arXiv subjects

S. T. Petcov

Publications and source records attributed to S. T. Petcov.

At least 55 records · Page 3Linked to original sources

arXiv2019

Leptogenesis from Low Energy $CP$ Violation

We revisit the possibility of producing the observed baryon asymmetry of the Universe via thermal leptogenesis, where $CP$ violation comes exclusively from the low-energy phases of the neutrino mixing matrix. We demonstrate the viability of thermal flavoured leptogenesis across seven orders of magnitude $\left(10^{6}<T \text{ (GeV)}< 10^{13}\right)$, using modern numerical machinery, where the lower bound can be reached only if flavour effects are taken into account and its value depends on the allowed degree of cancellation between the tree-level and radiative contributions to the light neutrino masses. At very high scales $\left( T \gg 10^{12} \text{ GeV} \right)$, we clarify that thermal leptogenesis is sensitive to the low-energy phases, in contradiction with what is usually claimed in the literature. In particular we demonstrate that Majorana-phase leptogenesis is in general viable while Dirac-phase leptogenesis requires some level of fine-tuning.

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arXiv2018

Discrete Flavour Symmetries, Neutrino Mixing and Leptonic CP Violation

The current status of our knowledge of the 3-neutrino mixing parameters and of the CP violation in the lepton sector is summarised. The discrete symmetry approach to understanding the observed pattern of neutrino mixing and the related predictions for neutrino mixing angles and leptonic Dirac CP violation are reviewed.

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arXiv2018

On Neutrino Mixing in Matter and CP and T Violation Effects in Neutrino Oscillations

Aspects of 3-neutrino mixing and oscillations in vacuum and in matter with constant density are investigated working with a real form of the neutrino Hamiltonian. We find the (approximate) equalities $θ^m_{23} = θ_{23}$ and $δ^m = δ$, $θ_{23}$ ($θ^m_{23}$) and $δ$ ($δ^m$) being respectively the atmospheric neutrino mixing angle and the Dirac CP violation phase in vacuum (in matter) of the neutrino mixing matrix, which are shown to represent excellent approximations for the conditions of the T2K (T2HK), T2HKK, NO$ν$A and DUNE neutrino oscillation experiments. A new derivation of the known relation $\sin2θ^m_{23} \sinδ^m = \sin2θ_{23} \sinδ$ is presented and it is used to obtain a correlation between the shifts of $θ_{23}$ and $δ$ due to the matter effect. A derivation of the relation between the rephasing invariants which determine the magnitude of CP and T violating effects in 3-flavour neutrino oscillations in vacuum, $J_{\rm CP}$, and of the T violating effects in matter with constant density, $J^{m}_{\rm T} \equiv J^{m}$, reported in \cite{Krastev:1988yu} without a proof, is presented. It is shown that the function $F$ which appears in this relation, $J^{m} = J_{\rm CP}\,F$, and whose explicit form was given in \cite{Krastev:1988yu}, coincides with the function $\tilde{F}$ in the similar relation $J^{m} = J_{\rm CP}\tilde{F}$ derived in \cite{Naumov:1991ju}, although $F$ and $\tilde{F}$ are expressed in terms of different sets of neutrino mass and mixing parameters and have completely different forms.

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arXiv2018

Three-Flavoured Non-Resonant Leptogenesis at Intermediate Scales

Leptogenesis can successfully explain the matter-antimatter asymmetry via out-of-equilibrium decays of heavy Majorana neutrinos in the early Universe. In this article, we focus on non-resonant thermal leptogenesis and the possibility of lowering its scale. In order to do so, we calculate the lepton asymmetry produced from the decays of one and two heavy Majorana neutrinos using three-flavoured density matrix equations in an exhaustive exploration of the model parameter space. We find regions of the parameter space where thermal leptogenesis is viable at intermediate scales, $T\sim 10^{6}$ GeV. However, the viability of thermal leptogenesis at such scales requires a certain degree of cancellation between the tree and one-loop level contribution to the light neutrino mass matrix and we quantify such fine-tuning.

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arXiv2018

Lepton Masses and Mixing from Modular $S_4$ Symmetry

We study models of lepton masses and mixing based on broken modular invariance. We consider invariance under the finite modular group $Γ_4 \simeq S_4$ and focus on the minimal scenario where the expectation value of the modulus is the only source of symmetry breaking, such that no flavons need to be introduced. After constructing a basis for the lowest weight modular forms, we build two minimal models, one of which successfully accommodates charged lepton masses and neutrino oscillation data, while predicting the values of the Dirac and Majorana CPV phases.

