arXiv subjects

S. T. Petcov

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

At least 91 records · Page 5Linked to original sources

arXiv2012

Revisiting Leptogenesis in a SUSY SU(5) x T' Model of Flavour

We investigate the generation of the baryon asymmetry of the Universe within a SUSY SU(5) x T' model of flavour, which gives rise to realistic masses and mixing patterns for quarks and leptons. The model employs the see-saw mechanism for generation of the light neutrino masses and the baryon asymmetry is produced via leptogenesis. We perform detailed calculations of both the CP violating lepton asymmetries, originating from the decays of the heavy Majorana neutrinos operative in the see-saw mechanism, and of the efficiency factors which account for the lepton asymmetry wash-out processes in the Early Universe. The latter are calculated by solving numerically the system of Boltzmann equations describing the generation and the evolution of the lepton asymmetries. The baryon asymmetry in the model considered is proportional to the J_{CP} factor, which determines the magnitude of CP violation effects in the oscillations of flavour neutrinos. The leptogenesis scale can be sufficiently low, allowing to avoid the potential gravitino problem.

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arXiv2011

Predictions for Neutrino Masses, $ββ_{0ν}$-Decay and Lepton Flavor Violation in a SUSY $SU(5) \times T^{\prime}$ Model of Flavour

We obtain predictions for the neutrino masses, the effective Majorana mass in neutrinoless double beta decay and for the rates of the lepton flavor violating processes $μ\rightarrow e + γ$, $τ\rightarrow e + γ$ and $ τ\rightarrow μ+ γ$ in a SUSY $SU(5) \times T^{\prime}$ Model of flavour, which gives rise to realistic masses and mixing patterns for quarks and leptons.

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arXiv2011

Low Energy Signatures of the TeV Scale See-Saw Mechanism

We study a type I see-saw scenario where the right-handed (RH) neutrinos, responsible for the light neutrino mass generation, lie at the electroweak scale. Under certain conditions, the strength of the charged and neutral current weak interactions of the Standard Model particles with the heavy RH neutrinos can be large enough to allow their production at the LHC, opening also the possibility of observing other low energy signatures of the new physics in the electroweak precision observables as well as in searches for rare leptonic decays or neutrinoless double beta decay. We argue that in this scenario the flavour structure of the neutrino Yukawa couplings is essentially determined by the low energy neutrino parameters, leading to fairly strong correlations among the new phenomena. In particular, we show that the present bound on the $μ\to e +γ$ decay rate makes very difficult the observation of the heavy RH neutrinos at the LHC or the observation of deviations from the Standard Model predictions in the electroweak precision data. We also argue that all present experimental constraints on this scenario still allow i) for an enhancement of the rate of neutrinoless double beta decay, which thus can be in the range of sensitivity of the GERDA experiment even when the light Majorana neutrinos possess a normal hierarchical mass spectrum, and ii) for the predicted $μ\to e+ γ$ decay rate to be within the sensitivity range of the MEG experiment.

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arXiv2011

Neutrino Mass Hierarchy Determination Using Reactor Antineutrinos

Building on earlier studies, we investigate the possibility to determine the type of neutrino mass spectrum (i.e., "the neutrino mass hierarchy") in a high statistics reactor electron antineutrino experiment with a relatively large KamLAND-like detector and an optimal baseline of 60 Km. We analyze systematically the Fourier Sine and Cosine Transforms (FST and FCT) of simulated reactor antineutrino data with reference to their specific mass hierarchy-dependent features discussed earlier in the literature. We perform also a binned χ^2 analysis of the sensitivity of simulated reactor electron antineutrino event spectrum data to the neutrino mass hierarchy, and determine, in particular, the characteristics of the detector and the experiment (energy resolution, visible energy threshold, exposure, systematic errors, binning of data, etc.), which would allow us to get significant information on, or even determine, the type of the neutrino mass spectrum. We find that if \sin^2 2θ_{13} is sufficiently large, \sin^2 2θ_{13} \gtap 0.02, the requirements on the set-up of interest are very challenging, but not impossible to realize.

