Details of the Researcher

PHOTO

Naoya Kitajima
Section
Frontier Research Institute for Interdisciplinary Sciences
Job title
Assistant Professor
Degree

  • PhD (The University of Tokyo)

Research Areas 1

  • Natural sciences / Theoretical studies related to particle-, nuclear-, cosmic ray and astro-physics /

Awards 2

  1. Outstanding presentation award Gold Prize

    2019/11　JGRG29

  2. 宇宙線研究所修士博士発表会 所長賞

    2014/02　東京大学宇宙線研究所

Papers 51

  1. Abelian-Higgs vortices in the oscillating axion background Peer-reviewed

    Naoya Kitajima, Shota Nakagawa

    Physics Letters B　871　139993-139993　2025/12

    Publisher: Elsevier BV

    DOI： 10.1016/j.physletb.2025.139993 　

    ISSN： 0370-2693

  2. Stability of domain walls with inflationary fluctuations under potential bias, and gravitational wave signatures Peer-reviewed

    Naoya Kitajima, Junseok Lee, Fuminobu Takahashi, Wen Yin

    Journal of Cosmology and Astroparticle Physics　2025　(07)　053-053　2025/07/01

    Publisher: IOP Publishing

    DOI： 10.1088/1475-7516/2025/07/053 　

    eISSN： 1475-7516

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    Abstract A recent study has shown that domain walls with inflationary initial fluctuations exhibit remarkable stability against population bias due to long-range correlations, challenging the claims of prior research. In this paper, we study the dynamics of these domain walls in the presence of potential bias and show that they collapse with a lifetime several times longer than that due to thermal fluctuations. This is interpreted as a difference in the average distance between domain walls, leading us to derive a new formula for the domain wall lifetime which depends on the area parameter in a qualitatively different way from previous studies. In addition, we compute the spectrum of gravitational waves generated by such domain walls and find that both the peak frequency and the peak abundance are lowered in a manner that depends on the area parameter. Based on these findings, we also determine the necessary degree of vacuum degeneracy for axion domain walls to explain the isotropic cosmic birefringence.

  3. A bound on light dark photon dark matter Peer-reviewed

    Naoya Kitajima, Shota Nakagawa, Fuminobu Takahashi, Wen Yin

    Physics Letters B　862　139304-139304　2025/03

    Publisher: Elsevier BV

    DOI： 10.1016/j.physletb.2025.139304 　

    ISSN： 0370-2693

  4. Primordial origin of supermassive black holes from axion bubbles Peer-reviewed

    Kentaro Kasai, Masahiro Kawasaki, Naoya Kitajima, Kai Murai, Shunsuke Neda, Fuminobu Takahashi

    Journal of Cosmology and Astroparticle Physics　2024　(05)　092-092　2024/05/01

    Publisher: IOP Publishing

    DOI： 10.1088/1475-7516/2024/05/092 　

    eISSN： 1475-7516

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    Abstract We study a modification of the primordial black hole (PBH) formation model from axion bubbles.We assume that the Peccei-Quinn scalar rolls down in the radial direction from a large field value to the potential minimum during inflation, which suppresses the axion fluctuations and weakens the clustering of PBHs on large scales.We find that the modified model can produce a sufficient number of PBHs that seed the supermassive black holes while avoiding the observational constraints from isocurvature perturbations and angular correlation of the high-redshift quasars.

