arXiv subjects

Takeo Moroi

Publications and source records attributed to Takeo Moroi.

At least 73 records · Page 4Linked to original sources

arXiv2016

Probing the origin of 750 GeV diphoton excess with the precision measurements at the ILC

The recently reported diphoton excess at the LHC may imply the existence of a new resonance with a mass of about 750 GeV which couples to photons via loops of new charged particles. In this letter, we study the possibility to test such models at the ILC, paying attention to the new charged particles responsible for the diphoton decay of the resonance. We show that they affect the scattering processes $e^+e^- \to f\bar{f}$ (with $f$ denoting Standard Model fermions) at the ILC, which makes it possible to indirectly probe the new charged particles even if they are out of the kinematical reach. We also show that the discriminations of the diphoton models may be possible based on a study of the angular distributions of $f\bar{f}$.

hep-ph↗

arXiv2016

Studying 750 GeV Di-photon Resonance at Photon-Photon Collider

Motivated by the recent LHC discovery of the di-photon excess at the invariant mass of ~ 750 GeV, we study the prospect of investigating the scalar resonance at a future photon-photon collider. We show that, if the di-photon excess observed at the LHC is due to a new scalar boson coupled to the standard-model gauge bosons, such a scalar boson can be observed and studied at the photon-photon collider with the center-of-mass energy of ~ 1 TeV in large fraction of parameter space.

hep-ph↗

arXiv2016

Production and Decay of Di-photon Resonance at Future $e^+e^-$ Colliders

Motivated by the ATLAS and CMS announcements of the excesses of di-photon events, we discuss the production and decay processes of di-photon resonance at future $e^+e^-$ colliders. We assume that the excess of the di-photon events at the LHC is explained by a scalar resonance decaying into a pair of photons. In such a case, the scalar interacts with standard model gauge bosons and, consequently, the production of such a scalar is possible at the $e^+e^-$ colliders. We study the production of the scalar resonance via the associated production with photon or $Z$, as well as via the vector-boson fusion, and calculate the cross sections of these processes. We also study the backgrounds, and discuss the detectability of the signals of scalar production with various decay processes of the scalar resonance. We also consider the case where the scalar resonance has an invisible decay mode, and study how the invisible decay can be observed at the $e^+e^-$ colliders.

hep-ph↗

arXiv2016

Diphoton Excess and Running Couplings

The recently observed diphoton excess at the LHC may suggest the existence of a singlet (pseudo-) scalar particle with a mass of 750 GeV which couples to gluons and photons. Assuming that the couplings to gluons and photons originate from loops of fermions and/or scalars charged under the Standard Model gauge groups, we show that here is a model-independent upper bound on the cross section $σ(pp\to S\to γγ)$ as a function of the cutoff scale $Λ$ and masses of the fermions and scalars in the loop. Such a bound comes from the fact that the contribution of each particle to the diphoton event amplitude is proportional to its contribution to the one-loop $β$ functions of the gauge couplings. We also investigate the perturbativity of running Yukawa couplings in models with fermion loops, and show the upper bounds on $σ(pp\to S\to γγ)$ for explicit models.

hep-ph↗

arXiv2016

Di-Higgs Decay of Stoponium at Future Photon-Photon Collider

We study the detectability of the stoponium in the di-Higgs decay mode at the photon-photon collider option of the International $e^+e^-$ Linear Collider (ILC), whose center-of-mass energy is planned to reach $\sim 1$ TeV. We find that $5σ$ detection of the di-Higgs decay mode is possible with the integrated electron-beam luminosity of $1 {\rm ab}^{-1}$ if the signal cross section, $σ(γγ\rightarrow σ_{\tilde{t}_1} \rightarrow hh)$, of ${\cal O}(0.1)$ fb is realized for the stoponium mass smaller than $\sim$ 800 GeV at 1 TeV ILC. Such a value of the cross section can be realized in the minimal supersymmetric standard model (MSSM) with relatively large trilinear stop-stop-Higgs coupling constant. Implication of the stoponium cross section measurement for the MSSM stop sector is also discussed.

hep-ph↗

arXiv2015

Renormalization-Scale Uncertainty in the Decay Rate of False Vacuum

We study radiative corrections to the decay rate of false vacua, paying particular attention to the renormalization-scale dependence of the decay rate. The decay rate exponentially depends on the bounce action. The bounce action itself is renormalization scale dependent. To make the decay rate scale-independent, radiative corrections, which are due to the field fluctuations around the bounce, have to be included. We show quantitatively that the inclusion of the fluctuations suppresses the scale dependence, and hence is important for the precise calculation of the decay rate. We also apply our analysis to a supersymmetric model and show that the radiative corrections are important for the Higgs-stau system with charge breaking minima.

