Searcharxiv⌕ Search

arXiv subjects

Kazunori Nakayama

Publications and source records attributed to Kazunori Nakayama.

At least 55 records · Page 3Linked to original sources
arXiv2023

Anisotropic cosmic optical background bound for decaying dark matter in light of the LORRI anomaly

Recently anomalous flux in the cosmic optical background (COB) has been reported by the New Horizon observations. The COB flux is $16.37\pm 1.47\, \rm nW m^{-2} sr^{-1}$, at the LORRI pivot wavelength of $0.608\,\rm μm$, which is $\sim 4σ$ level above the expected flux from the Hubble Space Telescope (HST) galaxy count. It would be great if this were a hint for the eV scale dark matter decaying into photons. In this paper, we point out that such a decaying dark matter model predicts a substantial amount of anisotropy in the COB flux, which is accurately measured by the HST. The data of the HST excludes the decay rate of the dominant cold dark matter larger than $10^{-24}$-$10^{-23}\,{\rm s}^{-1}$ in the mass range of $5$-$20$eV. As a result, the decaying cold dark matter explaining the COB excess is strongly disfavored by the anisotropy bound. We discuss some loopholes: e.g. warm/hot dark matter or two-step decay of the dark matter, to explain the COB excess.

hep-ph↗
arXiv2022

Instability of the Electroweak Vacuum in Starobinsky Inflation

We study the stability of the electroweak vacuum during and after the Starobinsky inflation, assuming the existence of the non-minimal Higgs coupling to the Ricci scalar. In the Starobinsky inflation, there exists $R^2$ term (with $R$ being the Ricci scalar), which modifies the evolution equation of the Higgs field. We consider the case that the non-minimal coupling is sizable so that the quantum fluctuation of the Higgs field is suppressed and that the Higgs amplitude is settled near the origin during the inflation. In such a case, the Higgs amplitude may be amplified in the preheating epoch after inflation because of the parametric resonance due to the non-minimal coupling. We perform a detailed analysis of the evolution of the Higgs field in the preheating epoch by a numerical lattice simulation and derive an upper bound on the non-minimal coupling constant $ξ$ in order to realize the electroweak vacuum in the present universe. We find that the upper bound on $ξ$ in the Starobinsky inflation model is more stringent than that in conventional inflation models without the $R^2$ term.

hep-ph↗
arXiv2022

Scalar field couplings to quadratic curvature and decay into gravitons

Any field, even if it lives in a completely hidden sector, interacts with the visible sector at least via the gravitational interaction. In this paper, we show that a scalar field in such a hidden sector generically couples to the quadratic curvature via dimension five operators, i.e., $ϕR^2$, $ϕR^{μν} R_{μν}$, $ϕR_{μνρσ} R^{μνρσ}$, $ϕR_{μνρσ} \tilde R^{μνρσ}$, because they can emerge if there exist particles heavier than it. We derive these scalar couplings to the quadratic curvature by integrating out heavy particles in a systematic manner. Such couplings are of phenomenological interest since some of them induce the scalar decay into the graviton pair. We point out that the decay of a scalar field can produce a substantial amount of the stochastic cosmic graviton background at high frequency since the suppression scale of these operators is given by the mass of heavy particles not by the Planck scale. The resultant graviton spectrum is computed in some concrete models.

hep-ph↗
arXiv2021

Revisiting CMB constraints on dark matter annihilation

The precision measurements of the cosmic microwave background power spectrum put a strong constraint on the dark matter annihilation cross section since the electromagnetic energy injection by the dark matter annihilation affects the ionization history of the universe. In this paper, we update our previous simulation code for calculating the ionization history with the effect of dark matter annihilation by including Helium interactions and improving the precision of calculations. We give an updated constraint on the annihilation cross section and mass of dark matter using the modified RECFAST code with the Planck 2018 datasets.

astro-ph.CO↗
arXiv2021

Constraints on small-scale primordial density fluctuation from cosmic microwave background through dark matter annihilation