  5. Gravitational waves from domain wall collapse, and application to nanohertz signals with QCD-coupled axions Peer-reviewed

    Naoya Kitajima, Junseok Lee, Kai Murai, Fuminobu Takahashi, Wen Yin

    Physics Letters B　851　138586-138586　2024/04

    Publisher: Elsevier BV

    DOI： 10.1016/j.physletb.2024.138586 　

    ISSN： 0370-2693

  6. Nanohertz gravitational waves from cosmic strings and dark photon dark matter Peer-reviewed

    Naoya Kitajima, Kazunori Nakayama

    Physics Letters B　846　138213-138213　2023/11

    Publisher: Elsevier BV

    DOI： 10.1016/j.physletb.2023.138213 　

    ISSN： 0370-2693

  7. Clustering of primordial black holes from QCD axion bubbles Peer-reviewed

    Kentaro Kasai, Masahiro Kawasaki, Naoya Kitajima, Kai Murai, Shunsuke Neda, Fuminobu Takahashi

    Journal of Cosmology and Astroparticle Physics　2023　(10)　049-049　2023/10/01

    Publisher: IOP Publishing

    DOI： 10.1088/1475-7516/2023/10/049 　

    eISSN： 1475-7516

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    Abstract We study the clustering of primordial black holes (PBHs) and axion miniclusters produced in the model proposed to explain the LIGO/Virgo events or the seeds of the supermassive black holes (SMBHs) in ref. [1]. It is found that this model predicts large isocurvature perturbations due to the clustering of PBHs and axion miniclusters, from which we obtain stringent constraints on the model parameters. Specifically, for the axion decay constant  fa = 1016 GeV, which potentially accounts for the seeds of the SMBHs, the PBH fraction in dark matter should be fPBH ≲ 7 × 10-10. Assuming that the mass of PBHs increases by more than a factor of 𝒪(10) due to accretion, this is consistent with the observed abundance of SMBHs. On the other hand, for  fa = 1017 GeV required to produce PBHs of masses detected in the LIGO/Virgo, the PBH fraction should be fPBH ≲ 6 × 10-8, which may be too small to explain the LIGO/Virgo events, although there is a significant uncertainty in calculating the merger rate in the presence of clustering.

  8. Stochastic gravitational wave background from early dark energy Peer-reviewed

    Naoya Kitajima, Tomo Takahashi

    Journal of Cosmology and Astroparticle Physics　2023　(10)　074-074　2023/10/01

    Publisher: IOP Publishing

    DOI： 10.1088/1475-7516/2023/10/074 　

    eISSN： 1475-7516

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    Abstract We study the production of stochastic gravitational wave background in early dark energy (EDE) models. It is generated by resonant amplification of scalar field fluctuations, which can easily take place for typical EDE potential based on the string axion or α-attractor model. The resultant spectrum of gravitational wave background is computed by performing 3D lattice simulations. We show that, specifically in some class of a generalized α-attractor EDE model, a significant amount of gravitational waves can be produced via tachyonic instability with a peak around femto-Hz frequency range. Models predicting such gravitational waves can be constrained by cosmic microwave background observations.

  9. Dark photon dark matter from cosmic strings and gravitational wave background Peer-reviewed

    Naoya Kitajima, Kazunori Nakayama

    Journal of High Energy Physics　2023　(8)　2023/08/14

    Publisher: Springer Science and Business Media LLC

    DOI： 10.1007/jhep08(2023)068 　

    eISSN： 1029-8479

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    Abstract Dark photon dark matter may be produced by the cosmic strings in association with the dark U(1) gauge symmetry breaking. We perform three-dimensional lattice simulations of the Abelian-Higgs model and follow the evolution of cosmic strings. In particular, we simulate the case of (very) light vector boson and find that such vector bosons are efficiently produced by the collapse of small loops while the production is inefficient in the case of heavy vector boson. We calculate the spectrum of the gravitational wave background produced by the cosmic string loops for the light vector boson case and find characteristic features in the spectrum, which can serve as a probe of the dark photon dark matter scenario. In particular, we find that the current ground-based detectors may be sensitive to such gravitational wave signals and also on-going/future pulsar timing observations give stringent constraint on the dark photon dark matter scenario.