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arXiv2011

Lepton Number Violation in TeV Scale See-Saw Extensions of the Standard Model

The low-energy neutrino physics constraints on the TeV scale type I see-saw scenarios of neutrino mass generation are revisited. It is shown that lepton charge (L) violation, associated to the production and decays of heavy Majorana neutrinos N_{j} having masses in the range of M_j \sim (100 ÷1000) GeV and present in such scenarios, is hardly to be observed at ongoing and future particle accelerator experiments, LHC included, because of very strong constraints on the parameters and couplings responsible for the corresponding |ΔL| = 2 processes. If the heavy Majorana neutrinos N_j are observed and they are associated only with the type I mechanism, they will behave effectively like pseudo-Dirac fermions. Conversely, the observation of effects proving the Majorana nature of N_j would imply that these heavy neutrinos have additional relatively strong couplings to the Standard Model particles or that light neutrino masses compatible with the observations are generated by a mechanism other than see-saw (e.g., radiatively at one or two loop level) in which the heavy Majorana neutrinos N_j are nevertheless involved.

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arXiv2010

A Case of Subdominant/Suppressed "High Energy" Contribution to the Baryon Asymmetry of the Universe in Flavoured Leptogenesis

The CP-violation necessary for the generation of the baryon asymmetry of the Universe $Y_B$ in the "flavoured" leptogenesis scenario can arise from the "low energy" PMNS neutrino mixing matrix $U$ and/or from the "high energy" part of neutrino Yukawa couplings, which can mediate CP-violating phenomena only at some high energy scale. The possible interplay between these two types of CP-violation is analysed. The type I see-saw model with three heavy right-handed Majorana neutrinos having hierarchical spectrum is considered. We show that in the case of inverted hierarchical light neutrino mass spectrum, there exist regions in the corresponding leptogenesis parameter space where the relevant "high energy" phases have large CP-violating values, but the purely "high energy" contribution in $Y_B$ plays a subdominant role in the production of baryon asymmetry compatible with the observations. In some of these regions the purely "high energy" contribution in $Y_B$ is so strongly suppressed that one can have successful leptogenesis only if the requisite CP-violation is provided by the Majorana phase(s) in the neutrino mixing matrix.

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arXiv2010

TeV Scale See-Saw Mechanisms of Neutrino Mass Generation, the Majorana Nature of the Heavy Singlet Neutrinos and $\betabeta$-Decay

It is shown that the Majorana nature of the heavy neutrinos $N_j$ having masses in the range of $M_j \sim (100 - 1000)$ GeV and present in the TeV scale type I and inverse see-saw scenarios of neutrino mass generation, is unlikely to be observable in the currently operating and future planned accelerator experiments (including LHC) due to the existence of very strong constraints on the parameters and couplings responsible for the corresponding $|ΔL| = 2$ processes, $L$ being the total lepton charge. If the heavy Majorana neutrinos $N_j$ are observed and they are associated only with the type I or inverse see-saw mechanisms and no additional TeV scale "new physics", they will behave like Dirac fermions to a relatively high level of precision, being actually pseudo-Dirac particles. The observation of effects proving the Majorana nature of $N_j$ would imply that these heavy neutrinos have additional relatively strong couplings to the Standard Model particles (as, e.g. in the type III see-saw scenario), or that light neutrino masses compatible with the observations are generated by a mechanism other than see-saw (e.g., radiatively at one or two loop level) in which the heavy Majorana neutrinos $N_j$ are nevertheless involved.

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arXiv2010

Charged Lepton Flavour Violating Radiative Decays $\ell_i \to \ell_j + γ$ in See-Saw Models with $A_4$ Symmetry

The charged lepton flavour violating (LFV) radiative decays, $μ\to e+γ$, $τ\to μ+γ$ and $τ\to e +γ$ are investigated in a class of supersymmetric $A_4$ models with three heavy right-handed (RH) Majorana neutrinos, in which the lepton (neutrino) mixing is predicted to leading order (LO) to be tri-bimaximal. The light neutrino masses are generated via the type I see-saw mechanism. The analysis is done within the framework of the minimal supergravity (mSUGRA) scenario, which provides flavour universal boundary conditions at the scale of grand unification $M_X \approx 2 \times 10^{16}$ GeV. Detailed predictions for the rates of the three LFV decays are obtained in two explicit realisations of the $A_4$ models due to Altarelli and Feruglio and Altarelli and Meloni, respectively.

