arXiv subjects

Takashi Shimomura

Publications and source records attributed to Takashi Shimomura.

At least 37 records · Page 2Linked to original sources

arXiv2020

Search for $Z'$ pair production from scalar boson decay in minimal $U(1)_{L_μ- L_τ}$ model at the LHC

We consider a model with gauged $L_μ- L_τ$ symmetry in which the symmetry is spontaneously broken by a scalar field. The decay of the scalar boson into two new gauge bosons is studied as a direct consequence of the spontaneous symmetry breaking. Then, a possibility of searches for the gauge and scalar bosons through such a decay at the LHC experiment is discussed. We consider the case that the mass range of the gauge boson is $\mathcal{O}(10)$ GeV, which is motivated by anomalies reported by LHCb. We will show that the signal significance of the searches for the gauge boson and scalar boson reach to $3$ and $5$ for the scalar mixing $0.012$ and $0.015$, respectively.

hep-ph↗

arXiv2020

Signal from sterile neutrino dark matter in extra $U(1)$ model at direct detection experiment

We examine the possibility that direct dark matter detection experiments find decay products from sterile neutrino dark mater in $U(1)_{B-L}$ and $U(1)_R$ models. This is possible if the sterile neutrino interacts with a light gauge boson and decays into a neutrino and the light gauge boson with a certain lifetime. This decay produces energetic neutrinos scattering off nuclei with a large enough recoil energy in direct dark matter detection experiments. We stress that direct dark matter detection experiments can explore not only WIMP but also sterile neutrino dark matter.

hep-ph↗

arXiv2020

Kinematical distributions of coherent neutrino trident production in gauged $L_μ-L_τ$ model

We analyze the distributions of energy, opening angle and invariant mass in muonic neutrino trident production processes, $ν_μ\to ν_μμ\overlineμ$, in a minimal gauged $U(1)_{L_μ- L_τ}$ model, in which the discrepancy of anomalous magnetic moment of muon can be solved. It is known that the total cross sections of the neutrino trident production are degenerate in new physics parameters, the new gauge coupling and gauge boson mass, and therefore other observables are needed to determine these parameters. From numerical analyses, we find that the muon energy and invariant mass distributions show the differences among the new physics parameter sets with which the total cross sections have the same value, while the anti-muon energy and opening angle distributions are not sensitive to the parameters.

hep-ph↗

arXiv2020

Type II seesaw models with modular $A_4$ symmetry

We discuss type-II seesaw models adopting modular $A_4$ symmetry in supersymmetric framework. In our approach, the models are classified by the assignment of $A_4$ representations and modular weights for leptons and triplet Higgs fields. Then neutrino mass matrix is characterized by modulus $τ$ and two free parameters. Carrying out numerical analysis, we find allowed parameter sets which can fit the neutrino oscillation data. For the allowed parameter sets, we obtain the predictions in neutrino sector such as CP violating phases and the lightest neutrino mass. Finally we also show the predictions for the branching ratios of doubly charged scalar boson focusing on the case where the doubly charged scalar boson dominantly decays into charged leptons.

hep-ph↗

arXiv2020

Atomki anomaly in gauged $U(1)_R$ symmetric model

The Atomki collaboration has reported that unexpected excesses have been observed in the rare decays of Beryllium nucleus. It is claimed that such excesses can suggest the existence of a new boson, called $X$, with the mass of about $17$ MeV. To solve the Atomki anomaly, we consider a model with gauged $U(1)_R$ symmetry and identify the new gauge boson with the $X$ boson. We also introduce two $SU(2)$ doublet Higgs bosons and one singlet Higgs boson, and discuss a very stringent constraint from neutrino-electron scattering. It is found that the $U(1)_R$ charges of the doublet scalars are determined to evade the constraint. In the end, we find the parameter region in which the Atomki signal and all experimental constraints can be simultaneously satisfied.

