arXiv subjects

Koji Tsumura

Publications and source records attributed to Koji Tsumura.

At least 37 records · Page 2Linked to original sources

arXiv2019

A $μ$-$τ$-philic scalar doublet under $Z_n$ flavor symmetry

We propose a minimal model which accommodates the long-standing anomaly of muon magnetic moment based on abelian discrete flavor symmetries. The standard model is extended by scalar doublets charged under a $Z_n$ lepton flavor symmetry. In these models, a large contribution to the muon magnetic moment can be obtained by the chirality enhancement from new scalar mediated diagrams without conflicting with the flavor symmetry. Thanks to the lepton flavor symmetry, these models automatically forbid lepton flavor violation. The minimal model is based on $Z_4$ symmetry with only one extra scalar doublet. In this model, we show that the parameter space favored by the muon $g-2$ can easily be consistent with experimental constraints and theoretical bounds such as the electroweak precision tests, lepton universality, potential stability condition and triviality bound as well as the LHC direct search mass bound. The new contributions to the muon electric dipole moment and the Higgs decay into $γγ$ can be indirect signals of the model.

hep-ph↗

arXiv2019

Minimal Gauged U(1)$_{L_α- L_β}$ Models Driven into a Corner

It is well known that the differences between the lepton numbers can be gauged with the Standard Model matter content. Such extended gauge theories, dubbed as the gauged U(1)$_{L_α- L_β}$ models, have been widely discussed so far as potential candidates for physics beyond the Standard Model. In this work, we study the minimal versions of these gauge theories, where three right-handed neutrinos as well as a single U(1)$_{L_α- L_β}$ symmetry breaking Higgs field---an SU(2)$_L$ singlet or doublet---are introduced. In these minimal models, the neutrino mass terms are constrained by the gauge symmetry, which result in the two-zero texture or two-zero minor structure of neutrino mass matrices. Such restrictive forms of neutrino mass matrices lead to non-trivial predictions for the neutrino oscillation parameters as well as the size of the mass eigenvalues. We find that due to this restriction the minimal gauged U(1)$_{L_α- L_β}$ models are either incompatible with the observed values of the neutrino parameters or in strong tension with the Planck 2018 limit on the sum of the neutrino masses. Only the U(1)$_{L_μ-L_τ}$ model with an SU(2)$_L$ singlet U(1)$_{L_μ-L_τ}$-breaking field barely evades the limit, which can be tested in the future neutrino experiments.

hep-ph↗

arXiv2018

Charged scalars confronting neutrino mass and muon g-2 anomaly

The present work introduces two possible extensions of the Standard Model Higgs sector. In the first case, the Zee-Babu type model for the generation of neutrino mass is augmented with a scalar triplet and additional singly charged scalar singlets. The second scenario, on the other hand, generalizes the Type-II seesaw model by replicating the number of the scalar triplets. A $\mathbb{Z}_3$ symmetry is imposed in case of both the scenarios, but, allowed to be violated by terms of mass dimension two and three for generating neutrino masses and mixings. We examine how the models so introduced can explain the experimental observation on the muon anomalous magnetic moment. We estimate the two-loop contribution to neutrino mass induced by the scalar triplet, in addition to what comes from the doubly charged singlet in the usual Zee-Babu framework, in the first model. On the other hand, the neutrino mass arises in the usual Type-II fashion in the second model. In addition, the role of the $\mathbb{Z}_3$ symmetry in suppressing lepton flavor violation is also elucidated.

hep-ph↗

arXiv2018

Model with a gauged lepton flavor SU(2) symmetry

We propose a model having a gauged $SU(2)$ symmetry associated with the second and third generations of leptons, dubbed $SU(2)_{μτ}$, of which $U(1)_{L_μ-L_τ}$ is an Abelian subgroup. In addition to the Standard Model fields, we introduce two types of scalar fields. One exotic scalar field is an $SU(2)_{μτ}$ doublet and SM singlet that develops a nonzero vacuum expectation value at presumably multi-TeV scale to completely break the $SU(2)_{μτ}$ symmetry, rendering three massive gauge bosons. At the same time, the other exotic scalar field, carrying electroweak as well as $SU(2)_{μτ}$ charges, is induced to have a nonzero vacuum expectation value as well and breaks mass degeneracy between the muon and tau. We examine how the new particles in the model contribute to the muon anomalous magnetic moment in the parameter space compliant with the Michel decays of tau.

hep-ph↗

arXiv2018

Syndetic Extension of Baryon and Lepton Numbers: Proton Decay and Long-Lived Dark Matter

The well-known baryon and lepton numbers of the standard model of quarks and leptons are extended to include new fermions and bosons in a simple structure with several essential features. The usual heavy right-handed neutrino singlets (for neutrino mass and leptogenesis) are related to the axion which solves the strong CP problem. At the same time, baryon number is broken softly, allowing the proton to decay. Associated with this breaking, a long-lived dark-matter candidate (called the pseudo-sakharon) emerges. This new insight connects proton decay to a new component of dark matter.

