arXiv subjects

Kazunori Nakayama

Publications and source records attributed to Kazunori Nakayama.

At least 19 recordsLinked to original sources

arXiv2026

Naturally Light Distortion

In the most general formulation of gravity, the metric and connection are independent degrees of freedom, and the connection may include torsion and non-metricity (or distortion, collectively) degrees of freedom, resulting in a huge number of possible dynamical fields. However, most fields are either non-dynamical or extremely heavy and the general relativity is recovered at low energy. We find a unique naturally light vector- or scalar-like distortion field, which can be dynamical and have phenomenological implications. In particular, a light scalar particle that mixes with the Higgs boson naturally appears.

gr-qc↗

arXiv2026

High-sensitivity Ultralight Dark Matter detector: Parametrically Amplified Casimir Devices

We propose the use of sphere-and-plate Casimir force measurement setups to detect ultralight vector dark matter. We demonstrate that sub-picometer signals can be parametrically amplified to be detectable. We present a novel improvement that allows the continuous tuning of the natural frequency of the system, opening new research opportunities at the submicron scale. We show that the improved setup can determine the dark matter mass with high accuracy, significantly expand the detectable mass range, and achieve the best sensitivity to date in the $8\times10^{-14}\sim1\times10^{-12}{\rm~ eV}$ mass range.

hep-ph↗

arXiv2026

Gravitational Decays of Secluded Scalars and Graviton Dark Radiation

We discuss graviton dark radiation produced by the decay of a secluded scalar field that couples to the Standard Model (SM) only through gravity. Such scalar fields are long-lived, and their decay channels generically include gravitons. If such particles existed and dominated the early universe, a sizable branching ratio into gravitons would yield non-negligible dark radiation that significantly alters the subsequent thermal history of the universe. In this work, we focus on the dark glueball as a representative secluded hidden scalar and compare the decay rates into SM particles via a non-minimal coupling to gravity with those into gravitons, paying attention to how the breaking of conformal invariance affects the amount of graviton dark radiation. We find that decays into the SM are dominated by two-body decay channels into Higgs bosons and gluons. In particular, when the Higgs field has a large non-minimal coupling to gravity, the production of graviton dark radiation is naturally suppressed in the metric formalism, and the SM sector is preferentially reheated and energy transfer to other hidden sectors is suppressed. Finally, we present the expected gravitational-wave spectrum resulting from dark glueball domination.

hep-ph↗

arXiv2026

Inflation and Reheating by Dynamical Torsion

In gravity theories with torsion, inflation can be driven by the dynamical torsion. Since it naturally has a derivative coupling to the axial current consisting of Standard Model particles, the reheating dynamics should be much different from conventional inflation models like Starobinsky inflation. We calculate the inflaton decay rate in detail and find that, although the 2 body decay into the fermion pair is vanishing in the massless fermion limit, the anomaly-induced 2 body decay into the gauge bosons and 3 body decay involving the Higgs boson are non-vanishing and they give sizable contributions to the total decay width of the inflaton. This gives a natural lower bound on the reheating temperature in the inflation model with dynamical torsion.

hep-ph↗

arXiv2026

Revisiting the Minimal Nelson-Barr Model

We revisit the minimal Nelson-Barr model for solving the strong CP problem through the idea of spontaneous CP breaking. The minimal model suffers from the quality problem, which means that the strong CP angle is generated by higher-dimensional operators and one-loop effects. Consequently, it has been considered that there is a cosmological domain wall problem and that leptogenesis does not work. We point out that just imposing an additional approximate global symmetry solves the quality problem. We also propose a simple solution to the domain wall problem and show that the thermal leptogenesis scenario works.

hep-ph↗

arXiv2026

Detecting dark matter using optically trapped Rydberg atom tweezer arrays

A new scheme for detecting wave-like dark matter (DM) using Rydberg atoms is proposed. Recent advances in trapping and manipulating Rydberg atoms make it possible to use Rydberg atoms trapped in optical tweezer arrays for DM detection. We propose to prepare a large ensemble of Rydberg atoms and to observe the excitations between Rydberg states by the DM-induced effective electric field. A scan over DM mass is enabled with the use of the Zeeman and diamagnetic shifts of energy levels under an applied external magnetic field. Taking dark-photon DM as an example, we demonstrate that our proposed experiment can have high enough sensitivity to probe previously unexplored regions of the parameter space of dark-photon coupling strengths and masses.

hep-ph↗

arXiv2026

High Frequency Spectrum of Primordial Gravitational Waves

During inflation long wavelength gravitational waves are produced, which form stochastic background in the present universe with very wide range of frequencies. Higher frequency gravitational waves never experience super-Hubble-radius regime, but they are also amplified after inflation due to the inflaton oscillation. Taking account of the inflaton dynamics after inflation, the spectrum of such quantum-mechanically produced gravitational waves may extend to much higher frequencies than previously thought. In this paper we calculate the spectrum of high frequency gravitational waves produced during and after inflation in detail, in particular focusing on the connection between the low and high frequency regime, and show that the detailed spectrum can distinguish inflation models.

