arXiv subjects

Nobuhiro Maekawa

Publications and source records attributed to Nobuhiro Maekawa.

At least 37 records · Page 2Linked to original sources

arXiv2012

Averaged Number of the Lightest Supersymmetric Particles in Decay of Superheavy Particle with Long Lifetime

We calculate the averaged number νof the lightest supersymmetric particles (LSPs) in a shower from the decay of superheavy particle X by generalized DGLAP equations. If the primary decayed particles have color charges and the virtuality is around 10^13-10^14 GeV, the averaged number of the LSPs can become O(100). As the result, the upper limit of the mass of the superheavy particle, whose decay can produce the observed abundance of the dark matter, can increase from 10^12 GeV to 10^14 GeV. Since the typical scale of the inflaton mass of the chaotic inflation is around 10^13 GeV, the decay of the inflaton can produce the observed dark matter abundance if the reheating temperature is of order 1 GeV. Even for the standard model particles with virtuality Q\sim 10-100 TeV, the averaged number of the LSPs becomes O(0.1) for gluon, and O(0.01) for Higgs, which strongly constrains the scenario of non-thermal LSP production from the decay of moduli with 10-100 TeV mass.

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arXiv2011

Baryogenesis by B - L generation due to superheavy particle decay

We have shown that the $B-L$ generation due to the decay of the thermally produced superheavy fields can explain the Baryon assymmetry in the universe if the superheavy fields are heavier than $10^{13-14}$ GeV. Note that although the superheavy fields have non-vanishing charges under the standard model gauge interactions, the thermally prduced baryon asymmetry is sizable. The $B-L$ violating effective operators induced by integrating the superheavy fields have dimension 7, while the operator in the famous leptogenesis has dimension 5. Therefore, the constraints from the nucleon stability can be easily satisfied.

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arXiv2011

E6 grand unified theory with three generations from heterotic string

We construct two more supersymmetric E6 grand unified models with three generations within the framework of Z12 asymmetric orbifold compactification of the heterotic string theory. Such an asymmetric orbifold is missing in the classification in the literature, which concludes that only one E6 model is possible. In both of the new models, an adjoint Higgs field is obtained in virtue of the diagonal embedding method. This method mods out the three E6 factors of an even self-dual momentum-lattice by a permutation symmetry. In order to realize the E6^3 even self-dual lattice, we utilize the lattice engineering technique. Among the eight possible orbifold actions in our setup, two lead to new E6 models. Though these models still share the unsatisfactory issues with the known one, our discovery raises hopes that excellent models that solve all the problems in the supersymmetric grand unified models will be found in this framework.

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arXiv2010

Flavor structure of E6 GUT models

It has been pointed out that in E6 grand unified theory with SU(2) family symmetry, the spontaneous CP violation can solve the supersymmetric CP problem. The scenario predicts V_{ub} \sim O(λ^4) rather than O(λ^3) which is naively expected value, because of a cancellation at the leading order. Since the experimental value of V_{ub} is O(λ^4), it must be important to consider the reason and the conditions for the cancellation. In this paper, we give a simple reason for the cancellation and show that in some E6 models such a cancellation requires that the vacuum expectation value (VEV) of the adjoint Higgs does not break U(1)_{B-L}. Note that this direction of the VEV plays an important role in solving the doublet-triplet splitting problem by Dimopoulos-Wilczek mechanism. In this E6 models, the experiments may measure the direction of the adjoint Higgs VEV by measuring the size of V_{ub} \sim O(λ^4).

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arXiv2010

Gauge-mediated supersymmetry breaking with generalized messenger sector at LHC

We consider the generalized gauge mediated supersymmetry breaking (GMSB) models with the messenger fields which do not form the complete multiplets of SU(5) GUT symmetry. Such a situation may happen in the anomalous U(1) GUT scenario because the mass spectrum of the superheavy particle does not respect SU(5) GUT symmetry, although the success of the gauge coupling unification can be explained. In this paper, we assume that one pair of the messenger fields gives the dominant contribution, and the LHC signature for the two possible messengers, X + \bar{X} and Q + \bar{Q}, are examined. We investigate the possibility to measure the deviation from the usual GUT relation of the gaugino masses which is one of the most important features of these scenarios.

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arXiv2009

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 E_6 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 p_T 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 M_{T2} and M_{T2}^{min} in some representative model points are presented.

