Publications (197)845.28 Total impact

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ABSTRACT: Using the fact that the neutrino mixing matrix $U = U^\dagger_{e}U_{\nu}$, where $U_{e}$ and $U_{\nu}$ result from the diagonalisation of the charged lepton and neutrino mass matrices, we analyse the predictions based on the sum rules which the Dirac phase $\delta$ present in $U$ satisfies when $U_{\nu}$ has a form dictated by, or associated with, discrete flavour symmetries and $U_e$ has a "minimal" form (in terms of angles and phases it contains) that can provide the requisite corrections to $U_{\nu}$, so that the reactor, atmospheric and solar neutrino mixing angles $\theta_{13}$, $\theta_{23}$ and $\theta_{12}$ have values compatible with the current data. 
Article: Predictions for the Leptonic Dirac CP Violation Phase: a Systematic Phenomenological Analysis
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ABSTRACT: We derive predictions for the Dirac phase $\delta$ present in the $3\times 3$ unitary neutrino mixing matrix $U = U_e^{\dagger} \, U_{\nu}$, where $U_e$ and $U_{\nu}$ are $3\times 3$ unitary matrices which arise from the diagonalisation respectively of the charged lepton and the neutrino mass matrices. After performing a systematic search, we consider forms of $U_e$ and $U_{\nu}$ allowing us to express $\delta$ as a function of the Pontecorvo, Maki, Nakagawa, Sakata (PMNS) mixing angles, $\theta_{12}$, $\theta_{13}$ and $\theta_{23}$, present in $U$, and the angles contained in $U_{\nu}$. We derive sum rules for $\cos\delta$ in the cases of forms for which the sum rules of interest do not exist in the literature. We consider several forms of $\tilde{U}_{\nu}$ determined by, or associated with, symmetries, tribimaximal (TBM), bimaximal (BM), etc., for which the angles in $\tilde{U}_{\nu}$ are fixed. For each of these forms and forms of $\tilde{U}_e$ allowing to reproduce the measured values of the angles $\theta_{12}$, $\theta_{13}$ and $\theta_{23}$, we construct the likelihood function for $\cos \delta$, using i) the latest results of the global fit analysis of neutrino oscillation data, and ii) the prospective sensitivities on the PMNS mixing angles. Our results, in particular, confirm the conclusion reached in earlier similar studies that the measurement of the Dirac phase in the PMNS mixing matrix, together with an improvement of the precision on the mixing angles $\theta_{12}$, $\theta_{13}$ and $\theta_{23}$, can provide unique information about the possible existence of symmetry in the lepton sector. Such measurements could also provide an indication about the structure of the matrix $\tilde U_e$ originating from the charged lepton sector, and thus about the charged lepton mass matrix. 
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ABSTRACT: In this paper we discuss a minor modification of a previous SU(5) x A5 flavour model which exhibits at leading order golden ratio mixing and sum rules for the heavy and the light neutrino masses. Although this model could predict all mixing angles well it fails in generating a sufficient large baryon asymmetry via the leptogenesis mechanism. We repair this deficit here, discuss model building aspects and give analytical estimates for the generated baryon asymmetry before we perform a numerical parameter scan. Our setup has only a few parameters in the lepton sector. This leads to specific constraints and correlations between the neutrino observables. For instance, we find that in the model considered only the neutrino mass spectrum with normal mass ordering and values of the lightest neutrino mass in the interval 1018 meV are compatible with the current data on the neutrino oscillation parameters. With the introduction of only one NLO operator, the model can accommodate successfully simultaneously even at 1$\sigma$ level the current data on neutrino masses, on neutrino mixing and the observed value of the baryon asymmetry. 
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ABSTRACT: Using the fact that the neutrino mixing matrix , where and result from the diagonalisation of the charged lepton and neutrino mass matrices, we consider a number of forms of associated with a variety of flavour symmetries: i) bimaximal (BM) and ii) tribimaximal (TBM) forms, the forms corresponding iii) to the conservation of the lepton charge (LC), iv) to golden ratio type A (GRA) mixing, v) golden ratio type B (GRB) mixing, and vi) to hexagonal (HG) mixing. Employing the minimal form of , in terms of angles and phases it contains, that can provide the requisite corrections to so that reactor, atmospheric and solar neutrino mixing angles , and have values compatible with the current data, including a possible sizable deviation of from , we discuss the possibility to obtain predictions for the CP violation phases in the neutrino mixing