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arXiv2009

Majorana Phases and Leptogenesis in See-Saw Models with A_4 Symmetry

The related issues of Majorana CP violation in the lepton sector and leptogenesis are investigated in detail in two rather generic supersymmetric models with type I see-saw mechanism of neutrino mass generation and A_4 flavour symmetry, which naturally lead at leading order to tri-bimaximal neutrino mixing. The neutrino sector in this class of models is described at leading order by just two real parameters and one phase. This leads, in particular, to significant low energy constraints on the Majorana phases α_{21} and α_{31} in the PMNS matrix, which play the role of leptogenesis CP violating parameters in the generation of the baryon asymmetry of the Universe. We find that it is possible to generate the correct size and sign of the baryon asymmetry in both A_4 models. The sign of the baryon asymmetry is directly related to the signs of sinα_{21} and/or sinα_{31}.

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arXiv2009

On the Interplay Between the 'Low' and 'High' Energy CP-Violation in Leptogenesis

The CP-violation necessary for the generation of the baryon asymmetry of the Universe in the "flavoured" leptogenesis scenario can arise from the "low energy" PMNS neutrino mixing matrix and/or from the "high energy" part of neutrino Yukawa couplings, which can mediate CP-violating phenomena only at some high energy scale. The possible interplay between these two types of CP-violation is discussed. The type I see-saw model with three heavy right-handed Majorana neutrinos having hierarchical spectrum is considered. The analysis shows that there exist regions in the leptogenesis parameter space where the relevant "high energy" phases have large CP-violating values, but the purely "high energy" contribution to the baryon asymmetry plays a subdominant/suppressed role in the production of baryon asymmetry compatible with the observations and one can have successful leptogenesis if the requisite CP-violation is provided by the Majorana phase(s) in the neutrino mixing matrix.

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arXiv2009

Neutrinoless Double Beta Decay and H^{\pm\pm}\to {l'}^\pm l^\pm Decays in the Higgs Triplet Model

The connection between the neutrinoless double beta ((ββ)_{0 ν}-) decay effective Majorana mass, |M_{ee}|, and the branching ratios of the decays H^{\pm\pm}\to l^\pm {l'}^\pm, l,{l'}= e,μ, of the doubly charged Higgs boson H^{\pm\pm} is analysed within the Higgs Triplet Model of neutrino mass generation. We work in the version of the model with explicit breaking of the total lepton charge conservation, in which H^{\pm\pm}\to l^\pm {l'}^\pm, l,{l'} = e,μ,τ, are the dominant decay modes of H^{\pm\pm}. It is assumed also that H^{\pm\pm} are relatively light so that they can be produced at LHC and the branching ratios of interest measured. Taking into account the current and prospective uncertainties in the values of the neutrino mixing parameters most relevant for the problem studied - the atmospheric neutrino mixing angle θ_{23} and the CHOOZ angle θ_{13}, and allowing the lightest neutrino mass and the CP violating Dirac and Majorana phases to vary in the intervals [0, 0.3 eV] and [0, 2π], respectively, we derive the regions of values of BR(H^{\pm\pm}\to e^\pm e^\pm) and BR(H^{\pm\pm}\to e^\pm μ^\pm) for which |M_{ee}|\geq 0.05 eV, or |M_{ee}| < 0.05 eV. This is done for neutrino mass spectrum with normal ordering, inverted ordering and in the case when the type of the spectrum is not known, and i) without using the possible additional data on BR(H^{\pm\pm}\to μ^\pm μ^\pm), ii) using prospective data on BR(H^{\pm\pm}\to μ^\pm μ^\pm). In the latter case results for several values of BR(H^{\pm\pm}\to μ^\pmμ^\pm) are presented.