hep-ph↗

arXiv2019

Low scale seesaw models for low scale $U(1)_{L_μ-L_τ}$ symmetry

We propose models for neutrino masses and mixing in the framework of low scale $U(1)_{L_μ-L_τ}$ gauge extension of the standard model. The models are designed to spontaneously break $U(1)_{L_μ-L_τ}$ so that the $U(1)_{L_μ-L_τ}$ gauge boson acquires an MeV scale mass, which is required to solve the long-standing problem of muon anomalous magnetic moment. Tiny neutrino masses are obtained by simultaneously invoking the linear and the inverse seesaw mechanism, and we succeed in realizing two types of one-zero textures in the active neutrino mass matrix. Both of the obtained textures favor inverted neutrino mass ordering and are testable in next generation experiments of neutrinoless double beta decay. We also show that some of extra scalar bosons can have MeV scale masses and would have significant impacts on observations of high energy cosmic neutrinos.

hep-ph↗

arXiv2019

Searching for scalar boson decaying into light $Z'$ boson at collider experiments in $U(1)_{L_μ-L_τ}$ model

We study a model with $U(1)_{L_μ- L_τ}$ gauge symmetry and discuss collider searches for a scalar boson, which breaks $U(1)_{L_μ- L_τ}$ symmetry spontaneously, decaying into light $Z'$ gauge boson. In this model, the new gauge boson, $Z'$, with a mass lighter than $\mathcal{O}(100)$ MeV, plays a role in explaining the anomalous magnetic moment of muon via one-loop contribution. For the gauge boson to have such a low mass, the scalar boson, $ϕ$ with $\mathcal{O}(100)$ GeV mass appears associated with the symmetry breaking. We investigate experimental constraints on $U(1)_{L_μ- L_τ}$ gauge coupling, kinetic mixing, and mixing between the SM Higgs and $ϕ$. Then collider search is discussed considering $ϕ$ production followed by decay process $ϕ\to Z' Z'$ at the large hadron collider and the international linear collider. We also estimate discovery significance at the linear collider taking into account relevant kinematical cut effects.

hep-ph↗

arXiv2018

A simple approach to substitution minimal subshifts

In the study of substitution minimal subshifts, some complicated trivialities have hindered simple and general approaches. Recently, Maloney and Rust introduced the term "tame," simplifying the study. We introduce another term "$l$-primitive" for the substitutions and show that the combination of these two conditions can characterize the minimality of substitution subshifts. We shall show that all substitution minimal subshifts can be generated by substitutions that satisfy both conditions; conversely, all substitutions that satisfy the two conditions always generate minimal subshifts. As an application, we show that the result by Damanik and Lenz that an admissible substitution subshift is minimal if and only if it is linearly repetitive is valid for all substitution subshifts. The above set of conditions can be checked by finite calculations (algorithms).

math.DS↗

arXiv2018

Bratteli--Vershik models and graph covering models

Based on our previous graph covering method, we introduce weighted graph covering models and flexible graph covering models that are almost equivalent to the well-known Bratteli--Vershik models. These models play important roles in showing that every invertible dynamical system on compact metrizable zero-dimensional space admits a non-trivial Bratteli--Vershik model and a basic set. We can also obtain an analogue of Krieger's lemma for compact metrizable zero-dimensional systems. The flexible graph covering models enable us to consider "stationary" graph covering models, by which some portion of the substitution subshifts can be expressed. As an application, we show a way of constructing some class of transitive substitution subshifts.

math.DS↗

arXiv2018

Big-bang nucleosynthesis and Leptogenesis in CMSSM

We have investigated the constrained minimal supersymmetric standard model with three right-handed Majorana neutrinos whether there still is a parameter region which is consistent with all existing experimental data/limits such as Leptogenesis and the dark matter abundance and we also can solve the Lithium problem. Using Casas-Ibarra parameterization, we have found that a very narrow parameter space of the complex orthogonal matrix elements where the lightest slepton can have a long lifetime, that is necessary for solving the Lithium problem. Further, under this condition, there is a parameter region that can give an explanation for the experimental observations. We have studied three cases of the right-handed neutrino mass ratio \mbox{\em (i)} $M_{2}=2 \times M_{1}$, \mbox{\em (ii)} $M_{2}=4 \times M_{1}$, \mbox{\em (iii)} $M_{2}=10 \times M_{1}$ while $M_{3}=40 \times M_{1}$ is fixed. We have obtained the mass range of the lightest right-handed neutrino mass that lies between $10^9$ GeV and $10^{11}$ GeV. The important result is that its upper limit is derived by solving the Lithium problem and the lower limit comes from Leptogenesis. Calculated low-energy observables of these parameter sets such as BR($μ\to e γ$) is not yet restricted by experiments and will be verified in the near future.