matrix. Considering the “standard ordering” of the the 12 and the 23 rotations in and following the approach developed in [1] we derive predictions for the Dirac phase δ and the rephasing invariant in the cases of GRA, GRB and HG forms of (results for the TBM and BM (LC) forms were obtained in [1]). We show also that under rather general conditions within the scheme considered the values of the Majorana phases in the PMNS matrix can be predicted for each of the forms of discussed. We give examples of these predictions and of their implications for neutrinoless double beta decay. In the GRA, GRB and HG cases, as in the TBM one, relatively large CP violation effects in neutrino oscillations are predicted ( ). Distinguishing between the TBM, BM (LC), GRA, GRB and HG forms of requires a measurement of or a relatively high precision measurement of . 
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ABSTRACT: The process of Radiative Emission of Neutrino Pair (RENP) in atoms is sensitive to the absolute neutrino mass scale, the type of spectrum neutrino masses obey and the nature  Dirac or Majorana  of massive neutrinos. We analyse the possibility to test the hypothesis of existence of neutrinos with masses at the eV scale coupled to the electron in the weak charged lepton current in an RENP experiment. The presence of eV scale neutrinos in the neutrino mixing is associated with the existence of sterile neutrinos which mix with the active flavour neutrinos. At present there are a number of hints for activesterile neutrino oscillations driven by $\Delta m^2 \sim 1~{\rm eV^2}$. We perform a detailed analysis of the RENP phenomenology within the "3 + 1" scheme with one sterile neutrino. 
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ABSTRACT: Using the fact that the neutrino mixing matrix $U = U^\dagger_{e}U_{\nu}$, where $U_{e}$ and $U_{\nu}$ result from the diagonalisation of the charged lepton and neutrino mass matrices, we analyse the sum rules which the Dirac phase $\delta$ present in $U$ satisfies when $U_{\nu}$ has a form dictated by flavour symmetries and $U_e$ has a "minimal" form (in terms of angles and phases it contains) that can provide the requisite corrections to $U_{\nu}$, so that reactor, atmospheric and solar neutrino mixing angles $\theta_{13}$, $\theta_{23}$ and $\theta_{12}$ have values compatible with the current data. The following symmetry forms are considered: i) tribimaximal (TBM), ii) bimaximal (BM) (or corresponding to the conservation of the lepton charge $L' = L_e  L_\mu  L_{\tau}$ (LC)), iii) golden ratio type A (GRA), iv) golden ratio type B (GRB), and v) hexagonal (HG). We investigate the predictions for $\delta$ in the cases of TBM, BM (LC), GRA, GRB and HG forms using the exact and the leading order sum rules for $\cos\delta$ proposed in the literature, taking into account also the uncertainties in the measured values of $\sin^2\theta_{12}$, $\sin^2\theta_{23}$ and $\sin^2\theta_{13}$. This allows us, in particular, to assess the accuracy of the predictions for $\cos\delta$ based on the leading order sum rules and its dependence on the values of the indicated neutrino mixing parameters when the latter are varied in their respective 3$\sigma$ experimentally allowed ranges. 
Article: The Daya Bay and T2K results on $\sin^2 2 \theta_{13}$ and NonStandard Neutrino Interactions
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ABSTRACT: We show that the relatively large best fit value of $\sin^2 2 \theta_{13} = 0.14 \, (0.17)$ measured in the T2K experiment for fixed values of i) the Dirac CP violation phase $\delta = 0$, and ii) the atmospheric neutrino mixing parameters $\theta_{23} = \pi/4$, $\Delta m^2_{32} = 2.4 \times 10^{3} \; {\rm eV}^2$, can be reconciled with the Daya Bay result $\sin^2 2 \theta_{13} = 0.090 \pm 0.009$ if the effects of nonstandard neutrino interactions (NSI) in the relevant $\bar \nu_e \to \bar \nu_e$ and $\nu_\mu \to \nu_e$ oscillation probabilities are taken into account.Nuclear Physics B 05/2014; 886. DOI:10.1016/j.nuclphysb.2014.06.014 · 3.95 Impact Factor 
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ABSTRACT: The phenomenology of 3neutrino mixing, the current status of our knowledge about the 3neutrino mixing parameters, including the absolute neutrino mass scale, and of the Dirac and Majorana CP violation in the lepton sector, are reviewed. The problems of CP violation in neutrino oscillations and of determining the nature — Dirac or Majorana — of massive neutrinos, are discussed. The seesaw mechanism of neutrino mass generation and the related leptogenesis scenario of generation of the baryon asymmetry of the universe, are considered. The results showing that the CP violation necessary for the generation of the baryon asymmetry of the universe in leptogenesis can be due exclusively to the Dirac and/or Majorana CPviolating phase(s) in the neutrino mixing matrix U, are briefly reviewed.International Journal of Modern Physics A 03/2014; 29(11n12). DOI:10.1142/S0217751X14300282 · 1.09 Impact Factor 
