Ryuichiro Kitano - Tohoku University

Ryuichiro Kitano
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Ryuichiro Kitano
Tohoku University

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High Energy Physics - Phenomenology (49)
High Energy Physics - Theory (17)
High Energy Physics - Experiment (4)
Cosmology and Nongalactic Astrophysics (4)
High Energy Physics - Lattice (3)

Publications Authored By Ryuichiro Kitano

We discuss the properties of the topological soliton, or the Electroweak-Skyrmion, in the system of the Standard Model Higgs Lagrangian with addition of general O(p^4) terms. We show that the upper bound on the mass of the Electroweak-Skyrmion is about 10 TeV, which is obtained from currently available experimental constraints on coefficients of O(p^4) terms. The impact on the properties of the Electroweak-Skyrmion due to further modification of the Lagrangian is also discussed, and comments on possible mechanisms for the generation of the Electroweak-Skyrmion in the early Universe as a dark matter are given. Read More

A massless up quark has long been proposed as a solution to the strong CP problem. While this solution is sometimes thought to have been excluded, it is actually still ill-defined. In this work, we study the mass dependence of the physical observable $\chi_t$, the topological susceptibility. Read More

The baryon asymmetry of the Universe should have been produced after the inflation era. We consider the possibility that the asymmetry is generated by the flavor oscillations in the reheating process after inflation, so that the baryon asymmetry is realized already at the beginning of the radiation dominated era. In the seesaw model, we show that the propagators of the left-handed leptons generically have flavor mixings in the thermal background, that can generate flavor-dependent lepton asymmetry through the $CP$ violation in the oscillation phenomena. Read More

It is often argued that the minimal supersymmetric standard model has O(100) free parameters and the generic parameter region is already excluded by the null observation of the flavor and CP violating processes as well as the constraints from the LHC experiments. This situation naturally leads us to consider the case where all the dangerous soft supersymmetry breaking terms such as the scalar masses and scalar couplings are absent, while only the unified gaugino mass term and the mu term are non-vanishing at the grand unification scale. We revisit this simple situation taking into account the observed Higgs boson mass, 125 GeV. Read More

We show the existence of a nontrivial topological configuration of the Higgs field in the Standard Model with the Skyrme term. It is shown that the current upper bound of the mass of the topological object is about 34 TeV. We discuss the impact of the existence of the topological object on cosmology. Read More

The current LHC results make weak scale supersymmetry difficult due to relatively heavy mass of the discovered Higgs boson and the null results of new particle searches. Geometrical supersymmetry breaking from extra dimensions, Scherk-Schwarz mechanism, is possible to accommodate such situations. A concrete example, the Compact Supersymmetry model, has a compressed spectrum ameliorating the LHC bounds and large mixing in the top and scalar top quark sector with $|A_t|\sim 2m_{\tilde{t}}$ which radiatively raises the Higgs mass. Read More

The Standard Model predicts that the Higgs boson couples to the fermions in the mass eigenstates. We consider the effects of lepton flavor violating (LFV) $Z$ boson couplings in the case where the Higgs boson has flavor non-diagonal Yukawa interactions with the muon and the tau lepton generated from physics beyond the Standard Model. We list the formulae of the couplings of the effective interactions among the $\tau$ lepton, the muon and the $Z$ boson. Read More

The temperature dependence of the topological susceptibility in QCD, chi_t, essentially determines the abundance of the QCD axion in the Universe, and is commonly estimated, based on the instanton picture, to be a certain negative power of temperature. While lattice QCD should be able to check this behavior in principle, the temperature range where lattice QCD works is rather limited in practice, because the topological charge is apt to freezes at high temperatures. In this work, two exploratory studies are presented. Read More

Phenomenological aspects of simple dark matter models are studied. We discuss ways to discriminate the dark matter models in future experiments. We find that the measurements of the branching fraction of the Higgs boson into two photons and the electric dipole moment of the electron as well as the direct detection experiments are quite useful in discriminating particle models of dark matter. Read More

In a class of theories where the Higgs field emerges as a pseudo Nambu-Goldstone boson, it is often assumed that interactions to generate the top Yukawa coupling provide the Higgs potential as well. Such a scenario generically requires a little cancellation in the leading contribution to the Higgs potential, and the electroweak scale is generated by the balance between the leading and the subleading contributions. We, instead, consider the possibility that the contribution from the dark matter particle balances against that from the top quark. Read More