hep-ph↗

arXiv2017

Relaxion window

We investigate cosmological constraints on the original relaxion scenario proposed by Graham, Kaplan and Rajendran. We first discuss the appropriate sign choice of the terms in the scalar potential, when the QCD axion is the relaxion with a relaxion-inflaton coupling proposed in the original paper. We next derive the cosmologically consistent ranges of the mass and a coupling of the relaxion for both the QCD relaxion and non-QCD relaxion. The mass range is obtained by $10^{-5}$ eV $\ll m_ϕ \lesssim 10^4$ eV. We also find that a strong correlation between the Hubble parameter at the relaxion stabilization and the scale $Λ$ of non-QCD strong dynamics, which generates the non-perturbative relaxion cosine potential. For a higher relaxion mass, a large scale $Λ$ becomes available. However, for its lower mass, $Λ$ should be small and constructing such a particle physics model is challenging.

astro-ph.CO↗

arXiv2017

Atomki anomaly and dark matter in a radiative seesaw model with gauged $B-L$ symmetry

Motivated by recently reported anomalies in a decay of an excited state of beryllium by the Atomki collaboration, we study a radiative seesaw model with gauged $B-L$ symmetry and a $Z_2$ parity. Assuming that the anomalies originate from the decay of the $B-L$ gauge boson followed by the nuclear decay, the mass of the lightest right-handed neutrino or the dark matter candidate can be determined below $10$ GeV. We show that for this mass range, the model can explain the anomalies in the beryllium decay and the relic dark matter abundance consistent with neutrino masses. We also predict its spin-independent cross section in direct detection experiments for this mass range.

hep-ph↗

arXiv2017

Proximal Cantor systems with topological rank 2 are residually scrambled

Downarowicz and Maass (2008) proposed topological ranks for all homeomorphic Cantor minimal dynamical systems using properly ordered Bratteli diagrams. In this study, we adopt this definition to the case of all essentially minimal zero-dimensional systems. We consider the cases in which topological ranks are 2 and unique minimal sets are fixed points. Akin and Kolyada (2003), in their study of Li--Yorke sensitivity, showed that if the unique minimal set of an essentially minimal system is a fixed point, then the system must be proximal. However, a finite topological rank implies expansiveness; furthermore, in the case of proximal Cantor systems with topological rank 2, the expansiveness is always from the lowest degree. Rank 2 zero-dimensional systems might be thought as a part of the rank 1 transformations that are considered in the vast field of ergodic theory. However, these systems are also interesting from the perspective of topological chaos theory; e.g., in this study, we show that all proximal Cantor systems with topological rank 2 are residually scrambled. In addition, we investigate the finite invariant measures on these systems. Evidently, such systems have at most two ergodic measures. We present a necessary and sufficient condition for the unique ergodicity of these systems. In addition, we show that the number of ergodic measures of systems that are topologically mixing can be 1 and 2. Moreover, we present examples that are topologically weakly mixing, not topologically mixing, and uniquely ergodic. Finally, we show that the number of ergodic measures of systems that are not weakly mixing can be 1 and 2.

math.DS↗

arXiv2017

On the possibility of search for $L_μ- L_τ$ gauge boson at Belle-II and neutrino beam experiments

We study the possibilities on the search of the light and weakly interacting gauge boson in the gauged $L_μ- L_τ$ model. Introducing the kinetic mixing at the tree-level, the allowed parameter regions for the gauge coupling and kinetic mixing parameter are presented. Then, we analyze one photon plus missing event within the allowed region and show that search for the light gauge boson will be possible at Belle-II experiment. We also analyze neutrino trident production process in neutrino beam experiments.

