Coupling Unification, GUT-Scale Baryogenesis and Neutron-Antineutron Oscillation in SO(10)

We show that unification of the three gauge couplings can be realized consistently in a class of non-supersymmetric SO(10) models with a one-step breaking to the Standard Model if a color-sextet scalar field survives down to the TeV scale. Such scalars, which should be accessible to the LHC for direct detection, arise naturally in SO(10) as remnants of the seesaw mechanism for neutrino masses. The diquark couplings of these scalars lead to \Delta B = 2 baryon number violating processes such as neutron-antineutron oscillation. We estimate the free neutron-antineutron transition time to be \tau_{n-\bar{n}} \approx (10^9-10^{12}) sec., which is in the interesting range for next generation n-\bar{n} oscillation experiments. These models also realize naturally the recently proposed (B-L)-violating GUT scale baryogenesis which survives to low temperatures unaffected by the electroweak sphaleron interactions.Comment: 15 pages, 4 eps figures, references added, to appear in Phys. Lett.

Testing the Zee-Babu model via neutrino data, lepton flavour violation and direct searches at the LHC

In this talk we discuss how the Zee-Babu model can be tested combining information from neutrino data, low-energy experiments and direct searches at the LHC. We update previous analysis in the light of the recent measurement of the neutrino mixing angle θ13 [1], the new MEG limits on μ→eγ [2], the lower bounds on doubly-charged scalars coming from LHC data [3, 4], and, of course, the discovery of a 125 GeV Higgs boson by ATLAS and CMS [5, 6]. In particular, we find that the new singly- and doubly-charged scalars are accessible at the second run of the LHC, yielding different signatures depending on the neutrino hierarchy and on the values of the phases. We also discuss in detail the stability of the potential

Finite SU(N)^k Unification

We consider N=1 supersymmetric gauge theories based on the group SU(N)_1 x SU(N)_2 x ... x SU(N)_k with matter content (N,N*,1,...,1) + (1,N,N*,...,1) + >... + (N*,1,1,...,N) as candidates for the unification symmetry of all particles. In particular we examine to which extent such theories can become finite and we find that a necessary condition is that there should be exactly three families. We discuss further some phenomenological issues related to the cases (N,k) = (3,3), (3,4), and (4,3), in an attempt to choose those theories that can become also realistic. Thus we are naturally led to consider the SU(3)^3 model which we first promote to an all-loop finite theory and then we study its additional predictions concerning the top quark mass, Higgs mass and supersymmetric spectrum.Comment: 15 page

Quark mixing from softly broken symmetries

Quark flavor mixing may originate in the soft breaking of horizontal symmetries. Those symmetries, which in the simplest case are three family U(1) groups, are obeyed only by the dimension-4 Yukawa couplings and lead, when unbroken, to the absence of mixing. Their breaking may arise from the dimension-3 mass terms of SU(2)-singlet vector-like quarks. Those gauge-singlet mass terms break the horizontal symmetries at a scale much higher than the Fermi scale, yet softly, leading to quark mixing while the quark masses remain unsuppressed.Comment: 9 pages, plain Latex, no figure

P-odd and CP-odd Four-Quark Contributions to Neutron EDM

In a class of beyond-standard-model theories, CP-odd observables, such as the neutron electric dipole moment, receive significant contributions from flavor-neutral P-odd and CP-odd four-quark operators. However, considerable uncertainties exist in the hadronic matrix elements of these operators strongly affecting the experimental constraints on CP-violating parameters in the theories. Here we study their hadronic matrix elements in combined chiral perturbation theory and nucleon models. We first classify the operators in chiral representations and present the leading-order QCD evolutions. We then match the four-quark operators to the corresponding ones in chiral hadronic theory, finding symmetry relations among the matrix elements. Although this makes lattice QCD calculations feasible, we choose to estimate the non-perturbative matching coefficients in simple quark models. We finally compare the results for the neutron electric dipole moment and P-odd and CP-odd pion-nucleon couplings with the previous studies using naive factorization and QCD sum rules. Our study shall provide valuable insights on the present hadronic physics uncertainties in these observables.Comment: 40 pages, 7 figures. This is the final version. A discussion of the uncertainty of the calculation is adde