The Standard Model of particle physics fails to explain the important pieces in the standard cosmology, such as inflation, baryogenesis, and dark matter of the Universe. We consider the possibility that the sector to generate small neutrino masses is responsible for all of them; the inflation is driven by the Higgs field to break $B-L$ gauge symmetry which provides the Majorana masses to the right-handed neutrinos, and the reheating process by the decay of the $B-L$ Higgs boson supplies the second lightest right-handed neutrinos whose CP violating decays produce $B-L$ asymmetry, a la, leptogenesis. The lightest right-handed neutrinos are also produced by the reheating process, and remain today as the dark matter of the Universe. Read More

We present a new scenario of gravitino dark matter which is compatible with the thermal leptogenesis. We confirm by an explicit calculation in supergravity that the relic abundance of thermally produced gravitino becomes insensitive to the reheating temperature once the temperature of the Universe exceeds the mass scale of the messenger fields. In such a situation, the correct baryon to dark matter ratio can be obtained by thermal leptogenesis when the reheating temperature after inflation is high enough. Read More

The color-flavor locking phenomenon in the magnetic picture can be the microscopic description of the quark confinement in QCD. We demonstrate it in an N=2 supersymmetric SU(Nc)xSU(Nc) quiver gauge theory coupled to Nf flavors of quarks (NfRead More

We propose a model to describe the low energy physics of partially composite Standard Model, in which the electroweak sector in the Standard Model is weakly coupled to some strong dynamics. The vector resonances in the strong sector are introduced as the effective degrees of freedom, W' and Z', which mixes with the W and Z bosons through the electroweak symmetry breaking. Through the coupling to the strong sector, the Standard Model Higgs boson becomes partially composite, and its properties are modified. Read More

Both the ATLAS and CMS experiments at the LHC have reported the observation of the particle of mass around 125GeV which is consistent to the Standard Model (SM) Higgs boson, but with an excess of events beyond the SM expectation in the diphoton decay channel at each of them. There still remains room for a logical possibility that we are not seeing the SM Higgs but something else. Here we introduce the minimal dilaton model in which the LHC signals are explained by an extra singlet scalar of the mass around 125GeV that slightly mixes with the SM Higgs heavier than 600GeV. Read More

In light of the discovery of the new particle at 125GeV and the strong lower limits on the masses of superparticles from LHC, we discuss a possible picture of weak scale supersymmetry. Read More

The Higgs mechanism well describes the electroweak symmetry breaking in nature. We consider a possibility that the microscopic origin of the Higgs field is UV physics of QCD. We construct a UV complete model of a higher dimensional Yang-Mills theory as a deformation of a deconstructed (2,0) theory in six dimensions, and couple the top and bottom (s)quarks to it. Read More

We construct a minimal calculable model of a light dilaton based on the scenario where only top and Higgs sectors are involved in a quasiconformal dynamics. The model consistently accommodates the electroweak precision tests even when the Higgs boson is very heavy, thereby allowing one to consider the possibility that the particle at around 125GeV, discovered at the LHC experiments, is identified as the light dilaton rather than the Higgs boson. We find that the current LHC data allow distinct parameter regions where the observed particle is either mostly the Higgs boson or the dilaton. Read More

Gauge mediation provides us with a complete picture of supersymmetry breaking and its mediation within the effective field theories, and thus allows us to discuss consistencies with low-energy particle physics as well as cosmological observations. We study in detail the cosmological evolution of the pseudo-modulus field in the supersymmetry breaking sector and also the production of the gravitinos in the early Universe in a simple (but a complete) model of gauge mediation. Under fairly reasonable assumptions, it is found that there exists a parameter region where dark matter of the Universe is explained by both thermally and non-thermally produced gravitinos, while the baryon asymmetry of the Universe is generated through the thermal leptogenesis. Read More

We propose a framework for natural breaking of electroweak symmetry in supersymmetric models, where elementary Higgs fields are semi-perturbatively coupled to a strong superconformal sector. The Higgs VEVs break conformal symmetry in the strong sector at the TeV scale, and the strong sector in turn gives important contributions to the Higgs potential, giving rise to a kind of Higgs bootstrap. A Higgs with mass $125\GeV$ can be accommodated without any fine tuning. Read More

