicted by the symmetries. We find that when the simulated data of DUNE and HK are fitted with the symmetry predictions, the θ23-δCP parameter space gets largely restricted near the charge parity conserving values of δCP. Finally, we illustrate the capability of these experiments to distinguish between the two cases leading to partial μ-τ symmetry, namely |Uμ1|=|Uτ1| and |Uμ2|=|Uτ2|. [ABSTRACT FROM AUTHOR]

The right-handed neutrinos within the type-I seesaw mechanism can induce large radiative corrections to the Higgs mass, and naturalness arguments can then be used to set limits on their mass scale and Yukawa couplings. Driven by minimality, we consider the presence of two degenerate right-handed neutrinos. We compare the limits from naturalness with the ones from the stability of the electroweak vacuum and from lepton flavor violation. Implications from neutrinoless double beta decay are also discussed and renormalization effects for the light neutrino parameters are presented. Adding small perturbations to the degenerate heavy neutrino spectrum allows for successful leptogenesis. [ABSTRACT FROM AUTHOR]