arXiv subjects

Tsutomu T. Yanagida

Publications and source records attributed to Tsutomu T. Yanagida.

At least 271 records · Page 15Linked to original sources

arXiv2011

Hermitian Flavor Violation

The fundamental constraint on two Higgs doublet models comes from the requirement of sufficiently suppressing flavor-changing neutral currents. There are various standard approaches for dealing with this problem, but they all tend to share a common feature; all of the Higgs doublets couple very weakly to the first generation quarks. Here we consider a simple two Higgs doublet model which is able to have large couplings to the first generation, while also being safe from flavor constraints. We assume only that there is an SU(3)_f flavor symmetry which is respected by the couplings of one of the Higgs doublets, and which is broken by Hermitian Yukawa couplings of the second doublet. As a result of the large permitted couplings to the first generation quarks, this scenario may be used to address the excess in W+dijet events recently observed by CDF at the Tevatron. Moreover, Hermitian Yukawa coupling matrices arise naturally in a broad class of solutions to the strong CP problem, providing a compelling context for the model.

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arXiv2011

A Scalar Boson as a Messenger of New Physics

Quantum corrections generate a quadratically divergent mass term for the Higgs boson in the Standard Model. Thus, if the Higgs boson has a mass of order 100 GeV, it implies the presence of a cut-off of the theory around TeV scale, and some particles associated with the new physics may appear around the cut-off scale $Λ$. However, if $Λ$ is several TeV, it may be difficult to find such particles at the LHC. In this paper, we consider a situation in which the new physics provides relatively light particles compared with the scale $Λ$. In such a situation, we show that diphoton event and four lepton event by the decay of the Higgs and/or a new particle have naturally large cross section, and LHC may test the new physics in a considerably broad parameter region even if $Λ$ is several TeV.

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arXiv2011

Number-Theory Dark Matter

We propose that the stability of dark matter is ensured by a discrete subgroup of the U(1)B-L gauge symmetry, Z_2(B-L). We introduce a set of chiral fermions charged under the U(1)B-L in addition to the right-handed neutrinos, and require the anomaly-cancellation conditions associated with the U(1)B-L gauge symmetry. We find that the possible number of fermions and their charges are tightly constrained, and that non-trivial solutions appear when at least five additional chiral fermions are introduced. The Fermat theorem in the number theory plays an important role in this argument. Focusing on one of the solutions, we show that there is indeed a good candidate for dark matter, whose stability is guaranteed by Z_2(B-L).

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arXiv2011

Gravitino Dark Matter and Light Gluino in an R-invariant Low Scale Gauge Mediation

We consider the simplest class of the R-invariant gauge mediation model with the gravitino mass in the one to ten keV range. We show that the entropy production from the supersymmetry breaking sector makes the gravitino into a warm dark matter candidate. We also discuss that the gluino mass can be lighter than the wino mass even when the messenger sector satisfies the GUT relations at the GUT scale.

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arXiv2011

Constraint on the gravitino mass in hybrid inflation

We revisit the F-term hybrid inflation model in supergravity. In particular, we point out that a constant term in the superpotential has significant effects on the inflaton dynamics. It is shown that the hybrid inflation model suffers from several potential problems: tuning of the initial condition, gravitino overproduction and formation of cosmic strings, for both minimal and non-minimal Kahler potentials. These problems can only be avoided in gauge-mediated SUSY breaking models where the gravitino is relatively light and the constant term in the superpotential is not important. Implications on the non-thermal leptogenesis scenario are also described.

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arXiv2011

Higgs Messengers

We explore the consequences of the Higgs fields acting as messengers of supersymmetry breaking. The hidden-sector paradigm in the gauge mediation framework is relaxed by allowing two types of gauge-invariant, renormalizable operators that are typically discarded: direct coupling between the Higgses and supersymmetry breaking singlets, and Higgs-messenger mixing terms. The most important phenomenological consequence is a flavor-dependent shift in sfermion masses. This is from a one-loop contribution, which we compute for a general set of weak doublet messengers. We also study a couple of explicit models in detail, finding that precision electroweak constraints can be satisfied with a spectrum significantly different from that of gauge mediation.

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arXiv2011

Why have supersymmetric particles not been observed?