The cosmic microwave background (CMB) observation by the Planck satellite precisely determines primordial curvature fluctuations on larger scales than $\mathcal O(1)\,\mathrm{Mpc}$, while the small-scale curvature fluctuation is still less constrained. The constraint on small-scale fluctuations is highly improved if we assume the standard thermal relic dark matter scenario. When small-scale fluctuations are large enough, dense regions collapse to form small halos even in a redshift $z\gtrsim10^3$, which is called ultracompact minihalos. These minihalos enhance the annihilation of the dark matter and it is constrained by observations such as extragalactic gamma rays and the CMB. We revisit the effect of minihalos formed by the small-scale density fluctuations and calculate the ionization history modified by the dark matter annihilation. We perform the Markov Chain Monte Carlo method to constrain the size of small-scale curvature fluctuations by the CMB power spectrum. It is found that the constraint from the CMB power spectrum is comparable to that from the extragalactic gamma rays. We confirm that our constraint mainly comes from the energy injection in early time ($z\gtrsim 100$) and hence it is independent of the uncertainty of minihalo properties in the late time.

astro-ph.CO↗
arXiv2021

Hidden Photon and Axion Dark Matter from Symmetry Breaking

A light hidden photon or axion-like particle is a good dark matter candidate and they are often associated with the spontaneous breaking of dark global or gauged U(1) symmetry. We consider the dark Higgs dynamics around the phase transition in detail taking account of the portal coupling between the dark Higgs and the Standard Model Higgs as well as various thermal effects. We show that the (would-be) Nambu-Goldstone bosons are efficiently produced via a parametric resonance with the resonance parameter $q\sim 1$ at the hidden symmetry breaking. In the simplest setup, which predicts a second order phase transition, this can explain the dark matter abundance for the axion or hidden photon as light as sub eV. Even lighter mass, as predicted by the QCD axion model, can be consistent with dark matter abundance in the case of first order phase transition, in which case the gravitational wave signals may be detectable by future experiments such as LISA and DECIGO.

hep-ph↗
arXiv2021

Hidden Dark Matter from Starobinsky Inflation

The Starobinsky inflation model is one of the simplest inflation models that is consistent with the cosmic microwave background observations. In order to explain dark matter of the universe, we consider a minimal extension of the Starobinsky inflation model with introducing the dark sector which communicates with the visible sector only via the gravitational interaction. In Starobinsky inflation model, a sizable amount of dark-sector particle may be produced by the inflaton decay. Thus, a scalar, a fermion or a vector boson in the dark sector may become dark matter. We pay particular attention to the case with dark non-Abelian gauge interaction to make a dark glueball a dark matter candidate. In the minimal setup, we show that it is difficult to explain the observed dark matter abundance without conflicting observational constraints on the coldness and the self-interaction of dark matter. We propose scenarios in which the dark glueball, as well as other dark-sector particles, from the inflaton decay become viable dark matter candidates. We also discuss possibilities to test such scenarios.

hep-ph↗
arXiv2021

Preheating from target space curvature and unitarity violation: Analysis in field space

We study particle production and unitarity violation caused by a curved target space right after inflation. We use the inflaton field value instead of cosmic time as the time variable, and derive a semiclassical formula for the spectrum of produced particles. We then derive a simple condition for unitarity violation during preheating, which we confirm by our semiclassical method and numerical solution. This condition depends not only on the target space curvature but also on the height of the inflaton potential at the end of inflation. This condition tells us, for instance, that unitarity is violated in running kinetic inflation and Higgs inflation, while unitarity is conserved in $α$-attractor inflation and Higgs-Palatini inflation.

hep-ph↗
arXiv2021

Axion/Hidden-Photon Dark Matter Conversion into Condensed Matter Axion

The QCD axion or axion-like particles are candidates of dark matter of the universe. On the other hand, axion-like excitations exist in certain condensed matter systems, which implies that there can be interactions of dark matter particles with condensed matter axions. We discuss the relationship between the condensed matter axion and a collective spin-wave excitation in an anti-ferromagnetic insulator at the quantum level. The conversion rate of the light dark matter, such as the elementary particle axion or hidden photon, into the condensed matter axion is estimated for the discovery of the dark matter signals.

hep-ph↗
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.

hep-ph↗
arXiv2020

Gravitational Production of Hidden Photon Dark Matter in Light of the XENON1T Excess