  10. Stability of domain wall network with initial inflationary fluctuations and its implications for cosmic birefringence Peer-reviewed

    Diego Gonzalez, Naoya Kitajima, Fuminobu Takahashi, Wen Yin

    Physics Letters B　843　137990-137990　2023/08

    Publisher: Elsevier BV

    DOI： 10.1016/j.physletb.2023.137990 　

    ISSN： 0370-2693

  11. Viable vector coherent oscillation dark matter Peer-reviewed

    Naoya Kitajima, Kazunori Nakayama

    Journal of Cosmology and Astroparticle Physics　2023　(07)　014-014　2023/07/01

    Publisher: IOP Publishing

    DOI： 10.1088/1475-7516/2023/07/014 　

    eISSN： 1475-7516

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    Abstract We construct a viable model of the vector coherent oscillation dark matter. The vector boson is coupled to the inflaton through the kinetic function so that the effective Hubble mass term is cancelled out. In order to avoid strong constraints from isocurvature perturbation and statistically anisotropic curvature perturbation, the inflaton is arranged so that it does not contribute to the observed large scale curvature perturbation and we introduce a curvaton. We found viable vector coherent oscillation dark matter scenario for the wide vector mass range from 10-21 eV to 1 eV.

  12. Resonant production of dark photons from axions without a large coupling Peer-reviewed

    Naoya Kitajima, Fuminobu Takahashi

    Physical Review D　107　(12)　2023/06/15

    Publisher: American Physical Society (APS)

    DOI： 10.1103/physrevd.107.123518 　

    ISSN： 2470-0010

    eISSN： 2470-0029

  13. Power spectrum of domain-wall network, and its implications for isotropic and anisotropic cosmic birefringence Peer-reviewed

    Naoya Kitajima, Fumiaki Kozai, Fuminobu Takahashi, Wen Yin

    Journal of Cosmology and Astroparticle Physics　2022　(10)　043-043　2022/10/01

    Publisher: IOP Publishing

    DOI： 10.1088/1475-7516/2022/10/043 　

    eISSN： 1475-7516

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    Abstract Recently, based on a novel analysis of the Planck satellite data, a hint of a uniform rotation of the polarization of cosmic microwave background photons, called isotropic cosmic birefringence, has been reported. The suggested rotation angle of polarization of about 0.2–0.4 degrees is close to the fine-structure constant, α ≃ 1/137 rad ≃ 0.42 deg. Interestingly, this coincidence can be naturally explained over a very wide parameter range by the domain walls of axion-like particles. Furthermore, the axion-like particle domain walls predict not only isotropic cosmic birefringence but also anisotropic one that reflects the spatial distribution of the axion-like particle field on the last scattering surface. In this paper, we perform lattice simulations of the formation and evolution of domain walls in the expanding universe and obtain for the first time the two-point correlation function and power spectrum of the scalar field that constitutes the domain walls. We find that for initial fluctuations at subhorizon scales, the power spectrum is roughly consistent with analytical predictions based on random wall distributions. However, there is some excess at scales corresponding to the Hubble radius. Applying our results to the anisotropic cosmic birefringence, we predict the power spectrum of the rotation angles induced by the axion-like particle domain walls for the similar initial condition, and show that it is within reach of future observations of the cosmic microwave background.

  14. Nonthermally trapped inflation by tachyonic dark photon production Peer-reviewed

    Naoya Kitajima, Shota Nakagawa, Fuminobu Takahashi

    Physical Review D　105　(10)　2022/05/11

    Publisher: American Physical Society (APS)

    DOI： 10.1103/physrevd.105.103011 　

    ISSN： 2470-0010

    eISSN： 2470-0029

  15. New scenario of QCD axion clump formation. Part I. Linear analysis Peer-reviewed

    Naoya Kitajima, Kazuhiro Kogai, Yuko Urakawa

    Journal of Cosmology and Astroparticle Physics　2022　(03)　039-039　2022/03/01

    Publisher: IOP Publishing

    DOI： 10.1088/1475-7516/2022/03/039 　

    eISSN： 1475-7516

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    Abstract The QCD axion acquires the potential through the non-perturbative effect of the QCD matters around the QCD phase transition. During this period, the direct interaction between the axion and the QCD matters sets in. Focusing on the impact of this direct interaction, we propose two scenarios where the fluctuation of the axion can rapidly grow, potentially leading to the formation of axion miniclusters even if the Peccei-Quinn (PQ) symmetry was already broken during inflation. The first scenario assumes that the primordial curvature perturbation at the horizon scale during the QCD epoch was significantly enhanced and the second one assumes that the initial misalignment was tuned around the hilltop of the potential.