Accidental stability of dark matter

We propose that dark matter is stable as a consequence of an accidental Z2 that results from a flavour-symmetry group which is the double-cover group of the symmetry group of one of the regular geometric solids. Although model-dependent, the phenomenology resembles that of a generic Higgs portal dark matter scheme.Comment: 12 pages, final version, published in JHE

Effective Lagrangian approach to neutrinoless double beta decay and neutrino masses

Neutrinoless double beta ($0\nu\beta\beta$) decay can in general produce electrons of either chirality, in contrast with the minimal Standard Model (SM) extension with only the addition of the Weinberg operator, which predicts two left-handed electrons in the final state. We classify the lepton number violating (LNV) effective operators with two leptons of either chirality but no quarks, ordered according to the magnitude of their contribution to \znbb decay. We point out that, for each of the three chirality assignments, $e_Le_L, e_Le_R$ and $e_Re_R$, there is only one LNV operator of the corresponding type to lowest order, and these have dimensions 5, 7 and 9, respectively. Neutrino masses are always induced by these extra operators but can be delayed to one or two loops, depending on the number of RH leptons entering in the operator. Then, the comparison of the $0\nu\beta\beta$ decay rate and neutrino masses should indicate the effective scenario at work, which confronted with the LHC searches should also eventually decide on the specific model elected by nature. We also list the SM additions generating these operators upon integration of the heavy modes, and discuss simple realistic examples of renormalizable theories for each case.Comment: Accepted for publication. Few misprints corrected and new references adde

Vanishing Minors in the Neutrino Mass Matrix from Abelian Gauge Symmetries

Augmenting the Standard Model by three right-handed neutrinos allows for an anomaly-free gauge group extension G_max = U(1)_(B-L) x U(1)_(L_e-L_mu) x U(1)_(L_mu-L_tau). While simple U(1) subgroups of G_max have already been discussed in the context of approximate flavor symmetries, we show how two-zero textures in the right-handed neutrino Majorana mass matrix can be enforced by the flavor symmetry, which is spontaneously broken very economically by singlet scalars. These zeros lead to two vanishing minors in the low-energy neutrino mass matrix after the seesaw mechanism. This study may provide a new testing ground for a zero-texture approach: the different classes of two-zero textures with almost identical neutrino oscillation phenomenology can in principle be distinguished by their different Z' interactions at colliders.Comment: 12 pages; Extended and clarified discussion; comments on finetuning in the textures; matches published versio

Relating quarks and leptons with the T-7 flavour group

In this letter we present a model for quarks and leptons based on T-7 as flavour symmetry, predicting a canonical mass relation between charged leptons and down-type quarks proposed earlier. Neutrino masses are generated through a Type-I seesaw mechanism, with predicted correlations between the atmospheric mixing angle and neutrino masses. Compatibility with oscillation results leads to lower bounds for the lightest neutrino mass as well as for the neutrinoless double beta decay rates, even for normal neutrino mass hierarchy

The Zee-Babu model revisited in the light of new data

We update previous analyses of the Zee-Babu model in the light of new data, e.g., the mixing angle On, the rare decay μ-> e gamma and the LHC results. We also analyze the possibility of accommodating the deviations in Gamma (H -> gamma gamma) hinted by the LHC experiments, and the stability of the scalar potential. We find that neutrino oscillation data and low energy constraints are still compatible with masses of the extra charged scalars accessible to LHC. Moreover, if any of them is discovered, the model can be falsified by combining the information on the singly and doubly charged scalar decay modes with neutrino data. Conversely, if the neutrino spectrum is found to be inverted and the CP phase delta is quite different from pi, the masses of the charged scalars will be well outside the LHC reach