Recently, the XENON1T experiment has reported an excess in the electronic recoil events. The excess is consistent with the interpretation of absorption of 3keV bosonic dark matter, for example, hidden photon dark matter with kinetic mixing of the order of $10^{-15}$. We point out that the minimally gravitational production provides a viable mechanism for obtaining a correct relic hidden photon abundance. We present parameter dependence of the hidden photon dark matter abundance on the inflationary scale $H_{\rm inf}$ and also the reheating temperature $T_{\rm R}$. We show that the inflationary Hubble scale and reheating temperature are both bounded from below, $H_{\rm inf}\gtrsim 7\times 10^{11}$GeV, $T_{\rm R}\gtrsim 10^2$GeV. In particular, the high-scale inflation is consistent with 3keV hidden photon dark matter.

hep-ph↗
arXiv2020

High-frequency Graviton from Inflaton Oscillation

We point out that there is a high-frequency tail of the stochastic inflationary gravitational wave background that scales as $f^{-1/2}$ with frequency $f$. This contribution comes from the graviton vacuum fluctuation amplified by the inflaton coherent oscillation during the reheating stage. It contains information on inflaton properties such as the inflaton mass as well as the thermal history of the early Universe.

astro-ph.CO↗
arXiv2020

Constraint on Vector Coherent Oscillation Dark Matter with Kinetic Function

A spatially uniform vector condensate can be formed during inflation if the vector boson is coupled to the inflaton through nontrivial kinetic function. The coherent oscillation of such a massive vector boson is a dark matter candidate. In this paper we consider the case where the vector boson energy density increases during inflation and show that the curvature/isocurvature perturbation gives stringent constraint on this scenario.

hep-ph↗
arXiv2020

Detecting Light Boson Dark Matter through Conversion into Magnon

Light boson dark matter such as axion or hidden photon can be resonantly converted into a magnon in a magnetic insulator under the magnetic field, which can be detected experimentally. We provide a quantum mechanical formulation for the magnon event rate and show that the result is consistent with that obtained by a classical calculation. Besides, it is pointed out that the experimental setup of the QUAX proposal for the axion detection also works as a detector of hidden photon dark matter. It has good sensitivity in the mass range around 1 meV, which is beyond astrophysical constraints.

hep-ph↗
arXiv2019

Signals of Axion Like Dark Matter in Time Dependent Polarization of Light

We consider the search for axion-like particles (ALPs) by using time series data of the polarization angle of the light. If the condensation of an ALP plays the role of dark matter, the polarization plane of the light oscillates as a function of time and we may be able to detect the signal of the ALP by continuously observing the polarization. In particular, we discuss that the analysis of the Fourier-transformed data of the time-dependent polarization angle is powerful to find the signal of the ALP dark matter. We pay particular attention to the light coming from astrophysical sources such as protoplanetary disks, supernova remnants, the foreground emission of the cosmic microwave background, and so on. We show that, for the ALP mass of $\sim 10^{-22}$--$10^{-19}\ {\rm eV}$, ALP searches in the Fourier space may reach the parameter region which is unexplored by other searches yet.

astro-ph.CO↗
arXiv2019

Aspects of Nonlinear Effect on Black Hole Superradiance

Under some conditions, light boson fields grow exponentially around a rotating black hole, called the superradiance instability. We discuss effects of nonlinear interactions of the boson on the instability. In particular, we focus on the effect of the particle production and show that the growth of the boson cloud may be saturated much before the black hole spin is extracted by the boson cloud, while the nonlinear interactions also induce the boson emission. For application, we revisit the superradiant instability of the standard model photon, axion and hidden photon.

hep-ph↗
arXiv2019

Novel Flavon Stabilization with Trimaximal Neutrino Mixing

We construct a supersymmetric $S_4$ flavor symmetry model with one of the trimaximal neutrino mixing patterns, the so-called TM$_1$, by using the novel way to stabilize flavons, which we proposed recently. The flavons are assumed to have tachyonic supersymmetry breaking mass terms and stabilized by higher-dimensional terms in the potential. We can obtain the desired alignment structure of the flavon vacuum expectation values to realize neutrino masses and mixings consistent with the current observations. This mechanism naturally avoids the appearance of dangerous cosmological domain walls.

hep-ph↗