hep-ph↗

arXiv2026

Anisotropy of Cosmic Background Photons from Annihilating/Decaying Dark Matter

We provide a detailed formulation for calculating the angular power spectrum of the cosmic background photons arising from the dark matter decay or annihilation in a comprehensive manner. We pay particular attention to the case of dark matter decaying or annihilating into line photons. It is pointed out that taking account of the energy resolution of the detector is essential to correctly evaluate the angular power spectrum. We apply our formulation to the observational data from infrared, optical, X-ray and gamma-ray telescopes.

hep-ph↗

arXiv2026

Entanglement-enhanced AC magnetometry in the presence of Markovian noises

Entanglement is a resource to improve the sensitivity of quantum sensors. In an ideal case, using an entangled state as a probe to detect target fields, we can beat the standard quantum limit by which all classical sensors are bounded. However, since entanglement is fragile against decoherence, it is unclear whether entanglement-enhanced metrology is useful in a noisy environment. Its benefit is indeed limited when estimating the amplitude of DC magnetic fields under the effect of parallel Markovian decoherence, where the noise operator is parallel to the target field. In this paper, on the contrary, we show an advantage to using an entanglement over the classical strategy under the effect of parallel Markovian decoherence when we try to detect AC magnetic fields. We consider a scenario to induce a Rabi oscillation of the qubits with the target AC magnetic fields. Although we can, in principle, estimate the amplitude of the AC magnetic fields from the Rabi oscillation, the signal becomes weak if the qubit frequency is significantly detuned from the frequency of the AC magnetic field. We show that, by using the GHZ states, we can significantly enhance the signal of the detuned Rabi oscillation even under the effect of parallel Markovian decoherence. Our method is based on the fact that the interaction time between the GHZ states and AC magnetic fields scales as $1/L$ to mitigate the decoherence effect where $L$ is the number of qubits, which contributes to improving the bandwidth of the detectable frequencies of the AC magnetic fields. Our results open up the way for new applications of entanglement-enhanced AC magnetometry.

quant-ph↗

arXiv2026

Reheating with Thermal Dissipation and Primordial Gravitational Waves

In order for an inflationary universe to evolve into a hot universe, a process known as reheating is required. However, the precise mechanism of reheating remains unknown. We show that if the reheating is triggered by thermal dissipation effects, distinctive features appear in the spectrum of primordial gravitational waves. This suggests a possible way to observationally probe the physics of reheating.

astro-ph.CO↗

arXiv2026

Dynamical Prevention of Topological Defect Formation

Topological defects can have significant cosmological consequences, so their production must be examined carefully. It is usually assumed that topological defects are produced if the temperature becomes sufficiently high, but in reality their formation depends on the post-inflationary dynamics of a symmetry-breaking scalar. We analyze the dynamics of a symmetry-breaking scalar field in the early universe within models that provide an effective negative mass term at the origin, and show that the symmetry can remain broken so that topological defects are never formed. In particular, we demonstrate that nonthermally produced particles (such as the Standard Model Higgs) during preheating can generate such an effective negative mass term, allowing the scalar field to follow a time-dependent minimum even in renormalizable models with a quartic coupling. We also discuss the implications of this result for the Peccei-Quinn scalar in axion models.

hep-ph↗

arXiv2025

Hybrid-spin decoupling for noise-resilient DC quantum sensing

The excellent sensitivities of quantum sensors are a double-edged sword: minuscule quantities can be observed, but any undesired signal acts as noise. This is challenging when detecting quantities that are obscured by such noise. Decoupling sequences improve coherence times and hence sensitivities, though only AC signals in narrow frequency bands are distinguishable. Alternatively, comagnetometers operate gaseous spin mixtures at high temperatures in the self-compensating regime to counteract slowly varying noise. These are applied with great success in various exotic spin-interaction searches. Here, we propose a method that decouples specific DC fields from DC and AC magnetic noise. It requires any spin cluster where the effect on each individual spin is different for the target field and local magnetic fields, which allows for a different approach compared to comagnetometers. The presented method has several key advantages, including an orders-of-magnitude increase in noise frequencies to which we are resistant. We explore electron-spin nuclear-spin pairs in nitrogen-vacancy centres in diamond, with a focus on their merit for light dark-matter searches. Other applications include gradient sensing, quantum memory, and gyroscopes.