If low-energy supersymmetry is the solution to the hierarchy problem, it is a puzzle why supersymmetric particles have not been observed experimentally to date. We show that supersymmetric particles in the TeV region can be explained if the fundamental cut-off scale of the theory is smaller than the 4-dimensional Planck scale and if thermal leptogenesis is the source of the observed baryon asymmetry. The supersymmetric particles such as sfermions and gauginos are predicted to be in the TeV region, while the gravitino is the LSP with mass of O(100)GeV and is a good candidate for dark matter. Interestingly, the cosmological moduli problem can be solved in the theory with the low cut-off scale.

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arXiv2011

Strong dynamics at the Planck scale as a solution to the cosmological moduli problem

We argue that strong dynamics at the Planck scale can solve the cosmological moduli problem. We discuss its implications for inflation models, and find that a certain type of multi-field inflation model is required for this mechanism to work, since otherwise it would lead to the serious eta-problem. Combined with the inflaton-induced gravitino problem, we show that a chaotic inflation with a discrete symmetry naturally avoids both problems. Interestingly, the focus point supersymmetry is predicted when this mechanism is applied to the Polonyi model.

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arXiv2011

Testing the Nambu-Goldstone Hypothesis for Quarks and Leptons at the LHC

The hierarchy of the Yukawa couplings is an outstanding problem of the standard model. We present a class of models in which the first and second generation fermions are SUSY partners of pseudo-Nambu-Goldstone bosons that parameterize a non-compact Kahler manifold, explaining the small values of these fermion masses relative to those of the third generation. We also provide an example of such a model. We find that various regions of the parameter space in this scenario can give the correct dark matter abundance, and that nearly all of these regions evade other phenomenological constraints. We show that for gluino mass ~700 GeV, model points from these regions can be easily distinguished from other mSUGRA points at the LHC with only 7 fb^(-1) of integrated luminosity at 14 TeV. The most striking signatures are a dearth of b- and tau-jets, a great number of multi-lepton events, and either an "inverted" slepton mass hierarchy, narrowed slepton mass hierarchy, or characteristic small-mu spectrum.

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arXiv2010

Saving fourth generation and baryon number by living long

Recent studies of precision electroweak observables have led to the conclusion that a fourth generation is highly constrained. However, we point out that a long-lived fourth generation can reopen a large portion of the parameter space. In addition, it preserves baryon and lepton asymmetries against sphaleron erasure even if $B-L=0$. It opens up the possibility of exact $B-L$ symmetry and hence Dirac neutrinos. The fourth generation can be observed at the LHC with unique signatures of long-lived particles in the near future.

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arXiv2010

A theory of extra radiation in the Universe

Recent cosmological observations, such as the measurement of the primordial 4He abundance, CMB, and large scale structure, give preference to the existence of extra radiation component, Delta N_nu > 0. The extra radiation may be accounted for by particles which were in thermal equilibrium and decoupled before the big bang nucleosynthesis. Broadly speaking, there are two possibilities: 1) there are about 10 particles which have very weak couplings to the standard model particles and decoupled much before the QCD phase transition; 2) there is one or a few light particles with a reasonably strong coupling to the plasma and it decouples after the QCD phase transition. Focusing on the latter case, we find that a light chiral fermion is a suitable candidate, which evades astrophysical constraints. Interestingly, such a scenario may be confirmed at the LHC. As a concrete example, we show that such a light fermion naturally appears in the E_6-inspired GUT.

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arXiv2010

Cascade supersymmetry breaking and low-scale gauge mediation

We propose a new class of models with gauge mediated supersymmetry breaking, the cascade supersymmetry breaking. This class of models is consistent with the gravitino mass as low as O(1)eV without having suppressed gaugino masses, nor the Landau pole problems of the gauge coupling constants of the Standard Model below the scale of the grand unification. In particular, there is no supersymmetric vacuum in the vicinity of the supersymmetry breaking vacuum even for such a low gravitino mass. Thus, the model does not have a vacuum stability problem decaying into supersymmetric vacua.

