arXiv subjects

Kento Asai

Publications and source records attributed to Kento Asai.

30 records · Page 2Linked to original sources

arXiv2022

The Forward Physics Facility at the High-Luminosity LHC

High energy collisions at the High-Luminosity Large Hadron Collider (LHC) produce a large number of particles along the beam collision axis, outside of the acceptance of existing LHC experiments. The proposed Forward Physics Facility (FPF), to be located several hundred meters from the ATLAS interaction point and shielded by concrete and rock, will host a suite of experiments to probe Standard Model (SM) processes and search for physics beyond the Standard Model (BSM). In this report, we review the status of the civil engineering plans and the experiments to explore the diverse physics signals that can be uniquely probed in the forward region. FPF experiments will be sensitive to a broad range of BSM physics through searches for new particle scattering or decay signatures and deviations from SM expectations in high statistics analyses with TeV neutrinos in this low-background environment. High statistics neutrino detection will also provide valuable data for fundamental topics in perturbative and non-perturbative QCD and in weak interactions. Experiments at the FPF will enable synergies between forward particle production at the LHC and astroparticle physics to be exploited. We report here on these physics topics, on infrastructure, detector, and simulation studies, and on future directions to realize the FPF's physics potential.

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arXiv2021

Resolving the Hubble tension in a U(1)$_{L_μ-L_τ}$ model with Majoron

In this paper, we explore possibilities of resolving the Hubble tension and $(g-2)_μ$ anomaly simultaneously in a U(1)$_{L_μ- L_τ}$ model with Majoron. We only focus on a case where the Majoron $ϕ$ does not exist at the beginning of the universe and is created by neutrino inverse decay $νν\to ϕ$ after electron-positron annihilation. In this case, contributions of the new gauge boson $Z'$ and Majoron $ϕ$ to the effective number of neutrino species $N_{\rm eff}$ can be calculated in separate periods. These contribution are labelled $N'_{\rm eff}$ for the U(1)$_{L_μ- L_τ}$ gauge boson and $ΔN_{\rm eff}^\prime$ for the Majoron. The effective number $N_{\rm eff} = N'_{\rm eff} + ΔN_{\rm eff}^\prime$ is evaluated by the evolution equations of the temperatures and the chemical potentials of light particles in each period. As a result, we found that the heavier $Z'$ mass $m_{Z^\prime}$ results in the smaller $N_{\mathrm{eff}}^\prime$ and requires the larger $ΔN_{\mathrm{eff}}^\prime$ to resolve the Hubble tension. Therefore, compared to previous studies, the parameter region where the Hubble tension can be resolved is slightly shifted toward the larger value of $m_{Z^\prime}$.

hep-ph↗

arXiv2021

New physics searches at the ILC positron and electron beam dumps

We study capability of the ILC beam dump experiment to search for new physics, comparing the performance of the electron and positron beam dumps. The dark photon, axion-like particles, and light scalar bosons are considered as new physics scenarios, where all the important production mechanisms are included: electron-positron pair-annihilation, Primakoff process, and bremsstrahlung productions. We find that the ILC beam dump experiment has higher sensitivity than past beam dump experiments, with the positron beam dump having slightly better performance for new physics particles which are produced by the electron-positron pair-annihilation.

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arXiv2021

Electron Beam Dump Constraints on Light Bosons with Lepton Flavor Violating Couplings

We study constraints on light and feebly interacting bosons with lepton flavor violation from electron beam dump experiments. Scalar, vector, and dipole interactions of the bosons are analyzed, respectively, and excluded regions from the searches for decays into electron-positron pairs are derived. It is found that parameter regions unconstrained by flavor violating decays of muon can be excluded using the results of the E137 experiment. We also discuss the impact of the search for flavor violating decays of the light bosons in electron beam dump experiments.

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arXiv2021

Leptophilic Gauge Bosons at ILC Beam Dump Experiment

We study the prospects of searching for leptophilic gauge bosons (LGBs) at the beam dump experiment using $e^\pm$ beams of International $e^+e^-$ Linear Collider (ILC). We consider LGBs in association of $U(1)_{e-μ}$, $U(1)_{e-τ}$, and $U(1)_{μ-τ}$ gauge symmetries, which are assumed to be light and long-lived. Utilizing the energetic electron and positron beams of the ILC, we show that the ILC beam dump experiment can cover the parameter regions which have not been explored before. We also discuss the possibility of distinguishing various models.

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arXiv2021

Search for U(1)$_{L_μ-L_τ}$ charged Dark Matter with neutrino telescope

We study a simple Dirac fermion dark matter model in U(1)$_{L_μ-L_τ}$ theory. The new light gauge boson $X$ plays important roles in both dark matter physics and the explanation for the muon $g-2$ anomaly. The observed dark matter relic density is realized by a large U(1)$_{L_μ-L_τ}$ charge without introducing a resonance effect of the $X$ boson. As a by-product of the model, characteristic neutrino signatures from sub-GeV dark matter $ψ$ are predicted depending on the mass spectrum. We formulate the analysis of $ψ\barψ\to ν\barν$, and of $ψ\barψ\to XX$ followed by $X\toν\barν$, in a model independent way. The energy spectrum of neutrinos in the former process is monochromatic while in the latter process is bowl-shape. We also evaluate sensitivity at Super-Kamiokande and future Hyper-Kamiokande detectors. The analysis is finally applied to the U(1)$_{L_μ-L_τ}$ dark matter model.