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arXiv2018

The $10^{-3}$ eV Frontier in Neutrinoless Double Beta Decay

The observation of neutrinoless double beta decay would allow to establish lepton number violation and the Majorana nature of neutrinos. The rate of this process in the case of 3-neutrino mixing is controlled by the neutrinoless double beta decay effective Majorana mass $|\langle m \rangle|$. For a neutrino mass spectrum with normal ordering, which is favoured over the spectrum with inverted ordering by recent global fits, $|\langle m \rangle|$ can be significantly suppressed. Taking into account updated data on the neutrino oscillation parameters, we investigate the conditions under which $|\langle m \rangle|$ in the case of spectrum with normal ordering exceeds $10^{-3}~(5\times 10^{-3})$ eV: $|\langle m \rangle|_\text{NO} > 10^{-3}~(5\times 10^{-3})$ eV. We analyse first the generic case with unconstrained leptonic CP violation Majorana phases. We show, in particular, that if the sum of neutrino masses is found to satisfy $Σ> 0.10$ eV, then $|\langle m \rangle|_\text{NO} > 5\times 10^{-3}$ eV for any values of the Majorana phases. We consider also cases where the values for these phases are either CP conserving or are in line with predictive schemes combining flavour and generalised CP symmetries.

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arXiv2018

Low-Scale Seesaw and the CP Violation in Neutrino Oscillations

We consider a version of the low-scale type I seesaw mechanism for generating small neutrino masses, as an alternative to the standard seesaw scenario. It involves two right-handed (RH) neutrinos $ν_{1R}$ and $ν_{2R}$ having a Majorana mass term with mass $M$, which conserves the lepton charge $L$. The RH neutrino $ν_{2R}$ has lepton-charge conserving Yukawa couplings $g_{\ell 2}$ to the lepton and Higgs doublet fields, while small lepton-charge breaking effects are assumed to induce tiny lepton-charge violating Yukawa couplings $g_{\ell 1}$ for $ν_{1R}$, $l=e,μ,τ$. In this approach the smallness of neutrino masses is related to the smallness of the Yukawa coupling of $ν_{1R}$ and not to the large value of $M$: the RH neutrinos can have masses in the few GeV to a few TeV range. The Yukawa couplings $|g_{\ell 2}|$ can be much larger than $|g_{\ell 1}|$, of the order $|g_{\ell 2}| \sim 10^{-4} - 10^{-2}$, leading to interesting low-energy phenomenology. We consider a specific realisation of this scenario within the Froggatt-Nielsen approach to fermion masses. In this model the Dirac CP violation phase $δ$ is predicted to have approximately one of the values $δ\simeq π/4,\, 3π/4$, or $5π/4,\, 7π/4$, or to lie in a narrow interval around one of these values. The low-energy phenomenology of the considered low-scale seesaw scenario of neutrino mass generation is also briefly discussed.

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arXiv2018

Addressing Neutrino Mixing Models with DUNE and T2HK

We consider schemes of neutrino mixing arising within the discrete symmetry approach to the well-known flavour problem. We concentrate on $3ν$ mixing schemes in which the cosine of the Dirac CP violation phase $δ_\mathrm{CP}$ satisfies a sum rule by which it is expressed in terms of three neutrino mixing angles $θ_{12}$, $θ_{23}$, and $θ_{13}$, and a fixed real angle $θ^ν_{12}$, whose value depends on the employed discrete symmetry and its breaking. We consider five underlying symmetry forms of the neutrino mixing matrix: bimaximal (BM), tri-bimaximal (TBM), golden ratio A (GRA) and B (GRB), and hexagonal (HG). For each symmetry form, the sum rule yields specific prediction for $\cosδ_\mathrm{CP}$ for fixed $θ_{12}$, $θ_{23}$, and $θ_{13}$. In the context of the proposed DUNE and T2HK facilities, we study (i) the compatibility of these predictions with present neutrino oscillation data, and (ii) the potential of these experiments to discriminate between various symmetry forms.

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arXiv2018

Assessing the Viability of $A_4$, $S_4$ and $A_5$ Flavour Symmetries for Description of Neutrino Mixing

We consider the $A_4$, $S_4$ and $A_5$ discrete lepton flavour symmetries in the case of 3-neutrino mixing, broken down to non-trivial residual symmetries in the charged lepton and neutrino sectors in such a way that at least one of them is a $Z_2$. Such symmetry breaking patterns lead to predictions for some of the three neutrino mixing angles and/or the leptonic Dirac CP violation phase $δ$ of the neutrino mixing matrix. We assess the viability of these predictions by performing a statistical analysis which uses as an input the latest global data on the neutrino mixing parameters. We find 14 phenomenologically viable cases providing distinct predictions for some of the mixing angles and/or the Dirac phase $δ$. Employing the current best fit values of the three neutrino mixing angles, we perform a statistical analysis of these cases taking into account the prospective uncertainties in the determination of the mixing angles, planned to be achieved in currently running (Daya Bay) and the next generation (JUNO, T2HK, DUNE) of neutrino oscillation experiments. We find that only six cases would be compatible with these prospective data. We show that this number is likely to be further reduced by a precision measurement of $δ$.