hep-ph↗

arXiv2017

Detecting the $L_μ-L_τ$ gauge boson at Belle II

We discuss the feasibility of detecting the gauge boson of the $U(1)_{L_μ-L_τ}$ symmetry, which possesses a mass in the range between MeV and GeV, at the Belle-II experiment. The kinetic mixing between the new gauge boson $Z'$ and photon is forbidden at the tree level and is radiatively induced. The leptonic force mediated by such a light boson is motivated by the discrepancy in muon anomalous magnetic moment and also the gap in the energy spectrum of cosmic neutrino. Defining the process $e^{+} e^{-} \rightarrow γZ' \rightarrow γν\barν~(missing~energy)$ to be the signal, we estimate the numbers of the signal and the background events and show the parameter region to which the Belle-II experiment will be sensitive. The signal process in the $L_μ-L_τ$ model is enhanced with a light $Z'$, which is a characteristic feature differing from the dark photon models with a constant kinetic mixing. We find that the Belle-II experiment with the design luminosity will be sensitive to the $Z'$ with the mass $M_{Z'} \lesssim 1 $ GeV and the new gauge coupling $g_{Z'} \gtrsim 8\cdot 10^{-4}$, which covers a half of the unconstrained parameter region that explains the discrepancy in muon anomalous magnetic moment. The possibilities to improve the significance of the detection are also discussed.

hep-ph↗

arXiv2016

Finite rank Bratteli--Vershik homeomorphisms are expansive

Downarowicz and Maass (2008) have shown that every Cantor minimal homeomorphism with finite topological rank $K > 1$ is expansive. Bezuglyi, Kwiatkowski, and Medynets (2009) extended the result to non-minimal aperiodic cases. In this paper, we show that all finite-rank zero-dimensional systems are expansive or have infinite odometer systems; this is an extension of the two aforementioned results. Nevertheless, the methods follow similar approaches.

math.DS↗

arXiv2016

Topological rank does not increase by natural extension of Cantor minimals

Downarowicz and Maass (2008) have defined the topological rank for all Cantor minimal homeomorphisms. On the other hand, Gambaudo and Martens (2006) have expressed all Cantor minimal continuous surjections as the inverse limits of certain graph coverings. Using the aforementioned results, we previously extended the notion of topological rank to all Cantor minimal continuous surjections. In this paper, we show that taking natural extensions of Cantor minimal continuous surjections does not increase their topological ranks. Further, we apply the result to the minimal symbolic case.

math.DS↗

arXiv2016

A Solution to Lithium Problem by Long-Lived Stau

We review a non-standard Big-Bang nucleosynthesis (BBN) scenario within the minimal supersymmetric standard model, and propose an idea to solve both ${}^{7}$Li and ${}^{6}$Li problems. Each problem is a discrepancy between the predicted abundance in the standard BBN and observed one. We focus on the stau, a supersymmetric partner of tau lepton, which is a long-lived charged particle when it is the next lightest supersymmetric particle and is degenerate in mass with the lightest supersymmetric particle. The long-lived stau forms a bound state with a nucleus, and provide non-standard nuclear reactions. One of those, the internal conversion process, accelerates the destruction of ${}^{7}$Be and ${}^{7}$Li, and leads to a solution to the ${}^{7}$Li problem. On the other hand, the bound state of the stau and ${}^{4}$He enhances productions of n, d, t, and ${}^{6}$Li. The over-production of ${}^{6}$Li could solve the ${}^{6}$Li problem. While, the over-productions of d and t could conflict with observations, and the relevant parameter space of the stau is strictly constrained. We therefore need to carefully investigate the stau-${}^{4}$He bound state to find a condition of solving the ${}^{6}$Li problem. The scenario of the long-lived stau simultaneously and successfully fit the abundances of light elements (d, t, ${}^{3}$He, ${}^{4}$He, ${}^{6}$Li, and ${}^{7}$Li) and the neutralino dark matter to the observed ones. Consequently parameter space both of the stau and the neutralino is determined with excellent accuracy.

hep-ph↗