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arXiv2015

Revisiting Big-Bang Nucleosynthesis Constraints on Dark-Matter Annihilation

We study the effects of dark-matter annihilation during the epoch of big-bang nucleosynthesis on the primordial abundances of light elements. We improve the calculation of the light-element abundances by taking into account the effects of anti-nucleons emitted by the annihilation of dark matter and the interconversion reactions of neutron and proton at inelastic scatterings of energetic nucleons. Comparing the theoretical prediction of the primordial light-element abundances with the latest observational constraints, we derive upper bounds on the dark-matter pair-annihilation cross section. Implication to some of particle-physics models are also discussed.

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arXiv2015

Beam Dump Experiment at Future Electron-Positron Colliders

We propose a new beam dump experiment at future colliders with electron ($e^-$) and positron ($e^+$) beams, BDee, which will provide a new possibility to search for hidden particles, like hidden photon. If a particle detector is installed behind the beam dump, it can detect the signal of in-flight decay of the hidden particles produced by the scatterings of $e^\pm$ beams off materials for dumping. We show that, compared to past experiments, BDee (in particular BDee at $e^+e^-$ linear collider) significantly enlarges the parameter region where the signal of the hidden particle can be discovered.

hep-ph↗

arXiv2015

Footprints of Supersymmetry on Higgs Decay

Motivated by future collider proposals that aim to measure the Higgs properties precisely, we study the partial decay widths of the lightest Higgs boson in the minimal supersymmetric standard model with an emphasis on the parameter region where all superparticles and heavy Higgs bosons are not accessible at the LHC. Taking account of phenomenological constraints such as the Higgs mass, flavor constraints, vacuum stability, and perturbativity of coupling constants up to the grand unification scale, we discuss how large the deviations of the partial decay widths from the standard model predictions can be. These constraints exclude large fraction of the parameter region where the Higgs widths show significant deviation from the standard model predictions. Nevertheless, even if superparticles and the heavy Higgses are out of the reach of 14TeV LHC, the deviation may be large enough to be observed at future $e^+e^-$ collider experiments.

hep-ph↗

arXiv2015

AMS-02 Antiprotons from Annihilating or Decaying Dark Matter

Recently the AMS-02 experiment reported an excess of cosmic ray antiprotons over the expected astrophysical background. We interpret the excess as a signal from annihilating or decaying dark matter and find that the observed spectrum is well fitted by adding contributions from the annihilation or decay of dark matter with mass of O(TeV) or larger. Interestingly, Wino dark matter with mass of around 3 TeV, whose thermal relic abundance is consistent with present dark matter abundance, can explain the antiproton excess. We also discuss the implications for the decaying gravitino dark matter with R-parity violation.

hep-ph↗

arXiv2014

Studying Inflation with Future Space-Based Gravitational Wave Detectors

Motivated by recent progress in our understanding of the $B$-mode polarization of cosmic microwave background (CMB), which provides important information about the inflationary gravitational waves (IGWs), we study the possibility to acquire information about the early universe using future space-based gravitational wave (GW) detectors. We perform a detailed statistical analysis to estimate how well we can determine the reheating temperature after inflation as well as the amplitude, the tensor spectral index, and the running of the inflationary gravitational waves. We discuss how the accuracies depend on noise parameters of the detector and the minimum frequency available in the analysis. Implication of such a study on the test of inflation models is also discussed.

astro-ph.CO↗

arXiv2014

Cosmological Implications of High-Energy Neutrino Emission from the Decay of Long-Lived Particle

We study cosmological scenarios in which high-energy neutrinos are emitted from the decay of long-lived massive particles at the cosmic time later than a redshift of 10^6. The high-energy neutrino events recently observed by the IceCube experiment suggest a new source of high-energy cosmic-ray neutrinos; decay of a heavy particle can be one of the possibilities. We calculate the spectrum of the high-energy neutrinos emitted from the decay of long-lived particles, taking account of the neutrino scattering processes with background neutrinos. Then, we derive bounds on the scenario using the observation of high-energy cosmic-ray neutrino flux. We also study constraints from the spectral distortions of the cosmic microwave background and the big-bang nucleosynthesis. In addition, we show that the PeV neutrinos observed by the IceCube experiment can originate from the decay of a massive particle with its mass as large as O(10^10 GeV).