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arXiv2018

Neutrino Mixing and Leptonic CP Violation from $S_4$ and Generalised CP Symmetries

We consider a class of models of neutrino mixing with $S_4$ flavour symmetry and generalised CP symmetry, broken to $Z_2$ and $Z_2 \times {\rm CP}$ residual symmetries in the charged lepton and neutrino sectors, respectively. In this scheme, and up to discrete ambiguities, the neutrino mixing matrix is determined by two angles and one phase. We classify the phenomenologically viable mixing patterns, deriving predictions for the Dirac and Majorana CPV phases and for the effective Majorana mass in neutrinoless double beta decay.

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arXiv2018

CP Violation in the Lepton Sector and Implications for Leptogenesis

We review the current status of the data on neutrino masses and lepton mixing and the prospects for measuring the CP-violating phases in the lepton sector. The possible connection between low energy CP violation encoded in the Dirac and Majorana phases of the Pontecorvo-Maki-Nakagawa-Sakata mixing matrix and successful leptogenesis is emphasized in the context of seesaw extensions of the Standard Model with a flavor symmetry Gf (and CP symmetry).

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arXiv2017

Neutrino Mixing and Leptonic CP Violation from $S_4$ Flavour and Generalised CP Symmetries

We consider a class of models of neutrino mixing with $S_4$ lepton flavour symmetry combined with a generalised CP symmetry, which are broken to residual $Z_2$ and $Z_2 \times H^ν_{\rm CP}$ symmetries in the charged lepton and neutrino sectors, respectively, $H^ν_{\rm CP}$ being a remnant CP symmetry of the neutrino Majorana mass term. In this set-up the neutrino mixing angles and CP violation (CPV) phases of the neutrino mixing matrix depend on three real parameters - two angles and a phase. We classify all phenomenologically viable mixing patterns and derive predictions for the Dirac and Majorana CPV phases. Further, we use the results obtained on the neutrino mixing angles and leptonic CPV phases to derive predictions for the effective Majorana mass in neutrinoless double beta decay.

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arXiv2017

Revisiting Minimal Lepton Flavour Violation in the Light of Leptonic CP Violation

The Minimal Lepton Flavour Violation (MLFV) framework is discussed after the recent indication for CP violation in the leptonic sector. Among the three distinct versions of MLFV, the one with degenerate right-handed neutrinos will be disfavoured, if this indication is confirmed. The predictions for leptonic radiative rare decays and muon conversion in nuclei are analysed, identifying strategies to disentangle the different MLFV scenarios. The claim that the present anomalies in the semi-leptonic $B$-meson decays can be explained within the MLFV context is critically re-examined concluding that such an explanation is not compatible with the present bounds from purely leptonic processes.

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arXiv2016

On the IceCube Result on $\barν_μ \to \barν_{s}$ Oscillations

We elucidate the mechanism of enhancement of the $\barν_μ \to \barν_{s}$ transitions at small mixing angles of Earth-core-crossing neutrinos, which is at the basis of the recent result of the IceCube experiment on atmospheric muon anti-neutrino oscillations into sterile anti-neutrino.

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arXiv2016

Renormalisation Group Corrections to Neutrino Mixing Sum Rules

Neutrino mixing sum rules are common to a large class of models based on the (discrete) symmetry approach to lepton flavour. In this approach the neutrino mixing matrix $U$ is assumed to have an underlying approximate symmetry form $\tilde{U}_ν$, which is dictated by, or associated with, the employed (discrete) symmetry. In such a setup the cosine of the Dirac CP-violating phase $δ$ can be related to the three neutrino mixing angles in terms of a sum rule which depends on the symmetry form of $\tilde{U}_ν$. We consider five extensively discussed possible symmetry forms of $\tilde{U}_ν$: i) bimaximal (BM) and ii) tri-bimaximal (TBM) forms, the forms corresponding to iii) golden ratio type A (GRA) mixing, iv) golden ratio type B (GRB) mixing, and v) hexagonal (HG) mixing. For each of these forms we investigate the renormalisation group corrections to the sum rule predictions for $δ$ in the cases of neutrino Majorana mass term generated by the Weinberg (dimension 5) operator added to i) the Standard Model, and ii) the minimal SUSY extension of the Standard Model.