hep-ph↗

arXiv2017

Effects of initial spatial phase in radiative neutrino pair emission

We study radiative neutrino pair emission in deexcitation process of atoms taking into account coherence effect in a macroscopic target system. In the course of preparing the coherent initial state to enhance the rate, a spatial phase factor is imprinted in the macroscopic target. It is shown that this initial spatial phase changes the kinematics of the radiative neutrino pair emission. We investigate effects of the initial spatial phase in the photon spectrum of the process. It turns out that the initial spatial phase provides us significant improvements in exploring neutrino physics such as the Dirac-Majorana distinction and the cosmic neutrino background.

hep-ph↗

arXiv2017

Ma-xion: Majoron as QCD axion in a radiative seesaw model

The smallness of neutrino mass, the strong CP problem, and the existence of dark matter are explained in an economical way. The neutrino mass is generated by the colored version of a radiative seesaw mechanism by using color adjoint mediators. The Majorana mass term of the adjoint fermion, which carries lepton number U(1)_L, is induced by its spontaneous breaking, resulting in a Majoron which doubles as the QCD (quantum chromodynamics) axion, thereby solving the strong CP problem. The breaking of U(1)_L sets simultaneously the seesaw scale for neutrino mass and the Peccei-Quinn breaking scale. This axion is a good candidate for dark matter as usually assumed.

hep-ph↗

arXiv2017

A Radiative Neutrino Mass Model with SIMP Dark Matter

We propose the first viable radiative seesaw model, in which the neutrino masses are induced radiatively via the two-loop Feynman diagram involving Strongly Interacting Massive Particles (SIMP). The stability of SIMP dark matter (DM) is ensured by a $\mathbb{Z}_5$ discrete symmetry, through which the DM annihilation rate is dominated by the $3 \to 2$ self-annihilating processes. The right amount of thermal relic abundance can be obtained with perturbative couplings in the resonant SIMP scenario, while the astrophysical bounds inferred from the Bullet cluster and spherical halo shapes can be satisfied. We show that SIMP DM is able to maintain kinetic equilibrium with thermal plasma until the freeze-out temperature via the Yukawa interactions associated with neutrino mass generation.

hep-ph↗

arXiv2017

Gravitational wave from dark sector with dark pion

In this work, we investigate the spectra of gravitational waves produced by chiral symmetry breaking in dark quantum chromodynamics (dQCD) sector. The dark pion ($π$) can be a dark matter candidate as weakly interacting massive particle (WIMP) or strongly interacting massive particle (SIMP). For a WIMP scenario, we introduce the dQCD sector coupled to the standard model (SM) sector with classical scale invariance and investigate the annihilation process of the dark pion via the $2π\to 2\,\text{SM}$ process. For a SIMP scenario, we investigate the $3π\to 2π$ annihilation process of the dark pion as a SIMP using chiral perturbation theory. We find that in the WIMP scenario the gravitational wave background spectra can be observed by future space gravitational wave antennas. On the other hand, when the dark pion is the SIMP dark matter with the constraints for the chiral perturbative limit and pion-pion scattering cross section, the chiral phase transition becomes crossover and then the gravitational waves are not produced.

hep-ph↗

arXiv2017

The reheating era leptogenesis in models with seesaw mechanism

Observed baryon asymmetry can be achieved not only by the decay of right-handed neutrinos but also by the scattering processes in the reheating era. In the latter scenario, new physics in high energy scale does not need to be specified, but only two types of the higher dimensional operator of the standard model particles are assumed in the previous work. In this paper, we examine the origin of the higher dimensional operators assuming models with a certain seesaw mechanism at the high energy scale. The seesaw mechanism seems to be a simple realization of the reheating era leptogenesis because the lepton number violating interaction is included. We show that the effective interaction giving CP violating phases is provided in the several types of models and also the reheating era leptogenesis actually works in such models. Additionally, we discuss a possibility for lowering the reheating temperature in the radiative seesaw models, where the large Yukawa coupling is naturally realized.

hep-ph↗

arXiv2017

Deterministic Quantum Annealing Expectation-Maximization Algorithm

Maximum likelihood estimation (MLE) is one of the most important methods in machine learning, and the expectation-maximization (EM) algorithm is often used to obtain maximum likelihood estimates. However, EM heavily depends on initial configurations and fails to find the global optimum. On the other hand, in the field of physics, quantum annealing (QA) was proposed as a novel optimization approach. Motivated by QA, we propose a quantum annealing extension of EM, which we call the deterministic quantum annealing expectation-maximization (DQAEM) algorithm. We also discuss its advantage in terms of the path integral formulation. Furthermore, by employing numerical simulations, we illustrate how it works in MLE and show that DQAEM outperforms EM.

stat.ML↗

arXiv2017

Towards Background-free RENP using a Photonic Crystal Waveguide

We study how to suppress multiphoton emission background in QED against radiative emission of neutrino pair (RENP) from atoms. We purse the possibility of background suppression using the photonic band structure of periodic dielectric media, called photonic crystals. The modification of photon emission rate in photonic crystal waveguides is quantitatively examined to clarify the condition of background-free RENP.

