56 research outputs found
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
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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
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