arXiv subjects

Atsushi Nishizawa

Publications and source records attributed to Atsushi Nishizawa.

At least 19 recordsLinked to original sources

arXiv2026

Signature of axion dark matter in low-frequency terrestrial electromagnetic fields: formulation and predictions

We develop a theoretical framework for axion dark matter (DM) searches using terrestrial electromagnetic (EM) fields, enabling a global and quantitative characterization of the signal across the Earth. Axions couple to the geomagnetic field and generate a monochromatic EM signal at a frequency set by the axion mass. Incorporating a realistic atmospheric conductivity, we describe the axion-induced EM waves confined in the Earth-ionosphere cavity, avoiding the divergences present in idealized treatments. Our semi-analytical method yields quantitative predictions for the axion-induced magnetic field near the Earth's surface, and we found that (i) at $m_{\rm a}\gtrsim10^{-14}$ eV, the signal exhibits resonance structures aligned with Schumann resonances, and the magnetic field amplitude is especially enhanced at $m_{\rm a}\sim3\times10^{-14}$ eV. (ii) The signal amplitude and orientation also vary with geographic location, with Southeast Asia offering the strongest sensitivity. These predictions are insensitive to uncertainties in conductivity models and boundary conditions. Those distinctive features provide a reliable template to distinguish axion-induced signals from natural or anthropogenic EM backgrounds, and the formalism can be extended to other DM candidates such as dark photons.

hep-ph↗

arXiv2026

Searching for dark photon dark matter from terrestrial magnetic fields

We present a novel search for dark photon dark matter (DM) using terrestrial magnetic field measurements at frequencies below 100 Hz. Coherently oscillating dark photon DM can induce a monochromatic magnetic field via kinetic mixing with ordinary photons. Notably, for dark photon masses $m_{A'}$ around $3 \times 10^{-14}$ eV, the signal can be resonantly amplified within a cavity formed by the Earth's surface and the ionosphere. We compute the expected signal incorporating the effect of atmospheric conductivity, and derive new upper limits on the kinetic mixing parameter $\varepsilon$ from long-term geomagnetic data. These limits improve upon previous ground-based constraints in the mass range of $1 \times 10^{-15}$ eV $\lesssim m_{A'} \lesssim 2 \times 10^{-13}$ eV.

hep-ph↗

arXiv2025

Subtracting compact binary foregrounds utilizing anisotropic statistic for third-generation gravitational-wave detectors

The astrophysical foreground from compact-binary coalescence signals is expected to be a dominant part of total gravitational wave (GW) energy density in the frequency band of the third-generation detectors. The detection of any other subdominant stochastic GW background (GWB), especially a primordial GWB, will be disturbed by the astrophysical foreground, which needs to be cleaned for further studies of other stochastic GWB. Although previous studies have proposed several cleaning methods, the foreground from subthreshold binary neutron stars (BNS) has been a major obstacle to remove. In this paper, we propose the novel idea to acquire better estimation of the unresolved foreground, by using the information about its anisotropies. We simulate the BNS population and compute its angular power spectrum and shot noise. We find that the shot noise from BNS is too faint to observe after subtracting loud signals due to the limited angular resolution of the third-generation detectors. This justifies the approximation regarding the unresolved foreground as an isotropic component. We also discuss the angular resolution necessary to make our method valid for the foreground subtraction.

gr-qc↗

arXiv2025

Proof-of-Principle Experiment on a Displacement-Noise-Free Neutron Interferometer for Gravitational Wave Detection

The displacement-noise-free interferometer (DFI) is designed to eliminate all displacement-induced noise while retaining sensitivity to gravitational wave (GW) signals. Ground-based DFIs suffer from physical arm-length limitations, resulting in poor sensitivity at frequencies below 1 kHz. To address this, previous research introduced a neutron-based DFI, which replaces laser light with neutrons and achieves exceptional sensitivity down to a few hertz. In this study, we conducted a proof-of-principle experiment using a pulsed neutron source at the Japan Proton Accelerator Research Complex (J- PARC). Despite practical constraints that led to deviations from the ideal experimental design, we optimized the setup and developed a novel analysis method that successfully cancels displacement noise while preserving simulated GW signals. This work presents the first successful demonstration of a neutron DFI and a neutron interferometer for GW detection.

physics.ins-det↗

arXiv2025

Physically consistent gravitational waveform for capturing beyond general relativity effects in the compact object merger phase