hep-ph↗

arXiv2014

Axion Models with High Scale Inflation

We revisit the cosmological aspects of axion models. In the high-scale inflation scenario, the Peccei-Quinn (PQ) symmetry is likely to be restored during/after inflation. If the curvature of the PQ scalar potential at the origin is smaller than its vacuum expectation value; for instance in a class of SUSY axion models, thermal inflation happens before the radial component of the PQ scalar (saxion) relaxes into the global minimum of the potential and the decay of saxion coherent oscillation would produce too much axion dark radiation. In this paper, we study how to avoid the overproduction of axion dark radiation with some concrete examples. We show that, by taking account of the finite-temperature dissipation effect appropriately, the overproduction constraint can be relaxed since the PQ scalar can take part in the thermal plasma again even after the PQ phase transition. We also show that it can be further relaxed owing to the late time decay of another heavy CP-odd scalar, if it is present.

hep-ph↗

arXiv2014

Isospin-Violating Dark Matter with Colored Mediators

In light of positive signals reported by the CDMS-II Si experiment and the recent results of the LUX and SuperCDMS experiments, we study isospin-violating dark matter scenarios assuming that the interaction of the dark matter is mediated by colored particles. We investigate the phenomenology of the model, including collider searches, flavor and CP phenomenology. A minimal possible scenario includes scalar dark matter and new vector-like colored fermions with masses of O(1) TeV as mediators. Such a scenario may be probed at the 14 TeV LHC, while flavor and CP constraints are stringent and severe tuning in the couplings is unavoidable. We also found that, as an explanation of the CDMS-II Si signal, isospin-violating fermionic dark matter models with colored scalar mediators are disfavored by the LHC constraints.

hep-ph↗

arXiv2014

Complexified Starobinsky Inflation in Supergravity in the Light of Recent BICEP2 Result

Motivated by the recent observation of the B-mode signal in the cosmic microwave background by BICEP2, we stuty the Starobinsky-type inflation model in the framework of old-minimal supergravity, where the inflaton field in the original (non-supersymmetric) Starobinsky inflation model becomes a complex field. We study how the inflaton evolves on the two-dimensional field space, varying the initial condition. We show that (i) one of the scalar fields has a very steep potential once the trajectory is off from that of the original Starobinsky inflation, and that (ii) the B-mode signal observed by BICEP2 is too large to be consistent with the prediction of the model irrespective of the initial condition. Thus, the BICEP2 result strongly disfavors the complexified Starobinsky inflation in supergravity.

hep-ph↗

arXiv2013

Cosmic-Ray Neutrinos from the Decay of Long-Lived Particle and the Recent IceCube Result

Motivated by the recent IceCube result, we study high energy cosmic-ray neutrino flux from the decay of a long-lived particle. Because neutrinos are so transparent, high energy neutrinos produced in the past may also contribute to the present neutrino flux. We point out that the PeV neutrino events observed by IceCube may originate in the decay of a particle much heavier than PeV if its lifetime is shorter than the present cosmic time. It is shown that the mass of the particle responsible for the IceCube event can be as large as $\sim 10^{10}\ {\rm GeV}$. We also discuss several possibilities to acquire information about the lifetime of the long-lived particle.

hep-ph↗

arXiv2013

Reconstructing Supersymmetric Contribution to Muon Anomalous Magnetic Dipole Moment at ILC

We study the possibility to determine the supersymmetric (SUSY) contribution to the muon anomalous magnetic dipole moment by using ILC measurements of the properties of superparticles. Assuming that the contribution is as large as the current discrepancy between the result of the Brookhaven E821 experiment and the standard-model prediction, we discuss how and how accurately the SUSY contribution can be reconstructed. We will show that, in a sample point, the reconstruction can be performed with the accuracy of ~ 13 % with the center-of-mass energy 500 GeV and the integrated luminosity ~ 500-1000 fb-1.

hep-ph↗

arXiv2013

Inflationary Gravitational Waves and the Evolution of the Early Universe

We study the effects of various phenomena which may have happened in the early universe on the spectrum of inflationary gravitational waves. The phenomena include phase transitions, entropy productions from non-relativistic matter, the production of dark radiation, and decoupling of dark matter/radiation from thermal bath. These events can create several characteristic signatures in the inflationary gravitational wave spectrum, which may be direct probes of the history of the early universe and the nature of high-energy physics.

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