We try to identify the light hadron world as the magnetic picture of QCD. We take both phenomenological and theoretical approaches to this hypothesis, and find that the interpretation seems to show interesting consistencies. In particular, one can identify the rho and omega mesons as the magnetic gauge bosons, and the Higgs mechanism for them provides a dual picture of the color confinement. Read More

The light mesons such as pi, rho, omega, f0, and a0 are possible candidates of magnetic degrees of freedom, if a magnetic dual picture of QCD exists. We construct a linear sigma model to describe spontaneous breaking of the magnetic gauge group, in which there is a stable vortex configuration of vector and scalar mesons. We numerically examine whether such a string can be interpreted as the confining string. Read More

The technique of Weinberg's spectral-function sum rule is a powerful tool for a study of models in which global symmetry is dynamically broken. It enables us to convert information on the short-distance behavior of a theory to relations among physical quantities which appear in the low-energy picture of the theory. We apply such technique to general supersymmetry breaking models to derive new sum rules. Read More

The hidden local symmetry is a successful model to describe the properties of the vector mesons in QCD. We point out that if we identify this hidden gauge theory as the magnetic picture of QCD, a linearized version of the model simultaneously describes color confinement and chiral symmetry breaking. We demonstrate that such a structure can be seen in the Seiberg dual picture of a softly broken supersymmetric QCD. Read More

Authors: Daniele Alves1, Nima Arkani-Hamed2, Sanjay Arora3, Yang Bai4, Matthew Baumgart5, Joshua Berger6, Matthew Buckley7, Bart Butler8, Spencer Chang9, Hsin-Chia Cheng10, Clifford Cheung11, R. Sekhar Chivukula12, Won Sang Cho13, Randy Cotta14, Mariarosaria D'Alfonso15, Sonia El Hedri16, Rouven Essig17, Jared A. Evans18, Liam Fitzpatrick19, Patrick Fox20, Roberto Franceschini21, Ayres Freitas22, James S. Gainer23, Yuri Gershtein24, Richard Gray25, Thomas Gregoire26, Ben Gripaios27, Jack Gunion28, Tao Han29, Andy Haas30, Per Hansson31, JoAnne Hewett32, Dmitry Hits33, Jay Hubisz34, Eder Izaguirre35, Jared Kaplan36, Emanuel Katz37, Can Kilic38, Hyung-Do Kim39, Ryuichiro Kitano40, Sue Ann Koay41, Pyungwon Ko42, David Krohn43, Eric Kuflik44, Ian Lewis45, Mariangela Lisanti46, Tao Liu47, Zhen Liu48, Ran Lu49, Markus Luty50, Patrick Meade51, David Morrissey52, Stephen Mrenna53, Mihoko Nojiri54, Takemichi Okui55, Sanjay Padhi56, Michele Papucci57, Michael Park58, Myeonghun Park59, Maxim Perelstein60, Michael Peskin61, Daniel Phalen62, Keith Rehermann63, Vikram Rentala64, Tuhin Roy65, Joshua T. Ruderman66, Veronica Sanz67, Martin Schmaltz68, Stephen Schnetzer69, Philip Schuster70, Pedro Schwaller71, Matthew D. Schwartz72, Ariel Schwartzman73, Jing Shao74, Jessie Shelton75, David Shih76, Jing Shu77, Daniel Silverstein78, Elizabeth Simmons79, Sunil Somalwar80, Michael Spannowsky81, Christian Spethmann82, Matthew Strassler83, Shufang Su84, Tim Tait85, Brooks Thomas86, Scott Thomas87, Natalia Toro88, Tomer Volansky89, Jay Wacker90, Wolfgang Waltenberger, Itay Yavin, Felix Yu, Yue Zhao, Kathryn Zurek
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This document proposes a collection of simplified models relevant to the design of new-physics searches at the LHC and the characterization of their results. Both ATLAS and CMS have already presented some results in terms of simplified models, and we encourage them to continue and expand this effort, which supplements both signature-based results and benchmark model interpretations. A simplified model is defined by an effective Lagrangian describing the interactions of a small number of new particles. Read More

We study effective theories of an axion in spontaneously broken supersymmetric theories. We consider a system where the axion supermultiplet is directly coupled to a supersymmetry breaking sector whereas the standard model sector is communicated with those sectors through loops of messenger fields. The gaugino masses and the axion-gluon coupling necessary for solving the strong CP problem are both obtained by the same effective interaction. Read More