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arXiv2009

The Interplay Between the "Low" and "High" Energy CP-Violation in Leptogenesis

We analyse within the "flavoured" leptogenesis scenario of baryon asymmetry generation, the interplay of the "low energy" CP-violation, originating from the PMNS neutrino mixing matrix $U$, and the "high energy" CP-violation which can be present in the matrix of neutrino Yukawa couplings, $λ$, and can manifest itself only in "high" energy scale processes. The type I see-saw model with three heavy right-handed Majorana neutrinos having hierarchical spectrum is considered. The "orthogonal" parametrisation of the matrix of neutrino Yukawa couplings, which involves a complex orthogonal matrix $R$, is employed. In this approach the matrix $R$ is the source of "high energy" CP-violation. Results for normal hierarchical (NH) and inverted hierarchical (IH) light neutrino mass spectrum are derived in the case of decoupling of the heaviest RH Majorana neutrino. It is shown that taking into account the contribution to $Y_B$ due to the CP-violating phases in the neutrino mixing matrix $U$ can change drastically the predictions for $Y_B$, obtained assuming only "high energy" CP-violation from the $R$-matrix is operative in leptogenesis. In the case of IH spectrum, in particular, there exist significant regions in the corresponding parameter space where the purely "high energy" contribution in $Y_B$ plays a subdominant role in the production of baryon asymmetry compatible with the observations.

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arXiv2009

CP Violation and Lightest Neutrino Mass Effects in Thermal Leptogenesis

Effects of the lightest neutrino mass in "flavoured" leptogenesis when the CP-violation necessary for the generation of the baryon asymmetry of the Universe is due exclusively to the Dirac and/or Majorana phases in the neutrino mixing matrix $U$ are discussed. The type I see-saw scenario with three heavy right-handed Majorana neutrinos having hierarchical spectrum is considered. The "orthogonal" parametrisation of the matrix of neutrino Yukawa couplings, which involves a complex orthogonal matrix $R$, is employed. Results for light neutrino mass spectrum with normal and inverted ordering (hierarchy) are reviewed.

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arXiv2008

Effects of Lightest Neutrino Mass in Leptogenesis

The effects of the lightest neutrino mass in ``flavoured'' leptogenesis are investigated in the case when the CP-violation necessary for the generation of the baryon asymmetry of the Universe is due exclusively to the Dirac and/or Majorana phases in the neutrino mixing matrix U. The type I see-saw scenario with three heavy right-handed Majorana neutrinos having hierarchical spectrum is considered. The ``orthogonal'' parametrisation of the matrix of neutrino Yukawa couplings, which involves a complex orthogonal matrix R, is employed. Results for light neutrino mass spectrum with normal and inverted ordering (hierarchy) are obtained. It is shown, in particular, that if the matrix R is real and CP-conserving and the lightest neutrino mass m_3 in the case of inverted hierarchical spectrum lies the interval 5 \times 10^{-4} eV < m_3 < 7 \times 10^{-3} eV, the predicted baryon asymmetry can be larger by a factor of \sim 100 than the asymmetry corresponding to negligible m_3 \cong 0. As consequence, we can have successful thermal leptogenesis for 5 \times 10^{-6} eV < m_3 < 5 \times 10^{-2} eV even if R is real and the only source of CP-violation in leptogenesis is the Majorana and/or Dirac phase(s) in U.

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arXiv2008

Neutrino Oscillation Parameters After High Statistics KamLAND Results

We do a re-analysis to asses the impact of the results of the Borexino experiment and the recent 2.8 KTy KamLAND data on the solar neutrino oscillation parameters. The current Borexino results are found to have no impact on the allowed solar neutrino parameter space. The new KamLAND data causes a significant reduction of the allowed range of $Δm^2_{21}$, determining it with an unprecedented precision of 8.3% at 3$σ$. The precision of $Δm^2_{21}$ is controlled practically by the KamLAND data alone. Inclusion of new KamLAND results also improves the upper bound on $\sin^2θ_{12}$, but the precision of this parameter continues to be controlled by the solar data. The third mixing angle is constrained to be $\sin^2θ_{13} < 0.063$ at $3σ$ from a combined fit to the solar, KamLAND, atmospheric and CHOOZ results. We also address the issue of how much further reduction of allowed range of $Δm^2_{21}$ and $\sin^2θ_{12}$ is possible with increased statistics from KamLAND. We find that there is a sharp reduction of the $3σ$ ``spread'' with enhanced statistics till about 10 KTy after which the spread tends to flatten out reaching to less than 4% with 15 KTy data. For $\sin^2θ_{12}$ however, the spread is more than 25% even after 20 KTy exposure and assuming $θ_{12} < π/4$, as dictated by the solar data. We show that with a KamLAND like reactor ``SPMIN'' experiment at a distance of $\sim$ 60 km, the spread of $\sin^2θ_{12}$ could be reduced to about 5% at $3σ$ level while $Δm_{21}^2$ could be determined to within 4%, with just 3 KTy exposure.