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arXiv2004

Horizontal symmetry in Higgs sector of GUT with U(1)_A symmetry

In a series of papers, we pointed out that an anomalous $U(1)_A$ gauge symmetry naturally solves various problems in grand unified theories (GUTs) and that a horizontal gauge symmetry, $SU(2)_H$ or $SU(3)_H$, not only realizes the unification of three generation quarks and leptons in fewer multiplets but also solves the supersymmetric flavor problem. In this paper, we examine the possibility that the Higgs sectors of the GUT symmetry and of the horizontal symmetry are unified, that is, there are some Higgs fields whose vacuum expectation values (VEVs) break both the GUT gauge symmetry and the horizontal symmetry at the same time. Although the scale of the VEVs become too large to suppress the flavor changing neutral current processes sufficiently, the unification is possible. In addition, for the $SU(3)_H$ models, the $SU(3)_H$ gauge anomaly is cancelled in the unified models without introducing additional fields in contrast with the previous models in which the Higgs sectors are not unified.

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arXiv2004

E6 Unification, Large Neutrino Mixings, and SUSY Flavor Problem

This study indicates that in E6 grand unified theories (GUTs), once a hierarchical structure of up-quark-type Yukawa couplings is given as a basic structure of flavor, larger lepton mixings than the quark mixings and milder down-quark-type (and charged-lepton-type) mass hierarchies than 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 E6 structure, which realizes bi-large neutrino mixings, also solves a problem that results from the large neutrino mixing angles.

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arXiv2003

GUT with Anomalous U(1) Suggests Heterotic M-Theory?

We show that a new GUT scenario we proposed indicates that heterotic M-theory is one of the most interesting possibility to describe our world because of the two features in the scenario. The first feature is that E_6 unified group plays an essential role not only in understanding larger mixing angles in lepton sector than in quark sector but also in solving the SUSY flavor Problem by introducing non-abelian horizontal symmetry SU(2)_H or SU(3)_H. The second feature is that the cutoff scale must be taken as the usual GUT scale Λ_G\sim 2\times 10^{16} GeV to realize natural gauge coupling unification. Because this cutoff scale is smaller than the Planck scale or string scale, it may suggest the existence of extra dimension in which only gravity modes can propagate. This talk is based on a dozen papars, some of which are collaborated with T. Yamashita, M. Bando and Q. Shafi.

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arXiv2003

Sliding Singlet Mechanism Revisited

We show that the unification of the doublet Higgs in the standard model (SM) and the Higgs to break the grand unified theory (GUT) group stabilizes the sliding singlet mechanism which can solve the doublet-triplet (DT) splitting problem. And we generalize this attractive mechanism to apply it to many unified scenarios. In this paper, we try to build various concrete E_6 unified models by using the generalized sliding singlet mechanism.

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arXiv2003

E_6 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 the scenario, the doublet-triplet splitting can be realized while the proton decay via dimension 5 operator is suppressed. The gauge coupling unification is also realized without finetuning. The GUT scale is generally lower than the usual GUT scale 2\times 10^{16} GeV, the proton decay via dimension 6 operator can be seen in near future experiment. The lifetime of nucleon in the model is roughly estimated as τ_p(p\to e^+π^0)\sim 3\times 10^{33} years. It is shown that the Higgs sector is compatible with the matter sector proposed by one of the authors, which reproduces the realistic quark and lepton mass matrices including bi-large neutrino mixing angle. Combining the Higgs sector and the matter sector, we can obtain the totally consistent E_6 GUT. The input parameters are only 8 integer anomalous U(1)_A charges (+3 for singlet Higgs) for the Higgs sector and 3 (half) integer charges for the matter sector.

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arXiv2003

Flipped SO(10) model

We show that as in the flipped SU(5) models, doublet-triplet splitting is realized by the missing partner mechanism in the flipped SO(10) models. The gauge group $SO(10)_F\times U(1)_{V'_F}$ includes $SU(2)_E$ gauge symmetry, that plays an important role in solving supersymmetric flavor problem by introducing non-abelian horizontal gauge symmetry and anomalous $U(1)_A$ gauge symmetry. The gauge group can be broken into the standard model gauge group by VEVs of only spinor fields, such models may be easier than $E_6$ models to be derived from the superstring theory.