The merger phase of compact binary coalescences is the strongest gravity regime that can be observed. To test the validity of general relativity (GR) in strong gravitational fields, we propose a gravitational waveform parameterized for deviations from GR in the dynamical and nonlinear regime of gravity. Our fundamental idea is that perturbative modifications to a GR waveform can capture possible deviations in the merger phase that are difficult to model in a specific theory of gravity. One of notable points is that our waveform is physically consistent in the sense that the additional radiative losses of energy and angular momentum associated with beyond-GR modifications are included. Our prescription to ensure physical consistency in the whole coalescence process is expected to be applicable to any deviation from the standard model of compact binary coalescence, such as the extended models of gravity or the environmental effects of compact objects, as long as perturbative modifications are considered. Based on the Fisher analysis and the compatibility with Einstein-dilaton Gauss-Bonnet waveforms, we show that our parameterization is a physically-consistent minimal one that captures the deviations in the nonlinear regime.

gr-qc↗

arXiv2025

Observational constraints on the nonlinear regime of gravity with a parametrized beyond-GR gravitational waveform model

Gravitational waves from compact binary coalescences provide unique opportunities to test general relativity (GR) in the strong-field regime. In particular, the merger phase, during which two compact objects finally coalesce, corresponds to the regime of the strongest gravitational fields accessible by direct observation and thus serves as a probe of the nonlinear nature of gravity. In this work, we test GR in the merger phase by analyzing GW150914 using a modified waveform proposed in [Watarai et al. 2024], which parametrizes possible deviations from GR during this stage. Within this framework, the inferred deviation parameters can be translated into model-independent constraints on physically meaningful quantities. For GW150914, we find that the additional energy radiated in the merger phase is constrained to be $0.26^{+0.75}_{-0.62}~\%$ of the total energy emitted over the entire coalescence predicted by GR, and the deviation in the coalescence time is $2.17^{+9.56}_{-9.90}~\mathrm{ms}$, both within the $90\%$ credible interval. These two constraints serve as observational benchmarks for deviations in the nonlinear gravity regime, offering guidance for theoretical investigations of beyond-GR models.

gr-qc↗

arXiv2025

Hunting axion dark matter signatures in low-frequency terrestrial magnetic fields

We show that Earth's natural environment can serve as a powerful probe for ultralight axion dark matter. In the presence of global geomagnetic fields, the axions with masses ranging from $10^{-15}\,{\rm eV}-10^{-13}\,{\rm eV}$ induce electromagnetic waves in the (sub-) extremely low-frequency band ($0.3-30\,{\rm Hz}$) through the axion-photon coupling. We predict the amplitude of induced magnetic fields in the Earth-ionosphere cavity, taking the finite conductivity of the atmosphere into account. This allows us to constrain the axion-photon coupling parameter, $g_{\rm aγ}$, from the long-term monitoring data of the low-frequency magnetic fields, resulting in a significant improvement from the previous constraints down to $g_{\rm aγ} \lesssim 4\times10^{-13}\,{\rm GeV}^{-1}$ for axion mass $\sim 3 \times 10^{-14}\,{\rm eV}$.

hep-ph↗

arXiv2025

Constraints on Logarithmic Model Extensions of Symmetric Teleparallel Gravity

We address various cosmological phenomenologies in the symmetric teleparallel framework both in background and perturbation such as cosmic expansion, gravitational coupling constant, gravitational waves propagation. Focusing on logarithmic extensions of $f(Q)$ models, we performed Bayesian analysis using the most-recent cosmological data, DESI DR2, Pantheon+. We also utilized a compilation of redshift space distortions ($f σ_8$) dataset to constrain the growth of structures in each of the models modulated by the effective gravitational coupling. We find that our extended Logarithmic $f(Q)$ models are well-constrained by the current cosmological data and are able to describe the late-time cosmic acceleration. The inverse Logarithmic model we introduce is also able to accommodate a phantom-like dark energy equation of state at late times, which is consistent with the recent DESI DR2 observations. We report explicitly predictions for the effective gravitational coupling ($μ$), and the amplitude damping parameter of gravitational wave ($ν$) solely based on the background data, which can be tested against future observations. While the two Log-based extensions we have introduced here perform equivalently on the background level, they provide contrasting predictions for the evolution of effective Gravitational constant and propagation of gravitational waves, which should be constrained against the future perturbation data.

