Mesodermal Nkx2.5 is necessary and sufficient for early second heart field development

AbstractThe vertebrate heart develops from mesoderm and requires inductive signals secreted from early endoderm. During embryogenesis, Nkx2.5 acts as a key transcription factor and plays essential roles for heart formation from Drosophila to human. In mice, Nkx2.5 is expressed in the early first heart field, second heart field pharyngeal mesoderm, as well as pharyngeal endodermal cells underlying the second heart field. Currently, the specific requirements for Nkx2.5 in the endoderm versus mesoderm with regard to early heart formation are incompletely understood. Here, we performed tissue-specific deletion in mice to dissect the roles of Nkx2.5 in the pharyngeal endoderm and mesoderm. We found that heart development appeared normal after endodermal deletion of Nkx2.5 whereas mesodermal deletion engendered cardiac defects almost identical to those observed on Nkx2.5 null embryos (Nkx2.5−/−). Furthermore, re-expression of Nkx2.5 in the mesoderm rescued Nkx2.5−/− heart defects. Our findings reveal that Nkx2.5 in the mesoderm is essential while endodermal expression is dispensable for early heart formation in mammals

The Interplay between Neutrinos and Charged Leptons in the Minimal SU(3)_LxU(1)_N Gauge Model

In the minimal SU(3)_LxU(1)_N gauge model with a global L_e-L_mu-L_tau (=L') symmetry and a discrete Z_4 symmetry, it is found that the interplay between neutrinos and charged leptons contained in triplets of \psi^i=(\nu^i_L, \ell^i_L, \ell^{ci}_L) (i=1,2,3) naturally leads to the large mixing angle (LMA) MSW solution. The model includes two (anti)sextet Higgs scalars, S^(0) with L'=0 and S^(+) with L'=2, which, respectively, couple to \psi^1\psi^{2,3} for the electron mass and masses of atmospheric neutrinos and to \psi^{2,3}\psi^{2,3} for the \mu- and \tau-masses and one-loop radiative neutrino masses relevant to solar neutrinos. This mechanism is realized by utilizing an additional residual discrete symmetry supplied by explicitly broken L', which guarantees the absence of tree-level neutrino mass terms of the \psi^{2,3}\psi^{2,3}-type. Pure rotation effects due to the diagonalization of neutrino and charged-lepton mass matrices are estimated to yield \Delta m^2_\odot/\Delta m^2_{atm} \leq (m_e/m_\mu)^{3/2}=O(10^{-4}) but the radiative effects supersede the rotation effects to yield \Delta m^2_\odot/\Delta m^2_{atm}=O(10^{-2}) as the LMA solution.Comment: 16 pages, RevTeX, including 2 figures with typos and misprints corrected and with modifications in sections II-B and V, accepted by Nuclear Physics

Phenomenology of Pseudo Dirac Neutrinos

We formulate general conditions on $3\times 3$ neutrino mass matrices under which a degenerate pair of neutrinos at a high scale would split at low scale by radiative corrections involving only the standard model fields. This generalizes the original observations of Wolfenstein on pseudo Dirac neutrinos to three generations. A specific model involving partially broken discrete symmetry and solving the solar and atmospheric anomalies is proposed. The symmetry pattern of the model naturally generates two large angles one of which can account for the large angle MSW solution to the solar neutrino problem.Comment: 15 pages LATE

Obtaining the large angle MSW solution to the solar neutrino problem in models

The large mixing angle (LMA) MSW solution to the solar neutrino problem seems favored by the data at the moment over the small mixing angle (SMA) MSW solution and the vacuum (VAC) solution. In this paper the various main types of models of neutrino masses and mixings are studied from the point of view of how naturally they can give the LMA solution. Special attention is given to a very simple type of "lopsided" SU(5) model.Comment: 29 pages, 2 Postscript figure

Bimaximal Neutrino Mixings from Lopsided Mass Matrices

Current solar and atmospheric neutrino oscillation data seem to favor a bimaximal pattern for neutrino mixings where the matrix elements U_{e2} and U_{\mu 3} are of order one, while U_{e3} is much smaller. We show that such a pattern can be obtained quite easily in theories with ``lopsided'' mass matrices for the charged leptons and the down type quarks. A relation connecting the solar and atmospheric neutrino mixing angles is derived, \tan^2\theta_{atm} \simeq 1+ \tan^2\theta_{sol}, which predicts \sin^2 2\theta_{atm} \simeq 0.97 corresponding to the best fit LMA solution for solar neutrinos. Predictive schemes in SO(10) realizing these ideas are presented. A new class of SO(10) models with lopsided mass matrices is found which makes use of an adjoint VEV along the I_{3R} direction, rather than the traditional B-L direction.Comment: 12 pages in LaTeX, no figure

Large Solar Neutrino Mixing and Radiative Neutrino Mechanism

We find that the presence of a global $L_e-L_\mu-L_\tau$ ($\equiv L^\prime$) symmetry and an $S_2$ permutation symmetry for the $\mu$- and $\tau$-families supplemented by a discrete $Z_4$ symmetry naturally leads to almost maximal atmospheric neutrino mixing and large solar neutrino mixing, which arise, respectively, from type II seesaw mechanism initiated by an $S_2$-symmetric triplet Higgs scalar $s$ with $L^\prime=2$ and from radiative mechanism of the Zee type initiated by two singly charged scalars, an $S_2$-symmetric $h^+$ with $L^\prime=0$ and an $S_2$-antisymmetric $h^{\prime +}$ with $L^\prime=2$. The almost maximal mixing for atmospheric neutrinos is explained by the appearance of the democratic coupling of $s$ to neutrinos ensured by $S_2$ and $Z_4$ while the large mixing for solar neutrinos is explained by the similarity of $h^+$- and $h^{\prime +}$-couplings described by $f^h_+\sim f^h_-$ and $\mu_+\sim\mu_-$, where $f^h_+$ ($f^h_-$) and $\mu_+$ ($\mu_-$) stand for $h^+$ ($h^{\prime +}$)-couplings, respectively, to leptons and to Higgs scalars.Comment: RevTex, 10 pages including one figure. In Ref.[25], the cited page number is correcte