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arXiv2021

Alternative Minimal U(1)$_{\rm B-L}$

We provide a minimal alternative gauged U(1)$_{\rm B-L}$ model in which three right-handed neutrinos with charges of (5,-4,-4) and only one B-L Higgs field with charge 1 are introduced. Consistent active neutrino masses and mixings can be obtained if the $Z_2$ symmetry on two of the right-handed neutrinos is introduced. It predicts two heavy degenerate right-handed neutrinos, which may realize resonant leptogenesis scenario, and one relatively light sterile neutrino, which is a good dark matter candidate.

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arXiv2020

Predictions for the neutrino parameters in the minimal model extended by linear combination of U(1)$_{L_e-L_μ}$, U(1)$_{L_μ-L_τ}$ and U(1)$_{B-L}$ gauge symmetries

We study the minimal extensions of the Standard Model by a linear combination of U(1)$_{L_e-L_μ}$, U(1)$_{L_μ-L_τ}$ and U(1)$_{B-L}$ gauge symmetries, where three right-handed neutrinos and one U(1)-breaking SU(2)$_L$ singlet or doublet scalar are introduced. Because of the dependence on the lepton flavor, the structures of both Dirac and Majorana mass matrices of neutrinos are restricted. In particular, the two-zero minor and texture structures in the mass matrix for the active neutrinos are interesting. Analyzing these structures, we obtain uniquely all the neutrino parameters, namely the Dirac CP phase $δ$, the Majorana CP phases $α_{2,3}$ and the mass eigenvalues of the light neutrinos $m_i$ as functions of the neutrino mixing angles $θ_{12}$, $θ_{23}$, and $θ_{13}$, and the squared mass differences $Δm^2_{21}$ and $Δm^2_{31}$. In 7 minimal models which are consistent with the recent neutrino oscillation data, we also obtain the predictions for the sum of the neutrino masses $Σ_i m_i$ and the effective Majorana neutrino mass $\langle m_{ββ} \rangle$ and compare them with the current experimental limits. In addition, we also discuss the implication of our results for leptogenesis.

hep-ph↗

arXiv2020

Leptogenesis in the minimal gauged U(1)$_{L_μ-L_τ}$ model and the sign of the cosmological baryon asymmetry

The minimal gauged U(1)$_{L_μ-L_τ}$ model is a simple extension of the Standard Model and has a strong predictive power for the neutrino sector. In particular, the mass spectrum and couplings of heavy right-handed neutrinos are determined as functions of three neutrino Dirac Yukawa couplings, with which we can evaluate the baryon asymmetry of the Universe generated through their decay, i.e., leptogenesis. In this letter, we study leptogenesis in the minimal gauged U(1)$_{L_μ-L_τ}$ model. It turns out that the sign of the resultant baryon asymmetry for the case with the Dirac CP phase, $δ$, larger than $π$ is predicted to be opposite to that for $δ< π$. In addition, if lepton asymmetry is dominantly produced by the decay of the lightest right-handed neutrino, then the correct sign of baryon asymmetry is obtained for $δ> π$, which is favored by the current neutrino-oscillation experiments, whilst the wrong sign is obtained for $δ< π$. We further investigate a non-thermal leptogenesis scenario where the U(1)$_{L_μ-L_τ}$ breaking field plays the role of inflaton and decays into right-handed neutrinos, as a concrete example. It is found that this simple framework offers a successful inflation that is consistent with the CMB observation. We then show that the observed amount of baryon asymmetry can be reproduced in this scenario, with its sign predicted to be positive in most of the parameter space.

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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

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↗

arXiv2017

Predictions for the neutrino parameters in the minimal gauged U(1)$_{L_μ-L_τ}$ model

We study the structure of the neutrino mass matrix in the minimal gauged U(1)$_{L_μ-L_τ}$ model, where three right-handed neutrinos are added to the Standard Model in order to obtain non-zero masses for active neutrinos. Because of the U(1)$_{L_μ-L_τ}$ gauge symmetry, the structure of both Dirac and Majorana mass terms of neutrinos is tightly restricted. In particular, the inverse of the neutrino mass matrix has zeros in the $(μ,μ)$ and $(τ,τ)$ components, namely, this model offers a symmetric realization of the so-called two-zero-minor structure in the neutrino mass matrix. Due to these constraints, all the CP phases-the Dirac CP phase $δ$ and the Majorana CP phases $α_2$ and $α_3$-as well as the mass eigenvalues of the light neutrinos $m_i$ are uniquely determined as functions of the neutrino mixing angles $θ_{12}$, $θ_{23}$, and $θ_{13}$, and the squared mass differences $Δm_{21}^2$ and $Δm_{31}^2$. We find that this model predicts the Dirac CP phase $δ$ to be $δ\simeq 1.59π$-$1.70π$ ($1.54π$-$1.78π$), the sum of the neutrino masses to be $\sum_{i}m_i \simeq 0.14$-0.22 eV ($0.12$-0.40 eV), and the effective mass for the neutrinoless double beta decay to be $\langle m_{ββ}\rangle \simeq 0.024$-0.055 eV ($0.017$-0.12 eV) at $1σ$ ($2σ$) level, which are totally consistent with the current experimental limits. These predictions can soon be tested in future neutrino experiments. Implications for leptogenesis are also discussed.

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