quant-ph↗

arXiv2025

PQ-ball and its Real Scalar Analogue in an Expanding Universe

We demonstrate the formation of quasi-stable localized scalar configurations in spontaneously symmetry breaking U(1) model by 3+1-dimensional classical lattice simulations. Such configurations are called PQ-balls, as the primary motivation of this kind of configuration is Peccei-Quinn theory under the kinetic misalignment mechanism. Our numerical simulations demonstrate that they can form if the PQ charge is generated through the coherent rotation of a complex scalar field in the complex plane, via dynamics analogous to the Affleck-Dine mechanism. These configurations subsequently decay due to the U(1)-breaking effect induced by spontaneous symmetry breaking. We also demonstrate the formation and decay of oscillons in a similar setup in a real scalar field theory.

hep-ph↗

arXiv2025

Light Dark Matter Detection with Sub-eV Transition-Edge Sensors

We present a comprehensive analysis of high-resolution transition-edge sensors (TESs) as a quantum sensing platform for detecting dark matter (DM). Operating near the thermodynamic noise limit with sub-eV energy resolution, TESs offer a powerful approach for probing light DM in the sub-GeV mass range. Optical TESs, realized on superconducting films with critical temperatures below 150 mK, achieve energy thresholds below 100 meV and enable precise calorimetric detection of individual energy depositions. We model TES response by incorporating fundamental noise sources and applying optimal filtering techniques, and evaluate their sensitivity across a range of DM interaction channels, accounting for in-medium effects in the target material. We show that even ng-month-scale exposures can reach previously unexplored DM-electron scattering cross sections below $10^{-27}$ cm$^2$ for sub-MeV masses, and can probe the MeV-scale mass range for DM-nucleon couplings. Combining high energy resolution, photon-number sensitivity, and scalability, optical TESs provide a compelling quantum sensing platform for rare-event searches at the intersection of particle physics and quantum metrology.

hep-ph↗

arXiv2025

Nuclear Spin Metrology with Nitrogen Vacancy Center in Diamond for Axion Dark Matter Detection

We present a method to directly detect the axion dark matter using nitrogen vacancy centers in diamonds. In particular, we use metrology leveraging the nuclear spin of nitrogen to detect axion-nucleus couplings. This is achieved through protocols designed for dark matter searches, which introduce a novel approach of quantum sensing techniques based on the nitrogen vacancy center. Although the coupling strength of the magnetic fields with nuclear spins is three orders of magnitude smaller than that with electron spins for conventional magnetometry, the axion interaction strength with nuclear spins is the same order of magnitude as that with electron spins. Furthermore, we can take advantage of the long coherence time by using the nuclear spins for the axion dark matter detection. We show that our method is sensitive to a broad frequency range $\lesssim 100\,\mathrm{Hz}$ corresponding to the axion mass $m_a \lesssim 4\times 10^{-13}\,\mathrm{eV}$. We present the detection limit of our method for both the axion-neutron and the axion-proton couplings and discuss its significance in comparison with other proposed ideas.

hep-ph↗

arXiv2025

Inflation with Gauss-Bonnet correction: beyond slow-roll

If a coupling between the inflaton and the Gauss-Bonnet term is introduced, many models of inflation that were ruled out by the most recent Planck data can be made viable again. The predictions for the scalar spectral index and tensor-to-scalar ratio are typically computed using the slow-roll approximation. In this paper we instead study the full equations of motion and determine the necessary initial conditions for reasonable inflation epoch. We derive the conditions under which the Friedmann equation admits positive solutions for the Hubble parameter. Then we study the possibility of the inflaton becoming trapped in a local potential minimum induced by the Gauss-Bonnet term. Finally we demonstrate the results on monomial potential models with a quadratic and a quartic potential and show that the slow-roll approximation becomes imprecise in the quartic case.

gr-qc↗

arXiv2025

Decay rate of PQ-ball

Q-balls are non-topological solitons that arise in theories with a complex scalar field possessing a conserved global U(1) charge. Their stability is ensured by this charge, making them potentially significant in cosmology. In this paper, we investigate Q-ball-like objects in scenarios where the scalar field acquires a finite vacuum expectation value, spontaneously breaking the global U(1) symmetry. A well-motivated example is the Peccei-Quinn field, where the U(1) symmetry is identified as the Peccei-Quinn symmetry, and hence we refer to such objects as PQ-balls. We first discuss the existence of stable PQ-ball solutions in a finite-density plasma and argue that they become unstable in vacuum. Using detailed numerical simulations under spherical symmetry, we confirm their formation, compute their decay rate, and derive an analytical formula for it. Our results have important implications for axion cosmology, particularly in the context of the kinetic misalignment mechanism.

hep-ph↗

arXiv2025

Light Dark Matter Search with Nitrogen-Vacancy Centers in Diamonds

We propose new ideas to directly search for light dark matter, such as the axion or the dark photon, by using magnetometry with nitrogen-vacancy centers in diamonds. If the dark matter couples to the electron spin, it affects the evolution of the Bloch vectors consisting of the spin triplet states, which may be detected through several magnetometry techniques. We give several concrete examples with the use of dc and ac magnetometry and estimate the sensitivity on dark matter couplings.

hep-ph↗