  16. Primordial black holes in peak theory with a non-Gaussian tail Peer-reviewed

    Naoya Kitajima, Yuichiro Tada, Shuichiro Yokoyama, Chul-Moon Yoo

    Journal of Cosmology and Astroparticle Physics　2021　(10)　053-053　2021/10/01

    Publisher: IOP Publishing

    DOI： 10.1088/1475-7516/2021/10/053 　

    eISSN： 1475-7516

  17. Nano-Hz Gravitational-Wave Signature from Axion Dark Matter Peer-reviewed

    Naoya Kitajima, Jiro Soda, Yuko Urakawa

    Physical Review Letters　126　(12)　2021/03/26

    Publisher: American Physical Society (APS)

    DOI： 10.1103/physrevlett.126.121301 　

    ISSN： 0031-9007

    eISSN： 1079-7114

  18. Can axion clumps be formed in a pre-inflationary scenario? Peer-reviewed

    Hayato Fukunaga, Naoya Kitajima, Yuko Urakawa

    Journal of Cosmology and Astroparticle Physics　2021　(02)　015-015　2021/02/09

    Publisher: IOP Publishing

    DOI： 10.1088/1475-7516/2021/02/015 　

    eISSN： 1475-7516

  19. Primordial black holes from QCD axion bubbles Peer-reviewed

    Naoya Kitajima, Fuminobu Takahashi

    Journal of Cosmology and Astroparticle Physics　2020　(11)　060-060　2020/11/27

    Publisher: IOP Publishing

    DOI： 10.1088/1475-7516/2020/11/060 　

    eISSN： 1475-7516

  20. Relic abundance of dark photon dark matter Peer-reviewed

    Prateek Agrawal, Naoya Kitajima, Matthew Reece, Toyokazu Sekiguchi, Fuminobu Takahashi

    Physics Letters B　801　135136-135136　2020/02

    Publisher: Elsevier BV

    DOI： 10.1016/j.physletb.2019.135136 　

    ISSN： 0370-2693

  21. Stochastic inflation with an extremely large number of e-folds Peer-reviewed

    Naoya Kitajima, Yuichiro Tada, Fuminobu Takahashi

    Physics Letters B　800　135097-135097　2020/01

    Publisher: Elsevier BV

    DOI： 10.1016/j.physletb.2019.135097 　

    ISSN： 0370-2693

  22. Efficient self-resonance instability from axions Peer-reviewed

    Hayato Fukunaga, Naoya Kitajima, Yuko Urakawa

    Journal of Cosmology and Astroparticle Physics　06　(055)　2019/06

    DOI： 10.1088/1475-7516/2019/06/055 　

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    It was recently shown that a coherent oscillation of an axion can cause an<br /> efficient parametric resonance, leading to a prominent emission of the<br /> gravitational waves (GWs). In this paper, conducting the Floquet analysis, we<br /> investigate the parametric resonance instability, which potentially triggers<br /> the emission of the GWs from axions. Such a resonance instability takes place,<br /> when the time evolution of the background field significantly deviates from the<br /> harmonic oscillation. Therefore, the resonance instability cannot be described<br /> by the Mathieu equation, whose stability/instability chart is well known. In<br /> this paper, introducing an explicitly calculable parameter $\tilde{q}$, which<br /> can be used to classify different types of the parametric resonance described<br /> by the general Hill&#039;s equation, we investigate the stability/instability chart<br /> for the general Hill&#039;s equation. This can also apply to the case where the<br /> background oscillation is anharmonic. We show that the flapping resonance<br /> instability, which takes place for $\tilde{q}=O(1)$, typically leads to the<br /> most significant growth of the inhomogeneous modes among the self-resonance<br /> instability. We also investigate whether the flapping resonance takes place for<br /> the cosine potential or not.