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ABSTRACT: We discuss how two birds—the little hierarchy problem of lowscale typeI seesaw models and the search for a viable dark matter candidate—are (proverbially) killed by one stone: a new inert scalar state.European Physical Journal C 01/2014; 74(2). DOI:10.1140/epjc/s100520142774x · 5.44 Impact Factor 
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ABSTRACT: We show that the relatively large best fit value of sin22θ13=0.14(0.17) measured in the T2K experiment for fixed values of i) the Dirac CP violation phase δ=0δ=0, and ii) the atmospheric neutrino mixing parameters θ23=π/4θ23=π/4, Δm322=2.4×10−3 eV2, can be reconciled with the Daya Bay result sin22θ13=0.090±0.009 if the effects of nonstandard neutrino interactions (NSI) in the relevant ν¯e→ν¯e and νμ→νeνμ→νe oscillation probabilities are taken into account. 
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ABSTRACT: We analyse the interplay of generalised CP transformations and the nonAbelian discrete group $T^{\prime}$ and use the semidirect product $G_f= T^{\prime}\rtimes H_{\text{CP}}$, as family symmetry acting in the lepton sector. The family symmetry is shown to be spontaneously broken in a geometrical manner. In the resulting flavour model, naturally small Majorana neutrino masses for the light active neutrinos are obtained through the type I seesaw mechanism. The known masses of the charged leptons, lepton mixing angles and the two neutrino mass squared differences are reproduced by the model with a good accuracy. The model allows for two neutrino mass spectra with normal ordering (NO) and one with inverted ordering (IO). For each of the three spectra the absolute scale of neutrino masses is predicted with relatively small uncertainty. The value of the Dirac CP violation (CPV) phase $\delta$ in the lepton mixing matrix is predicted to be $\delta \cong \pi/2~{\rm or}~ 3\pi/2$. Thus, the CP violating effects in neutrino oscillations are predicted to be maximal (given the values of the neutrino mixing angles) and experimentally observable. We present also predictions for the sum of the neutrino masses, for the Majorana CPV phases and for the effective Majorana mass in neutrinoless double beta decay. The predictions of the model can be tested in a variety of ongoing and future planned neutrino experiments.Journal of High Energy Physics 12/2013; 2014(2). DOI:10.1007/JHEP02(2014)050 · 6.22 Impact Factor 
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ABSTRACT: We perform a detailed analysis of lepton flavour violation (LFV) within minimal seesaw type extensions of the Standard Model (SM), which give a viable mechanism of neutrino mass generation and provide new particle content at the electroweak scale. We focus, mainly, on predictions and constraints set on each scenario from μ → eγ, μ → 3e and μ − e conversion in the nuclei. In this class of models, the flavour structure of the Yukawa couplings between the additional scalar and fermion representations and the SM leptons is highly constrained by neutrino oscillation measurements. In particular, we show that in some regions of the parameters space of type I and type II seesaw models, the Dirac and Majorana phases of the neutrino mixing matrix, the ordering and hierarchy of the active neutrino mass spectrum as well as the value of the reactor mixing angle θ 13 may considerably affect the size of the LFV observables. The interplay of the latter clearly allows to discriminate among the different low energy seesaw possibilities.Journal of High Energy Physics 09/2013; 2012(8). DOI:10.1007/JHEP08(2012)125 · 6.22 Impact Factor 
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ABSTRACT: We investigate the predictions for neutrinoless double beta ($(\beta \beta)_{0 \nu}$) decay effective Majorana mass $\left \langle \, m \, \rangle \right$ in the 3+1 and 3+2 schemes with one and two additional sterile neutrinos with masses at the eV scale. The two schemes are suggested by the neutrino oscillation interpretation of the reactor neutrino and Gallium "anomalies" and of the data of the LSND and MiniBooNE experiments. We analyse in detail the possibility of a complete or partial cancellation between the different terms in $\left \langle \, m \, \rangle \right$, leading to a strong suppression of $\left \langle \, m \, \rangle \right$. We determine the regions of the relevant parameter spaces where such a suppression can occure. This allows us to derive the conditions under which the effective Majorana mass satisfies $\left \langle \, m \, \rangle \right > 0.01$ eV, which is the range planned to be exploited by the next generation of $(\beta \beta)_{0 \nu}$experiments.Journal of High Energy Physics 08/2013; DOI:10.1007/JHEP11(2013)146 · 6.22 Impact Factor 