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arXiv2010

Dark Matter from Split Seesaw

The seesaw mechanism in models with extra dimensions is shown to be generically consistent with a broad range of Majorana masses. The resulting democracy of scales implies that the seesaw mechanism can naturally explain the smallness of neutrino masses for an arbitrarily small right-handed neutrino mass. If the scales of the seesaw parameters are split, with two right-handed neutrinos at a high scale and one at a keV scale, one can explain the matter-antimatter asymmetry of the universe, as well as dark matter. The dark matter candidate, a sterile right-handed neutrino with mass of several keV, can account for the observed pulsar velocities and for the recent data from Chandra X-ray Observatory, which suggest the existence of a 5 keV sterile right-handed neutrino.

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arXiv2010

Split Generation in the SUSY Mass Spectrum and B_s-{\bar B}_s Mixing

We show that the like-sign di-muon anomaly reported recently by the D0 Collaboration can be explained in the supersymmetric standard model (SM) if the squarks and the sleptons in the first two generations have relatively small, but degenerate mass spectrum, and those in the third generation are larger as O(1-10)TeV. This split generation model provides large contributions to the B_s-{\bar B}_s mixing, although most of the FCNC's are suppressed due to the large masses of the third generation squarks or the GIM mechanism partially acting on the first and second generations.

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arXiv2010

A CP-safe solution of the mu/ Bmu problem of gauge mediation

We construct a model that naturally generates mu and B of the same order without producing large CP violating phases. This is easily accomplished once one permits these mass scales to be determined independently of the ordinary gauge-mediated soft masses. The alignment of phases is shown to emerge dynamically upon coupling to supergravity and is not unique to the model presented here.

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arXiv2010

TeV scale horizontal gauge symmetry and its implications in B-physics

We propose a gauged $U(1)_{H}$ horizontal symmetry around TeV scale that is a subgroup of a $SU(3)_{H}$ horizontal gauge symmetry broken at ${\cal O}(10^{14}) \GeV$. The breaking generates right-handed Majorana neutrino masses through a $SU(3)_H$ sextet scalar. A particular Majorana right-handed neutrino mass matrix explicitly determines the remnant $U(1)_{H}$ at low energy which only couples to $b-s$ and $μ-τ$ in the gauge eigenstate. The dangerous $K-\bar{K}$, $D-\bar{D}$ mixing and $B_s \rightarrow μ^+ μ^-$ are kept to be safe because the relevant couplings are suppressed through high powers of small mixing angles in the fermion rotation matrix. Our analysis which applies to the general case shows that the Tevatron di-muon anomaly can be explained through the $B_{s}$ and $B_{d}$ mixing while keeping all the other experimental constraints within 90 \% C. L. For the $B$ meson decay, the $B_{s}\to μ^{\pm}τ^{\mp}$ is the leading leptonic decay channel which is several orders of magnitude below current experimental bound.

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arXiv2010

Dynamical Matter-Parity Breaking and Gravitino Dark Matter

Scenarios where gravitinos with GeV masses make up dark matter are known to be in tension with high reheating temperatures, as required by e.g. thermal leptogenesis. This tension comes from the longevity of the NLSPs, which can destroy the successful predictions of the standard primordial nucleosynthesis. However, a small violation of matter parity can open new decay channels for the NLSP, avoiding the BBN problems, while being compatible with experimental cosmic-ray constraints. In this paper, we propose a model where matter parity, which we assume to be embedded in the U(1)_{B-L} gauge symmetry, is broken dynamically in a hidden sector at low scales. This can naturally explain the smallness of the matter parity breaking in the visible sector. We discuss the dynamics of the corresponding pseudo Nambu--Goldstone modes of B-L breaking in the hidden sector, and we comment on typical cosmic-ray and collider signatures in our model.

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arXiv2010

A Conformal Gauge Mediation and Dark Matter with Only One Parameter

If the supersymmetry (SUSY) is a solution to the hierarchy problem, it is puzzling that any SUSY particle has not been discovered yet. We show that there is a low-scale conformal gauge mediation model which contains all necessary ingredients, i.e. not only a SUSY-breaking dynamics and a gauge mediation mechanism, but also a candidate for the dark matter. The model has only one free parameter, that is, the mass for messengers. In this model, the dark matter is provided by a composite particle in the SUSY-breaking sector, and the observed value of the dark matter density uniquely fixes the mass of messengers at the order of 10^2 TeV. Then, the sfermion and gaugino masses are fixed to be of order 10^2~10^3 GEV without any arbitrariness, thus the SUSY particles are expected not to be discovered at the Tevatron or LEP, while having a discovery possibility at the LHC.

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