We consider a supersymmetric QCD with soft supersymmetry breaking terms as the dynamics for the electroweak symmetry breaking. We find various advantages compared to the non-supersymmetric models, such as a natural incorporation of the dynamical top-quark mass generation (the topcolor mechanism), the existence of a boson-pair condensation (the composite Higgs fields) and a large anomalous dimension of the composite operator to cure the flavor-changing-neutral-current and the S-parameter crises of the technicolor theories. The knowledge of the weakly coupled description (the Seiberg duality) enables us to perform perturbative computations in strongly coupled theories. Read More

If the minimal supersymmetric standard model is the solution to the hierarchy problem, the scalar top quark (stop) and the Higgsino should weigh around the electroweak scale such as 200 GeV. A low messenger scale, which results in a light gravitino, is also suggested to suppress the quantum corrections to the Higgs mass parameters. Therefore the minimal model for natural supersymmetry is a system with stop/Higgsino/gravitino whereas other superparticles are heavy. Read More

Supersymmetry (SUSY) with a long-lived stau is an attractive scenario in the LHC experiments because one can directly observe stau tracks in each SUSY event, and thus precise measurements of SUSY particle masses are possible. In this scenario, we discuss the possibility to observe/measure parity violation in interactions among SUSY particles. Such a measurement will be important in determining spins and chiralities of SUSY particles. Read More

We review recent works on supersymmetry breaking and gauge mediation. We survey our current understanding of dynamical supersymmetry breaking mechanisms and describe new model building tools using duality, meta-stability, and stringy construction. We discuss phenomenological constraints and their solutions, paying attentions to issues with gaugino masses and electroweak symmetry breaking. Read More

We update the constraints on the minimal model of dark matter, where a stable real scalar field is added to the standard model Lagrangian with a renormalizable coupling to the Higgs field. Once we fix the dark matter abundance, there are only two relevant model parameters, the mass of the scalar field and that of the Higgs boson. The recent data from the CDMS II experiment have excluded a parameter region where the scalar field is light such as less than about 50 GeV. Read More

In supersymmetric scenarios with a long-lived stau, the LHC experiments provide us with a great environment for precise mass measurements of superparticles. We study a case in which the mass differences between the lightest stau and other sleptons are about 10 GeV or larger, so that the decay products of heavier sleptons are hard enough to be detected. We demonstrate that the masses of neutralinos, sleptons, and squarks can be measured with a good accuracy. Read More

We show that gravitinos produced by decays of a supersymmetry breaking scalar field (the pseudo-moduli field) can naturally explain the observed abundance of dark matter in a certain class of the gauge mediation models. We study the decay processes as well as cosmological constraints on this scenario in detail, particularly focusing on different behavior of the real and imaginary components of the pseudo-moduli field. Cosmologically viable scenario emerges when the gravitino and the pseudo-moduli masses are O(10-100) MeV and O(100) GeV, respectively. Read More

We present a simple formulation of non-linear supersymmetry where superfields and partnerless fields can coexist. Using this formalism, we propose a supersymmetric Standard Model without the Higgsino as an effective model for the TeV-scale supersymmetry breaking scenario. We also consider an application of the Hidden Local Symmetry in non-linear supersymmetry, where we can naturally incorporate a spin-two resonance into the theory in a manifestly supersymmetric way. Read More

We assess the model discriminating power of a combined phenomenological analysis of mu -> e gamma and mu -> e conversion on different target nuclei, including the current hadronic uncertainties. We find that the theoretical uncertainties can be largely reduced by using input from lattice QCD and do not constitute a limiting factor in discriminating models where one or at most two underlying operators (dipole, scalar, vector) provide the dominant source of lepton flavor violation. Our results show that a realistic discrimination among underlying mechanisms requires a measurement of the ratio of conversion rates at the 5% level (two light nuclei) or at the 20% level (one light and one heavy nucleus). Read More

We present a general scheme for finding or creating a metastable vacuum in supersymmetric theories. By using the formalism, we show that there is a parameter region where a metastable vacuum exists in the Wess-Zumino model coupled to messenger fields. This model serves as a perturbative renormalizable model of direct gauge mediation. Read More

We investigate the possibility that both the baryon asymmetry of the universe and the observed cold dark matter density are generated by decays of a heavy scalar field which dominates the universe before nucleosynthesis. Since baryons and cold dark matter have common origin, this mechanism yields a natural explanation of the similarity of the corresponding energy densities. The cosmological moduli and gravitino problems are avoided. Read More