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ABSTRACT: The lepton flavour violating (LFV) $\tau$ decays $\tau\to (e,\mu)\gamma$ and $\tau\to 3\mu$ are investigated in the frameworks of the TeV scale type I seesaw and Higgs Triplet (or type II seesaw) models. Predictions for the rates of these processes are obtained. The implications of the existing stringent experimental upper bounds on the $\mu\to e + \gamma$ and $\mu\to 3e$ decay branching ratios for the predictions of the $\tau\to (e,\mu)\gamma$ and $\tau\to 3\mu$ decay rates are studied in detail. The possibilities to observe the indicated LFV $\tau$ decays in present and future experiments are analysed.Journal of High Energy Physics 08/2013; DOI:10.1007/JHEP09(2013)086 · 6.22 Impact Factor 
Article: Lowscale seesaw and dark matter
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ABSTRACT: We discuss how two birdsthe little hierarchy problem of lowscale typeI seesaw models and the search for a viable dark matter candidateare (proverbially) killed by one stone: a new inert scalar state 
Article: The Nature of Massive Neutrinos
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ABSTRACT: The compelling experimental evidences for oscillations of solar, reactor, atmospheric, and accelerator neutrinos imply the existence of 3neutrino mixing in the weak charged lepton current. The current data on the 3neutrino mixing parameters are summarised and the phenomenology of 3 mixing is reviewed. The properties of massive Majorana neutrinos and of their various possible couplings are discussed in detail. Two models of neutrino mass generation with massive Majorana neutrinos—the type I seesaw and the Higgs triplet model—are briefly reviewed. The problem of determining the nature, Dirac or Majorana, of massive neutrinos is considered. The predictions for the effective Majorana mass in neutrinoless doublebeta() decay in the case of 3neutrino mixing and massive Majorana neutrinos are summarised. The physics potential of the experiments, searching for decay for providing information on the type of the neutrino mass spectrum, on the absolute scale of neutrino masses, and on the Majorana CPviolation phases in the PMNS neutrino mixing matrix, is also briefly discussed. The opened questions and the main goals of future research in the field of neutrino physics are outlined.Advances in High Energy Physics 04/2013; 2013. DOI:10.1155/2013/852987 · 2.62 Impact Factor 
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ABSTRACT: After the successful determination of the reactor neutrino mixing angle \theta_13 ~ 0.16 \neq 0, a new feature suggested by the current neutrino oscillation data is a sizeable deviation of the atmospheric neutrino mixing angle \theta_23 from \pi/4. Using the fact that the neutrino mixing matrix U = U_e^\dagger U_\nu, where U_e and U_\nu result from the diagonalisation of the charged lepton and neutrino mass matrices, and assuming that U_\nu has a i) bimaximal (BM), ii) tribimaximal (TBM) form, or else iii) corresponds to the conservation of the lepton charge L' = L_e  L_\mu  L_\tau (LC), we investigate quantitatively what are the minimal forms of U_e, in terms of angles and phases it contains, that can provide the requisite corrections to U_\nu so that \theta_13, \theta_23 and the solar neutrino mixing angle \theta_12 have values compatible with the current data. Two possible orderings of the 12 and the 23 rotations in U_e, "standard" and "inverse", are considered. The results we obtain depend strongly on the type of ordering. In the case of "standard" ordering, in particular, the Dirac CP violation phase \delta, present in U, is predicted to have a value in a narrow interval around i) \delta ~ \pi in the BM (or LC) case, ii) \delta ~ 3\pi/2 or \pi/2 in the TBM case, the CP conserving values \delta = 0, \pi, 2\pi being excluded in the TBM case at more than 4\sigma.Journal of High Energy Physics 02/2013; 2013(5). DOI:10.1007/JHEP05(2013)073 · 6.22 Impact Factor 
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ABSTRACT: We investigate the possibility to discriminate between different pairs of CP nonconserving mechanisms inducing the neutrinoless double beta $\betabeta$decay by using data on $\betabeta$decay halflives of nuclei with largely different nuclear matrix elements (NMEs). The mechanisms studied are: light Majorana neutrino exchange, heavy lefthanded (LH) and heavy righthanded (RH) Majorana neutrino exchanges, lepton charge nonconserving couplings in SUSY theories with Rparity breaking giving rise to the "dominant gluino exchange" and the "squarkneutrino" mechanisms. The nuclei considered are $^{76}$Ge, $^{82}$Se, $^{100}$Mo, $^{130}$Te and $^{136}$Xe. Four sets of nuclear matrix elements (NMEs) of the decays of these five nuclei, derived within the Selfconsistent Renormalized Quasiparticle Random Phase Approximation (SRQRPA), were employed in our analysis. While for each of the five single mechanisms discussed, the NMEs for $^{76}$Ge, $^{82}$Se, $^{100}$Mo and $^{130}$Te differ relatively little, the relative difference between the NMEs of any two nuclei not exceeding 10%, the NMEs for $^{136}Xe$ differ significantly from those of $^{76}$Ge, $^{82}Se$, $^{100}$Mo and $^{130}$Te, being by a factor $\sim (1.3  2.5)$ smaller. This allows, in principle, to draw conclusions about the pair of noninterfering (interfering) mechanisms possibly inducing the $\betabeta$decay from data on the halflives of $^{136}Xe$ and of at least one (two) more isotope(s) which can be, e.g., any of the four, $^{76}Ge$, $^{82}Se$, $^{100}Mo$ and $^{130}Te$. Depending on the sets of mechanisms considered, the conclusion can be independent of, or can depend on, the NMEs used in the analysis. The implications of the EXO lower bound on the halflife of $^{136}Xe$ for the problem studied are also exploited.Journal of High Energy Physics 12/2012; 2013(2). DOI:10.1007/JHEP02(2013)025 · 6.22 Impact Factor 
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ABSTRACT: We present a SUSY SU(5)×T′ unified flavor model with type I seesaw mechanism of neutrino mass generation, which predicts the reactor neutrino angle to be θ13≈0.14 close to the recent results from the Daya Bay and RENO experiments. The model predicts also values of the solar and atmospheric neutrino mixing angles, which are compatible with the existing data. The T′ breaking leads to tribimaximal mixing in the neutrino sector, which is perturbed by sizeable corrections from the charged lepton sector. The model exhibits geometrical CP violation, where all complex phases have their origin from the complex ClebschGordan coefficients of T′. The values of the Dirac and Majorana CP violating phases are predicted. For the Dirac phase in the standard parametrization of the neutrino mixing matrix we get a value close to 90°: δ≅π/20.45θc≅84.3°, θc being the Cabibbo angle. The neutrino mass spectrum can be with normal ordering (2 cases) or inverted ordering. In each case the values of the three light neutrino masses are predicted with relatively small uncertainties, which allows one to get also unambiguous predictions for the neutrinoless double beta decay effective Majorana mass.Physical review D: Particles and fields 12/2012; 86(11). DOI:10.1103/PhysRevD.86.113003 · 4.86 Impact Factor 
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ABSTRACT: The process of collective deexcitation of atoms in a metastable level into emission mode of a single photon plus a neutrino pair, called radiative emission of neutrino pair (RENP), is sensitive to the absolute neutrino mass scale, to the neutrino mass hierarchy and to the nature (Dirac or Majorana) of massive neutrinos. We investigate how the indicated neutrino mass and mixing observables can be determined from the measurement of the corresponding continuous photon spectrum taking the example of a transition between specific levels of the Yb atom. The possibility of determining the nature of massive neutrinos and, if neutrinos are Majorana fermions, of obtaining information about the Majorana phases in the neutrino mixing matrix, is analyzed in the cases of normal hierarchical, inverted hierarchical and quasidegenerate types of neutrino mass spectrum. We find, in particular, that the sensitivity to the nature of massive neutrinos depends critically on the atomic level energy difference relevant in the RENP.Physics Letters B 09/2012; 719(s 1–3). DOI:10.1016/j.physletb.2013.01.015 · 6.02 Impact Factor
Publication Stats
12k  Citations  
845.28  Total Impact Points  
Top Journals
Institutions

2010–2014

The University of Tokyo
Tōkyō, Japan


1981–2014

Bulgarian Academy of Sciences
 Institute for Nuclear Research and Nuclear Energy (INRNE)
Ulpia Serdica, SofiaCapital, Bulgaria


1991–2011

Scuola Internazionale Superiore di Studi Avanzati di Trieste
Trst, Friuli Venezia Giulia, Italy


2007

University of California, Berkeley
 Department of Physics
Berkeley, California, United States


1992–2007

INFN  Istituto Nazionale di Fisica Nucleare
Frascati, Latium, Italy


2006

Abdus Salam International Centre for Theoretical Physics
Trst, Friuli Venezia Giulia, Italy


2000

Medical University of Sofia
Ulpia Serdica, SofiaCapital, Bulgaria


1987

Institute for Nuclear Research and Nuclear Energy
Ulpia Serdica, SofiaCapital, Bulgaria


1979–1987

CERN
 Physics Department (PH)
Genève, GE, Switzerland


1984

Joint Institute for Nuclear Research
Dubno, Moskovskaya, Russia


1983

Stanford University
 SLAC National Accelerator Laboratory
Palo Alto, California, United States 
University of Delaware
Delaware, United States
