Sterile Neutrinos and Global Symmetries

We use an effective-field-theory approach to construct models with naturally light sterile neutrinos, due to either exact or accidental global symmetries. The most attractive models we find are based on gauge symmetries, either discrete or continuous. We give examples of simple models based on Z_N, U(1)', and SU(2)'.Comment: 8 pages. v2 (as in PRD): minor changes, typos corrected, two refs adde

Naturalness and the Neutrino Matrix

The observed pattern of neutrino mass splittings and mixing angles indicates that their family structure is significantly different from that of the charged fermions. We investigate the implications of these data for the fermion mass matrices in grand unified theories with a type-I seesaw mechanism. We show that, with simple assumptions, naturalness leads to a strongly hierarchical Majorana mass matrix for heavy right-handed neutrinos and a partially cascade form for the Dirac neutrino matrix. We consider various model building scenarios which could alter this conclusion, and discuss their consequences for the construction of a natural model. We find that including partially lopsided matrices can aid us in generating a satisfying model.Comment: Discussion about neutrinoless double beta decay expanded; typos corrected; references added and update

Proton Decay in a Consistent Supersymmetric SU(5) GUT Model

It is widely believed that minimal supersymmetric SU(5) GUTs have been excluded by the SuperKamiokande bound for the proton decay rate. In the minimal model, however, the theoretical prediction assumes unification of Yukawa couplings, Y_d=Y_e, which is known to be badly violated. We analyze the implications of this fact for the proton decay rate. In a consistent SU(5) model with higher dimensional operators, where SU(5) relations among Yukawa couplings hold, the proton decay rate can be several orders of magnitude smaller than the present experimental bound

Higher-dimensional operators in SUSY SO(10) GUT models

SO(10) GUT models with only small Higgs fields use higher-dimensional operators to generate realistic fermion mass matrices. In particular, a Higgs field in the spinor representation, 16^d_H, acquires a weak scale vev. We include the weak vev of the corresponding field \bar{16}^u_H and investigate the effect on two successful models, one by Albright and Barr (AB) and another by Babu, Pati and Wilczek (BPW). We find that the BPW model is a particular case within a class of models with identical fermion masses and mixings. In contrast, we expect corrections to the parameters of AB-type models.Comment: 3 page

Trinification, the Hierarchy Problem and Inverse Seesaw Neutrino Masses

In minimal trinification models light neutrino masses can be generated via a radiative see-saw mechanism, where the masses of the right-handed neutrinos originate from loops involving Higgs and fermion fields at the unification scale. This mechanism is absent in models aiming at solving or ameliorating the hierarchy problem, such as low-energy supersymmetry, since the large seesaw-scale disappears. In this case, neutrino masses need to be generated via a TeV-scale mechanism. In this paper, we investigate an inverse seesaw mechanism and discuss some phenomenological consequences.Comment: 10 pages, 11 figure

Minimal Trinification

We study the trinified model, SU(3)_C x SU(3)_L x SU(3)_R x Z_3, with the minimal Higgs sector required for symmetry breaking. There are five Higgs doublets, and gauge-coupling unification results if all five are at the weak scale, without supersymmetry. The radiative see-saw mechanism yields sub-eV neutrino masses, without the need for intermediate scales, additional Higgs fields, or higher-dimensional operators. The proton lifetime is above the experimental limits, with the decay modes p -> \bar\nu K^+ and p -> \mu^+ K^0 potentially observable. We also consider supersymmetric versions of the model, with one or two Higgs doublets at the weak scale. The radiative see-saw mechanism fails with weak-scale supersymmetry due to the nonrenormalization of the superpotential, but operates in the split-SUSY scenario.Comment: 23 pages, uses axodra

Flavour structure and proton decay in 6D orbifold GUTs

We study proton decay in a supersymmetric {\sf SO(10)} gauge theory in six dimensions compactified on an orbifold. The dimension-5 proton decay operators are forbidden by R-symmetry, whereas the dimension-6 operators are enhanced due to the presence of KK towers. Three sequential quark-lepton families are localised at the three orbifold fixed points, where {\sf SO(10)} is broken to its three GUT subgroups. The physical quarks and leptons are mixtures of these brane states and additional bulk zero modes. This leads to a characteristic pattern of branching ratios in proton decay, in particular the suppression of the p\to \m^+K^0 mode.Comment: 20 pages, 1 figur

Probing Yukawa Unification with K and B Mixing

We consider corrections to the unification of down-quark and charged-lepton Yukawa couplings in supersymmetric GUTs, which links the large nu_tau-nu_mu mixing angle to b -> s transitions. These corrections generically occur in simple grand-unified models with small Higgs representations and affect s -> d and b -> d transitions via the mixing of the corresponding right-handed superpartners. On the basis of a specific SUSY-SO(10) model, we analyze the constraints from K-Kbar and B-Bbar mixing on the additional \tilde{d}_R-\tilde{s}_R rotation angle theta. We find that epsilon_K already sets a stringent bound on theta, theta^{max}=O(1 degree), indicating a very specific flavor structure of the correction operators. The impact of the large neutrino mixings on the unitarity triangle analysis is also briefly discussed, as well as their ability to account for the sizeable CP-violating phase observed recently in B_s -> psi phi decays.Comment: 19 pages. Discussion in Sec. 5.2 slightly extended; minor numerical modifications in Secs. 5.1 to 5.4, conclusions unchanged. Version to appear in JHE

Mechanistic Understanding of the Heterogeneous, Rhodium-Cyclic (Alkyl)(Amino)Carbene-Catalyzed (Fluoro-)Arene Hydrogenation

Recently, chemoselective methods for the hydrogenation of fluorinated, silylated, and borylated arenes have been developed providing direct access to previously unattainable, valuable products. Herein, a comprehensive study on the employed rhodium-cyclic (alkyl)(amino)carbene (CAAC) catalyst precursor is disclosed. Mechanistic experiments, kinetic studies, and surface-spectroscopic methods revealed supported rhodium(0) nanoparticles (NP) as the active catalytic species. Further studies suggest that CAAC-derived modifiers play a key role in determining the chemoselectivity of the hydrogenation of fluorinated arenes, thus offering an avenue for further tuning of the catalytic properties. Copyright © 2020 American Chemical Society

Flavor Physics in an SO(10) Grand Unified Model

In supersymmetric grand-unified models, the lepton mixing matrix can possibly affect flavor-changing transitions in the quark sector. We present a detailed analysis of a model proposed by Chang, Masiero and Murayama, in which the near-maximal atmospheric neutrino mixing angle governs large new b -> s transitions. Relating the supersymmetric low-energy parameters to seven new parameters of this SO(10) GUT model, we perform a correlated study of several flavor-changing neutral current (FCNC) processes. We find the current bound on B(tau -> mu gamma) more constraining than B(B -> X_s gamma). The LEP limit on the lightest Higgs boson mass implies an important lower bound on tan beta, which in turn limits the size of the new FCNC transitions. Remarkably, the combined analysis does not rule out large effects in B_s-B_s-bar mixing and we can easily accomodate the large CP phase in the B_s-B_s-bar system which has recently been inferred from a global analysis of CDF and DO data. The model predicts a particle spectrum which is different from the popular Constrained Minimal Supersymmetric Standard Model (CMSSM). B(tau -> mu gamma) enforces heavy masses, typically above 1 TeV, for the sfermions of the degenerate first two generations. However, the ratio of the third-generation and first-generation sfermion masses is smaller than in the CMSSM and a (dominantly right-handed) stop with mass below 500 GeV is possible.Comment: 44 pages, 5 figures. Footnote and references added, minor changes, Fig. 2 corrected; journal versio