astro-ph.CO↗

arXiv2025

Statistical biases in parametrized searches for gravitational-wave polarizations

In tests of gravity using gravitational waves (GWs), GW events analyzed are often selected based on specific criteria, particularly the signal-to-noise ratio. However, such event selection can introduce bias into parameter estimation unless the selection effect is appropriately taken into account in the analysis. In this paper, we investigate how event selection with certain prior information affects parameter inference within the scalar-tensor polarization framework, focusing on the measurement of the scalar mode amplitude parameters. We find that for the Tensor+Scalar(dipole) model, the amplitude of the scalar dipole radiation is overestimated when its true value is nonzero while there is no false deviation in the absence of the scalar mode. The same bias is expected to occur also for the Tensor+Scalar(quadrupole) model. However, the error typically exceeds the bias as the scalar quadrupole mode is difficult to be distinguished from the tensor mode.

gr-qc↗

arXiv2025

Axion dark matter search from terrestrial magnetic fields at extremely low frequencies

The natural environment of the Earth can act as a sensitive detector for dark matter in ultralight axions. When axions with masses between $1\times10^{-15}\,{\rm eV}$ and $1\times10^{-13}\,{\rm eV}$ pass through the Earth, they interact with the global geomagnetic field, generating electromagnetic (EM) waves in the extremely low-frequency range ($0.3$--$30\,{\rm Hz}$) through axion-photon coupling. This paper is one of a series of companion papers for~\cite{Taruya:2025zql}, focusing on the data analysis method and search results for an axion signal. Utilizing the theoretical predictions of axion-induced EM spectra from a companion study, we analyzed long-term observational data of terrestrial magnetic fields in this frequency band to search for axion-induced signals. Our analysis identified 65 persistent signal candidates with a signal-to-noise ratio (SNR) greater than 3. Aside from these candidates, we placed a new upper bound on the axion-photon coupling parameter, significantly refining the previous constraint from CAST by at most two orders of magnitude down to $g_{aγ} \lesssim 4\times10^{-13} \,{\rm GeV}^{-1}$ for the axion mass around $3 \times 10^{-14}\,{\rm eV}$.

hep-ph↗

arXiv2025

Searches for ultralight vector and axion dark matter with KAGRA

We have proposed using laser interferometric gravitational wave detectors to search for ultralight vector and axion dark matter. Vector dark matter can be probed through oscillating forces on suspended mirrors, while axion dark matter can be detected via oscillating polarization rotation of laser beams. This paper reviews these searches with the KAGRA detector in Japan, including the first vector dark matter search with KAGRA's 2020 data and installation of polarization optics for axion dark matter search during the upcoming 2025 observing run.

hep-ph↗

arXiv2024

First results of axion dark matter search with DANCE

Axions are one of the well-motivated candidates for dark matter, originally proposed to solve the strong CP problem in particle physics. Dark matter Axion search with riNg Cavity Experiment (DANCE) is a new experimental project to broadly search for axion dark matter in the mass range of $10^{-17}~\mathrm{eV} < m_a < 10^{-11}~\mathrm{eV}$. We aim to detect the rotational oscillation of linearly polarized light caused by the axion-photon coupling with a bow-tie cavity. The first results of the prototype experiment, DANCE Act-1, are reported from a 24-hour observation. We found no evidence for axions and set 95% confidence level upper limit on the axion-photon coupling $g_{a γ} \lesssim 8 \times 10^{-4}~\mathrm{GeV^{-1}}$ in $10^{-14}~\mathrm{eV} < m_a < 10^{-13}~\mathrm{eV}$. Although the bound did not exceed the current best limits, this optical cavity experiment is the first demonstration of polarization-based axion dark matter search without any external magnetic field.

hep-ex↗

arXiv2024

Gravitational-wave constraints on scalar-tensor gravity from a neutron star and black-hole binary GW200115

In nonminimally coupled theories where a scalar field is coupled to the Ricci scalar, neutron stars (NSs) can have scalar charges through an interaction with matter mediated by gravity. On the other hand, the same theories do not give rise to hairy black hole (BH) solutions. The observations of gravitational waves (GWs) emitted from an inspiralling NS-BH binary system allows a possibility of constraining the NS scalar charge. Moreover, the nonminimally coupled scalar-tensor theories generate a breathing scalar mode besides two tensor polarizations. Using the GW200115 data of the coalescence of a BH-NS binary, we place observational constraints on the NS scalar charge as well as the nonminimal coupling strength for a subclass of massless Horndeski theories with a luminal GW propagation. Unlike past related works, we exploit a waveform for a mixture of tensor and scalar polarizations. Taking the breathing mode into account, the scalar charge is more tightly constrained in comparison to the analysis of the tensor GWs alone. In nonminimally coupled theories including Brans-Dicke gravity and spontaneous scalarization scenarios with/without a kinetic screening, we put new bounds on model parameters of each theory.

gr-qc↗

arXiv2024

Sagnac-type neutron displacement-noise-free interferometeric gravitational-wave detector