The differential cross section of the chargino-neutralino production, q qbar --> chi+- chi0, followed by their decays into scalar tau leptons, chi+- chi0 --> (stau+- nu)(stau-+ tau+-) --> (stau+- nu)(stau-+ l+- nu nubar), is calculated including the effect of spin correlations. In the case where stau is long-lived, this final state can be fully reconstructed in a hadron-collider experiment up to a discrete two-fold ambiguity. Distributions of various kinematic variables can thus be observable and tell us about masses and spins of superparticles and also parity/CP violation in interactions by comparing with the cross-section formula. Read More

In supersymmetric scenarios where the scalar tau lepton is stable or long-lived, a search for a decay mode chi0 --> stau + mu at the LHC has a good sensitivity to the flavor mixing in the scalar lepton sector. We demonstrate that the sensitivities to the mixing angle at the level of sin(theta)=0.15 are possible with an integrated luminosity of 100fb^{-1} if the total production cross section of supersymmetric particles is of the order of 1pb. Read More

It has been more than twenty years since theorists started discussing supersymmetric model building/phenomenology. We review mechanisms of supersymmetry breaking/mediation and problems in each scenario. We propose a simple model to address those problems and discuss its phenomenology. Read More

We propose a simple model of gauge mediation where supersymmetry is broken by a strong dynamics at O(100)TeV. Read More

Sweet spot supersymmetry is a phenomenologically and cosmologically perfect framework to realize a supersymmetric world at short distance. We discuss a class of dynamical models of supersymmetry breaking and its mediation whose low-energy effective description falls into this framework. Hadron fields in the dynamical models play a role of the messengers of the supersymmetry breaking. Read More

We find that there is no supersymmetric flavor/CP problem, mu-problem, cosmological moduli/gravitino problem or dimension four/five proton decay problem in a class of supersymmetric theories with O(1) GeV gravitino mass. The cosmic abundance of the non-thermally produced gravitinos naturally explains the dark matter component of the universe. A mild hierarchy between the mass scale of supersymmetric particles and electroweak scale is predicted, consistent with the null result of a search for the Higgs boson at the LEP-II experiments. Read More

The supersymmetric SU(Nc) Yang-Mills theory coupled to Nf matter fields in the fundamental representation has meta-stable vacua with broken supersymmetry when Nc < Nf < 3/2 Nc. By gauging the flavor symmetry, this model can be coupled directly to the standard model. We show that it is possible to make a slight deformation to the model so that gaugino masses are generated and the Landau pole problem can be avoided. Read More

Gravitinos and hidden sector fields often cause a cosmological disaster in supersymmetric models. We find that a model with gravitational gauge mediation solves such a problem quite naturally. The mu-problem is also absent in the model. Read More

It is often the case that the naive introduction of the messenger sector to supersymmetry breaking models causes restoration of supersymmetry. We discuss a possibility of stabilizing the supersymmetry breaking vacuum by gravitational interaction. Read More

Weak scale supersymmetric theories often suffer from several naturalness problems: the problems of reproducing the correct scale for electroweak symmetry breaking, the correct abundance for dark matter, and small rates for flavor violating processes. We argue that the first two problems point to particular regions of parameter space in models with weak scale supersymmetry: those with a small mu term. This has an interesting implication on direct dark matter detection experiments. Read More

Models for dynamical breaking of supersymmetric grand unified theories are presented. The doublet-triplet splitting problem is absent since the Higgs doublet superfields can be identified with the massless mesons of the strong gauge group whereas there are no massless states corresponding to the colored Higgs fields. Various strong gauge groups SU(Nc), Sp(Nc) and SO(Nc) are examined. Read More

One proposed solution of the moduli problem of string cosmology requires that the moduli are quite heavy, their decays reheating the universe to temperatures above the scale of nucleosynthesis. In many of these scenarios, the moduli are approximately supersymmetric; it is then crucial that the decays to gravitinos are helicity suppressed. In this paper, we discuss situations where these decays are, and are not, suppressed. Read More

Weak scale supersymmetry is often said to be fine-tuned, especially if the matter content is minimal. This is not true if there is a large A term for the top squarks. We present a systematic study on fine-tuning in minimal supersymmetric theories and identify low energy spectra that do not lead to severe fine-tuning. Read More