  23. Distribution of primordial black holes and 21cm signature Peer-reviewed

    Jinn-Ouk Gong, Naoya Kitajima

    Journal of Cosmology and Astroparticle Physics　11　(041)　2018/11

    DOI： 10.1088/1475-7516/2018/11/041 　

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    We show that the number of primordial black holes (PBHs) which is originated<br /> from primordial density perturbations with moderately-tilted power spectrum<br /> fluctuates following the log-normal distribution, while it follows the Poisson<br /> distribution if the spectrum is steeply blue. The log-normal, as well as the<br /> Poisson, fluctuation of the PBH number behaves as an isocurvature mode and<br /> affects the matter power spectrum and the halo mass function in a different way<br /> from those for the Poisson case. The future 21cm observation can potentially<br /> put a stronger constraint on the PBH fraction than the current one in a wide<br /> mass range, $10^{-5}M_\odot$--$10M_\odot$.

  24. Gravitational wave forest from string axiverse Peer-reviewed

    Naoya Kitajima, Jiro Soda, Yuko Urakawa

    Journal of Cosmology and Astroparticle Physics　10　(008)　2018/10

    DOI： 10.1088/1475-7516/2018/10/008 　

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    Axions predicted in string theory may have a scalar potential which has a<br /> much shallower potential region than the conventional cosine potential. We<br /> first show that axions which were located at such shallow potential regions<br /> generically undergo prominent resonance instabilities: the well-known narrow<br /> resonance and/or the flapping resonance, which has not been well investigated.<br /> We also study non-linear dynamics of axions caused by these resonance<br /> instabilities based on lattice simulation. We find that string axions in<br /> various mass ranges generate gravitational waves (GWs) with peaks at various<br /> frequencies determined by the mass scales, dubbed the GW forest. This may allow<br /> us to explore string axiverse through future multi-frequency GW observations.<br /> We also investigate GWs produced by the axion which accounts for present dark<br /> matter component.

  25. Electromagnetic effects induced by a time-dependent axion field Peer-reviewed

    Katsuhisa Taguchi, Tatsushi Imaeda, Tetsuya Hajiri, Takuya Shiraishi, Yukio Tanaka, Naoya Kitajima, Tatsuhiro Naka