The detection of low-frequency gravitational waves on Earth requires the reduction of displacement noise, which dominates the low-frequency band. One method to cancel test mass displacement noise is a neutron displacement-noise-free interferometer (DFI). This paper proposes a new neutron DFI configuration, a Sagnac-type neutron DFI, which uses a Sagnac interferometer in place of the Mach-Zehnder interferometer. We demonstrate that a sensitivity of the Sagnac-type neutron DFI is higher than that of a conventional neutron DFI with the same interferometer scale. This configuration is particularly significant for neutron DFIs with limited space for construction and limited flux from available neutron sources.

gr-qc↗

arXiv2023

Circular Polarization of the Astrophysical Gravitational Wave Background

The circular polarization of gravitational waves is a powerful observable to test parity violation in gravity and to distinguish between the primordial or the astrophysical origin of the stochastic background. This property comes from the expected unpolarized nature of the homogeneous and isotropic astrophysical background, contrary to some specific cosmological sources that can produce a polarized background. However, in this work we show that there is a non-negligible amount of circular polarization also in the astrophysical background, generated by Poisson fluctuations in the number of unresolved sources, which can be detected by the third-generation interferometers with signal-to-noise ratio larger than one. We also explain in which cases the gravitational wave maps can be cleaned from this extra source of noise, exploiting the frequency and the angular dependence, in order to search for signals from the early Universe. Future studies about the detection of polarized cosmological backgrounds with ground- and space-based interferometers should account for the presence of such a foreground contribution.

astro-ph.CO↗

arXiv2023

Observational constraint on axion dark matter in a realistic halo profile with gravitational waves

Axions are considered as a candidate of dark matter. The axions form coherent clouds which delay and amplify gravitational waves (GWs) at a resonant frequency. That is, the coherent axion clouds produce secondary GWs following a primary wave. All GWs from compact binary mergers detected so far propagate in the Milky Way halo composed of dark matter so that such GWs are followed by the secondary GWs. Since the properties of the secondary GWs depend on the axion mass and coupling with the parity violating sector of gravity, we can search for a characteristic signal produced by axions. In our previous study, we have developed a search method optimized for the axion signals, and obtained about ten times stronger constraint on the coupling than the previous best one for the axion mass range, $[1.7 \times 10^{-13},\,8.5 \times 10^{-12}]\,\mathrm{eV}$. However, in our previous search, we assume that dark matter is homogeneously distributed in the Milky Way halo, which is unrealistic. In this paper, we extend the dark matter profile to a more realistic one called the core NFW profile, and find that the constraint in our previous study is robust to the dark matter profile.

hep-ph↗

arXiv2023

Distinguishing a stochastic gravitational-wave signal from correlated noise with joint parameter estimation: Fisher analysis for ground-based detectors

Search sensitivity to a stochastic gravitational-wave background (SGWB) is enhanced by cross-correlating detector signals. However, one of the most serious concerns is the environmental noise correlated between detectors. The global electromagnetic fields on the Earth, known as Schumann resonances, produce the correlated noise through the instrumental magnetic couplings. In this paper, we study the detectability of a SGWB in the presence of the correlated magnetic noise, using the Fisher analysis based on the analytical model of the correlated magnetic noise. We find that there is no significant degeneracy between the SGWB and noise parameters. Marginalizing over the correlated noise parameters degrades the constraint on each SGWB parameter by a factor of $\sim2$ at most in the four-detector case, irrespective of the strength of the magnetic coupling. We also confirm that the forecast results are robust against the variation of correlated noise parameters and can vary up to $40\%$ in the realistic range of the coupling parameters for the second-generation detectors. However, ignoring the correlated noise in parameter estimation generally leads to a biased constraint on the SGWB parameters. If the coupling strength is twice as large as expected, this could result in a serious bias.

gr-qc↗

arXiv2022

Displacement-noise-free interferometeric gravitational-wave detector using unidirectional neutrons with four speeds

For further gravitational wave (GW) detections, it is significant to invent a technique to reduce all kinds of mirror displacement noise dominant at low frequencies for ground-based detectors. The neutron displacement-noise-free interferometer (DFI) is one of the tools to reduce all the mirror displacement noise at lower frequencies. In this paper, we describe a further simplified configuration of a neutron DFI in terms of neutron incidence direction. In the new configuration, neutrons enter the interferometer with unidirectional incidence at four speeds as opposed to two bidirectional incidences of opposite directions at two speeds as reported previously. This simplification of the neutron DFI is significant for proof-of-principle experiments.

gr-qc↗