Bilarge Neutrino Mixing and \mu - \tau Permutation Symmetry for Two-loop Radiative Mechanism

The presence of approximate electron number conservation and \mu-\tau permutation symmetry of S_2 is shown to naturally provide bilarge neutrino mixing. First, the bimaximal neutrino mixing together with U_{e3}=0 is guaranteed to appear owing to S_2 and, then, the bilarge neutrino mixing together with |U_{e3}|<<1 arises as a result of tiny violation of S_2. The observed mass hierarchy of \Delta m^2_{\odot}<<\Delta m^2_{atm} is subject to another tiny violation of the electron number conservation. This scenario is realized in a specific model based on SU(3)_L x U(1)_N with two-loop radiative mechanism for neutrino masses. The radiative effects from heavy leptons contained in lepton triplets generate the bimaximal structure and those from charged leptons, which break S_2, generate the bilarge structure together with |U_{e3}|<<1. To suppress dangerous flavor-changing neutral current interactions due to Higgs exchanges especially for quarks, this S_2 symmetry is extended to a discrete Z_8 symmetry, which also ensures the absence of one-loop radiative mechanism.Comment: 18 pages, 7 figures, to appear in Phys. Rev.

A texture of neutrino mass matrix in view of recent neutrino experimental results

In view of recent neutrino experimental results such as SNO, Super-Kamiokande (SK), CHOOZ and neutrinoless double beta decay $(\beta\beta_{0\nu})$, we consider a texture of neutrino mass matrix which contains three parameters in order to explain those neutrino experimental results. We have first fitted parameters in a model independent way with solar and atmospheric neutrino mass squared differences and solar neutrino mixing angle which satisfy LMA solution. The maximal value of atmospheric neutrino mixing angle comes out naturally in the present texture. Most interestingly, fitted parameters of the neutrino mass matrix considered here also marginally satisfy recent limit on effective Majorana neutrino mass obtained from neutrinoless double beta decay experiment. We further demonstrate an explicit model which gives rise to the texture investigated by considering an $SU(2)_L\times U(1)_Y$ gauge group with two extra real scalar singlets and discrete $Z_2\times Z_3$ symmetry. Majorana neutrino masses are generated through higher dimensional operators at the scale $M$. We have estimated the scales at which singlets get VEV's and M by comparing with the best fitted results obtained in the present work.Comment: Journal Ref.: Phys. Rev. D66, 053004 (2002

Two-loop Radiative Neutrino Mechanism in an SU(3)L×U(1)NSU(3)_L\times U(1)_N Gauge Model

By using the $L_e$ - $L_\mu$ - $L_\tau$ symmetry, we construct an $SU(3)_L\times U(1)_N$ gauge model that provides two-loop radiative neutrino masses as well as one-loop radiative neutrino masses. The generic smallness of two-loop neutrino masses leading to $\Delta m^2_\odot$ compared with one-loop neutrino masses leading to $\Delta m^2_{atm}$ successfully explains $\Delta m^2_{atm}$ $>>$ $\Delta m^2_{\odot}$ by invoking the $L_e$ - $L_\mu$ - $L_\tau$ breaking. The Higgs scalar ($h^+$) that initiates radiative mechanisms is unified into a Higgs triplet together with the standard Higgs scalar ($\phi^+$, $\phi^0$) to form ($\phi^+$, $\phi^0$, $h^+$), which calls for three families of lepton triplets: ($\nu^i_L$, $\ell^i_L$, $\omega^i_L$) (i = 1,2,3), where $\omega^i$ denote heavy neutral leptons. The two-loop radiative mechanism is found possible by introducing a singly charged scalar, which couples to $\ell^i_R\omega^j_R$ (i,j = 2,3).Comment: with 10 pages, revtex, including 2 figures, accepted for publication in Phys. Rev. D (with undefined latex citation indices removed

Low-Scale See-Saw Mechanisms for Light Neutrinos

Alternatives to the see-saw mechanism are explored in supersymmetric models with three right-handed or sterile neutrinos. Tree-level Yukawa couplings can be drastically suppressed in a natural way to give sub-eV Dirac neutrino masses. If, in addition, a B-L gauge symmetry broken at a large scale M_G is introduced, a wider range of possibilities opens up. The value of the right-handed neutrino mass M_R can be easily disentangled from that of M_G. Dirac and Majorana neutrino masses at the eV scale can be generated radiatively through the exchange of sneutrinos and neutralinos. Dirac masses m_D owe their smallness to the pattern of light-heavy scales in the neutralino mass matrix. The smallness of the Majorana masses m_L is linked to a similar see-saw pattern in the sneutrino mass matrix. Two distinct scenarios emerge. In the first, with very small or vanishing M_R, the physical neutrino eigenstates are, for each generation, either two light Majorana states with mixing angle ranging from very small to maximal, depending on the ratio m_D/M_R, or one light Dirac state. In the second scenario, with a large value of M_R, the physical eigenstates are two nearly unmixed Majorana states with masses \sim m_L and \sim M_R. In both cases, the (B-L)-breaking scale M_G is, in general, much smaller than that in the traditional see-saw mechanism.Comment: 31 pages, Latex, references added, version to appear in Phys. Rev.