    Physical Review B　97　(21)　214409　2018/06/11

    Publisher: American Physical Society

    DOI： 10.1103/PhysRevB.97.214409 　

    ISSN： 2469-9969 2469-9950

  26. Cosmological abundance of the QCD axion coupled to hidden photons Peer-reviewed

    Naoya Kitajima, Toyokazu Sekiguchi, Fuminobu Takahashi

    Physics Letters B　781　684-687　2018/06

    Publisher: Elsevier BV

    DOI： 10.1016/j.physletb.2018.04.024 　

    ISSN： 0370-2693

  27. Refined study of isocurvature fluctuations in the curvaton scenario Peer-reviewed

    Naoya Kitajima, David Langlois, Tomo Takahashi, Shuichiro Yokoyama

    JOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS　12　(12)　2017/12

    DOI： 10.1088/1475-7516/2017/12/042 　

    ISSN： 1475-7516

  28. Cosmological stochastic Higgs field stabilization Peer-reviewed

    Jinn-Ouk Gong, Naoya Kitajima

    PHYSICAL REVIEW D　96　(6)　063521　2017/09

    DOI： 10.1103/PhysRevD.96.063521 　

    ISSN： 2470-0010

    eISSN： 2470-0029

  29. Small-scale structure and 21cm fluctuations by primordial black holes Peer-reviewed

    Jinn-Ouk Gong, Naoya Kitajima

    JOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS　08　(8)　2017/08

    DOI： 10.1088/1475-7516/2017/08/017 　

    ISSN： 1475-7516

  30. Curvaton as dark matter with secondary inflation Peer-reviewed

    Jinn-Ouk Gong, Naoya Kitajima, Takahiro Terada

    JOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS　03　(3)　2017/03

    DOI： 10.1088/1475-7516/2017/03/053 　

    ISSN： 1475-7516

  31. Topological defects and nano-Hz gravitational waves in aligned axion models Peer-reviewed

    Tetsutaro Higaki, Kwang Sik Jeong, Naoya Kitajima, Toyokazu Sekiguchi, Fuminobu Takahashi

    JOURNAL OF HIGH ENERGY PHYSICS　08　(8)　2016/08

    DOI： 10.1007/JHEP08(2016)044 　

    ISSN： 1029-8479

  32. Quality of the Peccei-Quinn symmetry in the aligned QCD axion and cosmological implications Peer-reviewed

    Tetsutaro Higaki, Kwang Sik Jeong, Naoya Kitajima, Fuminobu Takahashi

    JOURNAL OF HIGH ENERGY PHYSICS　06　(6)　2016/06

    DOI： 10.1007/JHEP06(2016)150 　

    ISSN： 1029-8479

  33. Level crossing between the QCD axion and an axionlike particle Peer-reviewed

    Ryuji Daido, Naoya Kitajima, Fuminobu Takahashi

    PHYSICAL REVIEW D　93　(7)　075027　2016/04

    DOI： 10.1103/PhysRevD.93.075027 　

    ISSN： 2470-0010

    eISSN： 2470-0029

  34. The QCD axion from aligned axions and diphoton excess Peer-reviewed

    Tetsutaro Higaki, Kwang Sik Jeong, Naoya Kitajima, Fuminobu Takahashi

    Physics Letters B　755　13-16　2016/04

    DOI： 10.1016/j.physletb.2016.01.055 　

    ISSN： 0370-2693

    eISSN： 1873-2445

  35. Disappearing inflaton potential via heavy field dynamics Peer-reviewed

    Naoya Kitajima, Fuminobu Takahashi

    JOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS　02　(2)　2016/02

    DOI： 10.1088/1475-7516/2016/02/041 　

    ISSN： 1475-7516

  36. Domain wall formation from level crossing in the axiverse Peer-reviewed

    Ryuji Daido, Naoya Kitajima, Fuminobu Takahashi

    PHYSICAL REVIEW D　92　(6)　063512　2015/09

    DOI： 10.1103/PhysRevD.92.063512 　

    ISSN： 1550-7998

    eISSN： 1550-2368

  37. Axion domain wall baryogenesis Peer-reviewed

    Ryuji Daido, Naoya Kitajima, Fuminobu Takahashi

    JOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS　07　(7)　2015/07

    DOI： 10.1088/1475-7516/2015/07/046 　

    ISSN： 1475-7516

  38. Degenerate spectrum in the neutrino mass anarchy with Wishart matrices and implications for 0 nu beta beta and delta(CP) Peer-reviewed

    Kwang Sik Jeong, Naoya Kitajima, Fuminobu Takahashi

    PHYSICAL REVIEW D　91　(11)　113010　2015/06

    DOI： 10.1103/PhysRevD.91.113010 　

    ISSN： 2470-0010

    eISSN： 2470-0029

  39. Gravitational waves from Higgs domain walls Peer-reviewed

    Naoya Kitajima, Fuminobu Takahashi

    PHYSICS LETTERS B　745　(18)　112-117　2015/05

    DOI： 10.1016/j.physletb.2015.04.040 　

    ISSN： 0370-2693

    eISSN： 1873-2445

  40. 26aSD-5 Domain Wall Formation from Level Crossing in the Axiverse

    Daido Ryuji, Kitajima Naoya, Takahashi Fuminobu

    Meeting Abstracts of the Physical Society of Japan　70　18-18　2015

    Publisher: The Physical Society of Japan (JPS)