physics.optics↗

arXiv2017

Relaxation of the EM Algorithm via Quantum Annealing for Gaussian Mixture Models

We propose a modified expectation-maximization algorithm by introducing the concept of quantum annealing, which we call the deterministic quantum annealing expectation-maximization (DQAEM) algorithm. The expectation-maximization (EM) algorithm is an established algorithm to compute maximum likelihood estimates and applied to many practical applications. However, it is known that EM heavily depends on initial values and its estimates are sometimes trapped by local optima. To solve such a problem, quantum annealing (QA) was proposed as a novel optimization approach motivated by quantum mechanics. By employing QA, we then formulate DQAEM and present a theorem that supports its stability. Finally, we demonstrate numerical simulations to confirm its efficiency.

stat.ML↗

arXiv2016

Systematic U(1)_{B-L} Extensions of Loop-Induced Neutrino Mass Models with Dark Matter

We study the gauged U(1)_{B-L} extensions of the models for neutrino masses and dark matter. In this class of models, tiny neutrino masses are radiatively induced through the loop diagrams, while the origin of the dark matter stability is guaranteed by the remnant of the gauge symmetry. Depending on how the lepton number is violated in the neutrino mass diagrams, these models are systematically classified. We present a complete list for the one-loop Z_2 and the two-loop Z_3 neutrino mass models as examples of the classification. These underlying gauge symmetries and its breaking patterns can be probed at future high energy colliders by looking at the width of the new gauge boson.

hep-ph↗

arXiv2016

Models of LHC Diphoton Excesses Valid up to the Planck scale

We discuss a possibility to explain the LHC diphoton excesses at $750$GeV by the new scalar $X$ that couples to the gauge bosons through the loop of new massive particles with Standard Model charges. We assume that the new particles decay into the Standard Model particles at the tree level. We systematically examine the models that preserve the vacuum stability and the perturbativity up to the Planck scale. When we take scalars for the new particles, we find that only a few diquark and dilepton models can explain the observed diphoton cross section without conflicting the experimental mass bounds. When we take vector-like fermions for the new particles, we find rather different situations depending on whether their couplings to $X$ are scalar or pseudoscalar type. In the former case, a few models are allowed if we introduce only one species of fermions. The more fermions we introduce, the more models are allowed. In the later case, the most of the models are allowed because of the large coupling between $X$ and photon. It is interesting that the allowed mass regions of the scalar particles might be reached by the next lepton colliders.

hep-ph↗

arXiv2016

Relaxation of the EM Algorithm via Quantum Annealing

The EM algorithm is a novel numerical method to obtain maximum likelihood estimates and is often used for practical calculations. However, many of maximum likelihood estimation problems are nonconvex, and it is known that the EM algorithm fails to give the optimal estimate by being trapped by local optima. In order to deal with this difficulty, we propose a deterministic quantum annealing EM algorithm by introducing the mathematical mechanism of quantum fluctuations into the conventional EM algorithm because quantum fluctuations induce the tunnel effect and are expected to relax the difficulty of nonconvex optimization problems in the maximum likelihood estimation problems. We show a theorem that guarantees its convergence and give numerical experiments to verify its efficiency.

stat.ML↗

arXiv2015

Survey of Higgs interpretations of the diboson excesses

We investigate diboson signals in the Standard Model (SM) with an extended Higgs sector, motivated by the excesses in the diboson channels at the LHC. We begin with the unitarity sum-rules of the weak gauge boson scattering assuming the Higgs sector is extended. According to the sum-rules, we discuss the Higgs interpretations of the diboson signals and the consistency with the ATLAS diboson anomaly and other experimental constraints. As a concrete example, we propose a two-Higgs-doublet model where the Yukawa coupling of an extra CP-even scalar with up-type quark is relatively large. The diboson (WW and ZZ) signals can be explained by 2 TeV CP-even Higgs boson, while the partners, the CP-odd and the charged Higgs bosons in the extra doublet, are severely constrained by both the LHC direct search and the indirect search via flavor observables. Especially, in order to avoid the diboson resonance search in the Vh channel, further extensions of the model are required. The diboson excess is correlated with the SM Higgs signals in our framework, so that the precise measurement of the SM Higgs boson is also important to test the Higgs interpretation.

hep-ph↗

arXiv2015

Landau pole in the Standard Model with weakly interacting scalar fields

We consider the Standard Model with a new scalar field $X$ which is a $n_X^{}$ representation of the $SU(2)_L$ with a hypercharge $Y_X$. The renormalization group running effects on the new scalar quartic coupling constants are evaluated. Even if we set the scalar quartic coupling constants to be zero at the scale of the new scalar field, the coupling constants are induced by the one-loop effect of the weak gauge bosons. Once non-vanishing couplings are generated, the couplings rapidly increase by renormalization group effect of the quartic coupling constant itself. As a result, the Landau pole appears below Planck scale if $n_X^{}\geq 4$. We find that the scale of the obtained Landau pole is much lower than that evaluated by solving the one-loop beta function of the gauge coupling constants.

hep-ph↗