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arXiv2003

Grand Unification with Anomalous U(1) Symmetry and Non-abelian Horizontal Symmetry

Non-abelian horizontal symmetry has been considered to solve potentially SUSY flavor problem, but simple models are suffering from various problems. In this talk, we point out that (anomalous) U(1)_A gauge symmetry solves all the problems in a natural way, especially, in the E_6 grand unified theories. Combining the GUT scenario with anomalous U(1)_A gauge symmetry, in which doublet-triplet splitting and natural gauge coupling unification are realized, and realistic quark and lepton mass matrices are obtained including bi-large neutrino mixings, complete E_6* SU(3)_H (or E_6*SU(2)_H) GUTs can be obtained, in which all the three generation quarks and leptons are unified into a single multiplet (27,3) (or two multiplets (27,2+1)). This talk is based on Ref.[1].

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arXiv2003

Simple $E_6$ Unification with Anomalous $U(1)_A$ Symmetry

We propose simpler Higgs sectors in $E_6$ grand unified theory with anomalous $U(1)_A$ gauge symmetry than the previous model in Ref.1). As in the previous model, the doublet-triplet splitting can be realized in a natural way, while proton decay via dimension 5 operators is suppressed and gauge coupling unification is also realized without fine-tuning. Combining the matter sector, simple complete GUTs can be obtained. Since the Higgs sector is simpler, the gauge coupling constant at the cutoff scale can be in perturbative region, and therefore, the estimated value of the lifetime of a nucleon in this model, $τ_p(p\to e^+π^0)\sim 5\times 10^{33}$ years, becomes more reliable.

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arXiv2003

Gauge Coupling Unification in GUT with Anomalous U(1) Symmetry

We show that in the framework of grand unified theory (GUT) with anomalous $U(1)_A$ gauge symmetry, the success of the gauge coupling unification in the minimal SU(5) GUT is naturally explained, even if the mass spectrum of superheavy fields does not respect SU(5) symmetry. Because the unification scale for most realizations of the theory becomes smaller than the usual GUT scale, it suggests that the present level of experiments is close to that sufficient to observe proton decay via dimension 6 operators, $p\to e+π$.

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arXiv2002

Non-Abelian Horizontal Symmetry and Anomalous U(1) Symmetry for the Supersymmetric Flavor Problem

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 E_6 GUT. Combining the Higgs sector, in which doublet-triplet splitting is realized, a complete E_6\times SU(3)_H GUT, in which three generations are unified into a single multiplet, Psi(27,3), is obtained.

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arXiv2002

Grand Unification with Anomalous U(1) Symmetry

In this talk, we introduce a new scenario of grand unified theory (GUT) with anomalous $U(1)_A$ gauge symmetry. Since generic interactions (including non-renormalizable interactions) are introduced, once we fix the symmetry of the theory, we can define the theory without O(1) coefficients. The number of parameters to determine the symmetry is just 11 or 12 integer parameters which are anomalous $U(1)_A$ charges(integer) for introduced multiplets. It is surprising that the GUT scenario can explain doublet-triplet splitting, quark and lepton masses and mixing angles. In neutrino sector, the scenario realizes LMA solution for solar neutrino problem and large $U_{e3}=O(0.1)$. Moreover, the scenario predicts that the unification scale becomes smaller than the usual GUT scale, namely proton decay via dimension 6 operators may be seen in future experiments. This talk is based on the papers\cite{maekawa,maekawa2,maekawa3,BM,MY}.

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arXiv2002

Two-Loop Analysis of Gauge Coupling Unification with Anomalous U(1) Symmetry and Proton Decay

Recently, a new mechanism, which explains why the three gauge coupling constants meet at a certain scale in the minimal supersymmetric standard model, has been proposed in a scenario of grand unified theories with anomalous $U(1)_A$ gauge symmetry. It is a non-trivial result that although there are many superheavy fields whose mass scales are below the unification scale, this mechanism explains this fact by means of one-loop renormalization group equations. Since the unification scale generically becomes below the usual GUT scale, $2\times 10^{16}$ GeV, and proton decay via dimension 5 operators is suppressed, the scenario predicts that proton decay via dimension 6 operators, $p\to e π^0$, will be observed in the near future. In this paper, we attempt to estimate a reasonable range of values of the lifetime of the proton predicted within this scenario by using a two-loop renormalization group calculation and the ambiguities of \order1 coefficients.

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