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arXiv2008

Flavour physics of leptons and dipole moments

This chapter of the report of the ``Flavour in the era of the LHC'' Workshop discusses the theoretical, phenomenological and experimental issues related to flavour phenomena in the charged lepton sector and in flavour-conserving CP-violating processes. We review the current experimental limits and the main theoretical models for the flavour structure of fundamental particles. We analyze the phenomenological consequences of the available data, setting constraints on explicit models beyond the Standard Model, presenting benchmarks for the discovery potential of forthcoming measurements both at the LHC and at low energy, and exploring options for possible future experiments.

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arXiv2007

Majorana Neutrinos, Neutrino Mass Spectrum and the | | ~ 0.001 eV Frontier in Neutrinoless Double Beta Decay

If future neutrino oscillation experiments show that the neutrino mass spectrum is with normal ordering, m1 < m2 < m3, and the searches for neutrinoless double beta-decay with sensitivity to values of the effective Majorana mass | | > 0.01 eV give negative results, the next frontier in the quest for neutrinoless double beta-decay will correspond to | | ~ 0.001 eV. Assuming that massive neutrinos are Majorana particles and their exchange is the dominant mechanism generating neutrinoless double beta-decay, we analise the conditions under which | |, in the case of three neutrino mixing and neutrino mass spectrum with normal ordering, would satisfy | | > 0.001 eV. We consider the specific cases of i) normal hierarchical neutrino mass spectrum, ii) of relatively small value of the CHOOZ angle theta13 as well as iii) the general case of spectrum with normal ordering, partial hierarchy and a value of theta13 close to the existing upper limit. We study the ranges of the lightest neutrino mass m1 and/or of sin^2 theta13, for which | |> 0.001 eV and discuss the phenomenological implications of such scenarios. We provide also an estimate of | | when the three neutrino masses and the neutrino mixing originate from neutrino mass term of Majorana type for the (left-handed) flavour neutrinos and m1 Ue1^2 + m2 U_e2^2 + m3 Ue3^2 =0, but there does not exist a symmetry which forbids the neutrinoless double beta-decay.

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arXiv2007

U_{PMNS} = U_ell^dagger U_nu

We consider corrections to vanishing U_{e3} and maximal atmospheric neutrino mixing originating from the relation U = U_ell^dagger U_nu, where U is the PMNS mixing matrix and U_ell (U_nu) is associated with the diagonalization of the charged lepton (neutrino) mass matrix. We assume that in the limit of U_ell or U_nu being the unit matrix, one has U_{e3} = 0 and theta_{23} = pi/4, while the solar neutrino mixing angle is a free parameter. Well-known special cases of the indicated scenario are the bimaximal and tri-bimaximal mixing schemes. If U_{e3} \neq 0 and theta_{23} \neq pi/4 due to corrections from the charged leptons, |U_{e3}| can be sizable (close to the existing upper limit) and we find that the value of the solar neutrino mixing angle is linked to the magnitude of CP violation in neutrino oscillations. In the alternative case of the neutrino sector correcting U_{e3} = 0 and theta_{23} = pi/4, we obtain a generically smaller |U_{e3}| than in the first case. Now the magnitude of CP violation in neutrino oscillations is connected to the value of the atmospheric neutrino mixing angle theta_{23}. We find that both cases are in agreement with present observations. We also introduce parametrization independent "sum-rules" for the oscillation parameters.

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