Nobuhiro Maekawa

Neutrino Masses, Anomalous U(1) Gauge Symmetry and Doublet-Triplet Splitting

We propose an attractive scenario of grand unified theories in which doublet-triplet splitting is naturally realized in SO(10) unification using the Dimopoulos-Wilczek mechanism. The anomalous U (1)A gauge symmetry plays an essential role in the doublet-triplet splitting mechanism. It is interesting that the anomalous U (1)A charges determine the unification scale and mass spectrum of additional particles, as well as the order of the Yukawa couplings of quarks and leptons. For the neutrino sector, bi-maximal mixing angles are naturally obtained, and proton decay via dimension 5 operators is suppressed. It is suggestive that the anomalous U (1)A gauge symmetry motivated by superstring theory effectively solves the two biggest problems in grand unified theories, the fermion mass hierarchy problem and doublet-triplet splitting problem.

Improving the effective potential

Abstract A g ene ral procedure is presented how to improve the effective potential by using the renormalization group equation (RGE) in the MS scheme. If one knows the L-loop effective potential and the RGE coefficient functions up to the (L + 1)-loop level, this procedure gives an improved potential which satisfies the RGE and contains all of the leading, nest-to-leading, …, and Lth-to-leading log terms.

«Spontaneous» CP violation in the minimal supersymmetric standard model

Abstract We show in the minimal supersymmetric standard model that spontaneous CP violation can occur. The point is that the soft SUSY breaking terms can induce finite breaking effects even in dimension-four operators at the loop level.

E6 Unification, Doublet-Triplet Splitting and Anomalous U(1)A Symmetry

We propose a natural Higgs sector in E 6 grand unified theory (GUT) with anomalous U(1) A gauge symmetry. In this scenario, the doublet-triplet splitting can be realized, while proton decay via dimension 5 operators is suppressed. Gauge coupling unification is also realized without fine-tuning. The GUT scale obtained in this scenario is generally lower than the usual GUT scale, 2 x 10 1 6 GeV, and therefore it should be possible to observe proton decay via dimension 6 operators in near future experiments. The lifetime of a nucleon in this model is roughly estimated as T p (p → e + π 0 ) ∼ 3 x 10 3 3 years. It is shown that the Higgs sector is compatible with the matter sector proposed by one of the present authors, which reproduces realistic quark and lepton mass matrices, including a bi-large neutrino mixing angle. Combining the Higgs sector and the matter sector, we can obtain a completely consistent E 6 GUT. The input parameters for this model are only eight integer anomalous U(1) A charges (+3 for singlet Higgs) for the Higgs sector and three (half) integer charges for the matter sector.

Improving the Effective Potential Multi-Mass-Scale Case

Previously proposed procedure for improving the effective potential by using renormalization group equation (RGE) is generalized so as to be applicable to any system containing several different mass scales. If one knows L-loop effective potential and (L + 1)-loop RGE coefficient functions, this procedure gives an improved potential which satisfies the RGE and contains all of the leading, next-to-leading,..., and L-th-to-leading log terms. Our procedure here also clarifies how naturally the so-called effective field theory can be incorporated in the RGE in MS scheme

E6 Unification with Bi-Large Neutrino Mixing

The scenario of SO(10) grand unified theory (GUT) proposed by one of the authors is extended to E 6 unification. This gives realistic quark and lepton mass matrices. In the neutrino sector, the model reproduces the large mixing angle (LMA) MSW solution as well as the large mixing angle for the atmospheric neutrino anomaly. In this model, the right-handed down quark and the left-handed lepton of the first and second generations belong to a single multiplet 27. This causes natural suppression of the flavour changing neutral current (FCNC).

Gauge Coupling Unification with Anomalous U(1)A Gauge Symmetry

Recently we proposed a natural scenario of grand unified theories with anomalous U(1)_A gauge symmetry, in which doublet-triplet splitting is realized in SO(10) unification using Dimopoulos-Wilczek mechanism and realistic quark and lepton mass matrices can be obtained in a simple way. The scenario has an additional remarkable feature that the symmetry breaking scale and the mass spectrum of super heavy particles are determined essentially by anomalous U(1)_A charges. Therefore once all the anomalous U(1)_A charges are fixed, the gauge coupling flows can be calculated. We examine several models in which the gauge coupling unification is realized. Examining the conditions for the coupling unification, we show that when all the fields except those of the minimal SUSY standard model become super-heavy, the unification scale generically becomes just below the usual GUT scale \Lambda_G\sim 2\times 10^{16} GeV and the cutoff scale becomes around \Lambda_G. Since the lower GUT scale leads to shorter life time of nucleon, the proton decay via dimension six operator p\to e^+\pi^0 can be seen in future experiment. On the other hand, the lower cutoff scale than the Planck scale may imply the existence of extra dimension in which only gravity modes can propagate.

Non-Abelian horizontal symmetry and anomalous U(1) symmetry for the supersymmetric flavor problem

Abstract It is shown that using non-Abelian horizontal gauge symmetry and anomalous U(1)A symmetry in grand unified theories (GUTs), realistic quark and lepton mass matrices including large neutrino mixings can be obtained, while the differences among the scalar fermion masses are sufficiently small for suppression of various flavor changing neutral current (FCNC) processes, especially in E6 GUT. Combining the Higgs sector, in which doublet–triplet splitting is realized, a complete E6×SU(3)H GUT, in which three generations are unified into a single multiplet, Ψ( 27 , 3 ) , is obtained.

E6 Unification, Large Neutrino Mixings and SUSY Flavor Problem

The results of this study indicate that in E 6 grand unified theories (GUTs), once a hierarchical structure of up-quark-type Yukawa couplings is given as a basic structure of flavor, lepton mixings that are larger than the quark mixings and down-quark-type (and charged-lepton-type) mass hierarchies that are milder than the up-quark-type mass hierarchy can generically be obtained under a few natural assumptions. The basic flavor structure is compatible with non-Abelian horizontal symmetry, which can solve the SUSY flavor problem. It is shown that in solving the SUSY flavor problem, the E 6 structure, which realizes bi-large neutrino mixings, also solves a problem that results from the large neutrino mixing angles.

LHC signature of supersymmetric models with non-universal sfermion masses

We study the LHC signature of the minimal supersymmetric standard model with non-universal sfermion masses. In the model, soft masses of gauginos and the 3rd generation of 10 of SU(5) are around the weak scale, while other sfermion soft mass is universal and around a few TeV. Such sfermion mass spectrum is motivated not only from flavor, CP and naturalness constraints but also from E6 grand unified model with non-Abelian horizontal (flavor) symmetry. The characteristic signature of the model at the LHC is the dominance of the events with 4 b partons in the final state together with high rate of mildly boosted top quark arising from gluino decay. The prominent high pT jet also arises from squark decay. We show it is possible to find the characteristic signature in the early stage of the LHC. The discrimination of our scenario from some CMSSM model points with similar signature may be possible with large integrated luminosity. The result of sparticle mass measurement using exclusive channel with the help of hemisphere analysis, and inclusive measurement of gluino and squark masses using MT2 and MT2min in some representative model points are presented.