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arXiv2016

Enhancing Mechanisms of Neutrino Transitions in a Medium of Nonperiodic Constant - Density Layers and in the Earth

The extrema of the probabilities of the transitions $ν_μ (ν_e) \to ν_e (ν_{μ;τ})$, $ν_{2} \to ν_e$, etc. of neutrinos passing through a medium consisting of i) two layers of different constant densities or ii) three layers of constant density, of which the first and the third are identical but differ in density from the second (e.g., mantle-core-mantle in the Earth), are analyzed. We show that in addition to the MSW and neutrino oscillation length resonance (NOLR) local maxima previously discussed, there exist new resonance-like absolute maxima of interference nature. The latter correspond to a new effect of total neutrino conversion. The conditions for existence of the new maxima are fulfilled, in particular, for the Earth-core-crossing solar and atmospheric neutrinos. We prove that the NOLR and the new resonance-like enhancement are caused by a maximal constructive interference between the amplitudes of the neutrino transitions in the different density layers. The strong resonance-like enhancement of the transitions in the Earth of the Earth-core-crossing solar and atmospheric neutrinos is due to the new effect of total neutrino conversion.

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arXiv2016

Predictions for the Majorana CP Violation Phases in the Neutrino Mixing Matrix and Neutrinoless Double Beta Decay

We obtain predictions for the Majorana phases $α_{21}/2$ and $α_{31}/2$ of the $3\times 3$ unitary neutrino mixing matrix $U = U_e^{\dagger} \, U_ν$, $U_e$ and $U_ν$ being the $3\times 3$ unitary matrices resulting from the diagonalisation of the charged lepton and neutrino Majorana mass matrices, respectively. We focus on forms of $U_e$ and $U_ν$ permitting to express $α_{21}/2$ and $α_{31}/2$ in terms of the Dirac phase $δ$ and the three neutrino mixing angles of the standard parametrisation of $U$, and the angles and the two Majorana-like phases $ξ_{21}/2$ and $ξ_{31}/2$ present, in general, in $U_ν$. The concrete forms of $U_ν$ considered are fixed by, or associated with, symmetries (tri-bimaximal, bimaximal, etc.), so that the angles in $U_ν$ are fixed. For each of these forms and forms of $U_e$ that allow to reproduce the measured values of the three neutrino mixing angles $θ_{12}$, $θ_{23}$ and $θ_{13}$, we derive predictions for phase differences $(α_{21}/2 - ξ_{21}/2)$, $(α_{31}/2 - ξ_{31}/2)$, etc., which are completely determined by the values of the mixing angles. We show that the requirement of generalised CP invariance of the neutrino Majorana mass term implies $ξ_{21} = 0$ or $π$ and $ξ_{31} = 0$ or $π$. For these values of $ξ_{21}$ and $ξ_{31}$ and the best fit values of $θ_{12}$, $θ_{23}$ and $θ_{13}$, we present predictions for the effective Majorana mass in neutrinoless double beta decay for both neutrino mass spectra with normal and inverted ordering.

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arXiv2015

Leptonic Dirac CP Violation Predictions from Residual Discrete Symmetries

Assuming that the observed pattern of 3-neutrino mixing is related to the existence of a (lepton) flavour symmetry, corresponding to a non-Abelian discrete symmetry group $G_f$, and that $G_f$ is broken to specific residual symmetries $G_e$ and $G_ν$ of the charged lepton and neutrino mass terms, we derive sum rules for the cosine of the Dirac phase $δ$ of the neutrino mixing matrix $U$. The residual symmetries considered are: i) $G_e = Z_2$ and $G_ν = Z_n$, $n > 2$ or $Z_n \times Z_m$, $n,m \geq 2$; ii) $G_e = Z_n$, $n > 2$ or $Z_n \times Z_m$, $n,m \geq 2$ and $G_ν = Z_2$; iii) $G_e = Z_2$ and $G_ν = Z_2$; iv) $G_e$ is fully broken and $G_ν = Z_n$, $n > 2$ or $Z_n \times Z_m$, $n,m \geq 2$; and v) $G_e = Z_n$, $n > 2$ or $Z_n \times Z_m$, $n,m \geq 2$ and $G_ν$ is fully broken. For given $G_e$ and $G_ν$, the sum rules for $\cosδ$ thus derived are exact, within the approach employed, and are valid, in particular, for any $G_f$ containing $G_e$ and $G_ν$ as subgroups. We identify the cases when the value of $\cosδ$ cannot be determined, or cannot be uniquely determined, without making additional assumptions on unconstrained parameters. In a large class of cases considered the value of $\cosδ$ can be unambiguously predicted once the flavour symmetry $G_f$ is fixed. We present predictions for $\cosδ$ in these cases for the flavour symmetry groups $G_f = S_4$, $A_4$, $T^\prime$ and $A_5$, requiring that the measured values of the 3-neutrino mixing parameters $\sin^2θ_{12}$, $\sin^2θ_{13}$ and $\sin^2θ_{23}$, taking into account their respective $3σ$ uncertainties, are successfully reproduced.

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