    DOI： 10.11316/jpsgaiyo.70.2.0_18 　

    ISSN： 2189-079X

  41. Resonant conversions of QCD axions into hidden axions and suppressed isocurvature perturbations Peer-reviewed

    Naoya Kitajima, Fuminobu Takahashi

    JOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS　01　(1)　2015/01

    DOI： 10.1088/1475-7516/2015/01/032 　

    ISSN： 1475-7516

  42. Hidden axion dark matter decaying through mixing with QCD axion and the 3.5 keV X-ray line Peer-reviewed

    Tetsutaro Higaki, Naoya Kitajima, Fuminobu Takahashi

    JOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS　12　(12)　2014/12

    DOI： 10.1088/1475-7516/2014/12/004 　

    ISSN： 1475-7516

  43. Relaxing isocurvature bounds on string axion dark matter Peer-reviewed

    Masahiro Kawasaki, Naoya Kitajima, Fuminobu Takahashi

    PHYSICS LETTERS B　737　(7)　178-184　2014/10

    DOI： 10.1016/j.physletb.2014.08.017 　

    ISSN： 0370-2693

    eISSN： 1873-2445

  44. Thermal effects and sudden decay approximation in the curvaton scenario Peer-reviewed

    Naoya Kitajima, David Langlois, Tomo Takahashi, Tomohiro Takesako, Shuichiro Yokoyama

    JOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS　10　(10)　2014/10

    DOI： 10.1088/1475-7516/2014/10/032 　

    ISSN： 1475-7516

  45. Gravitational waves from a curvaton model with blue spectrum Peer-reviewed

    Masahiro Kawasaki, Naoya Kitajima, Shuichiro Yokoyama

    JOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS　08　(8)　2013/08

    DOI： 10.1088/1475-7516/2013/08/042 　

    ISSN： 1475-7516

  46. Heavy gravitino in hybrid inflation Peer-reviewed

    Masahiro Kawasaki, Naoya Kitajima, Kazunori Nakayama, Tsutomu T. Yanagida

    Journal of Cosmology and Astroparticle Physics　2013　(6)　2013/06

    DOI： 10.1088/1475-7516/2013/06/037 　

    ISSN： 1475-7516

  47. Primordial black hole formation from an axionlike curvaton model Peer-reviewed

    Masahiro Kawasaki, Naoya Kitajima, Tsutomu T. Yanagida

    Physical Review D　87　(6)　2013/03/18

    Publisher: American Physical Society (APS)

    DOI： 10.1103/physrevd.87.063519 　

    ISSN： 1550-7998

    eISSN： 1550-2368

  48. Smooth hybrid inflation in a supersymmetric axion model Peer-reviewed

    Masahiro Kawasaki, Naoya Kitajima, Kazunori Nakayama

    PHYSICAL REVIEW D　87　(3)　035010　2013/02

    DOI： 10.1103/PhysRevD.87.035010 　

    ISSN： 1550-7998

  49. Revisiting the cosmological coherent oscillation Peer-reviewed

    Masahiro Kawasaki, Naoya Kitajima, Kazunori Nakayama

    PHYSICAL REVIEW D　87　(2)　023513　2013/01

    DOI： 10.1103/PhysRevD.87.023513 　

    ISSN： 1550-7998

  50. Cosmological aspects of inflation in a supersymmetric axion model Peer-reviewed

    Masahiro Kawasaki, Naoya Kitajima, Kazunori Nakayama

    PHYSICAL REVIEW D　83　(12)　123521　2011/06

    DOI： 10.1103/PhysRevD.83.123521 　

    ISSN： 1550-7998

  51. Inflation from a supersymmetric axion model Peer-reviewed

    Masahiro Kawasaki, Naoya Kitajima, Kazunori Nakayama

    PHYSICAL REVIEW D　82　(12)　123531　2010/12

    DOI： 10.1103/PhysRevD.82.123531 　

    ISSN： 2470-0010

    eISSN： 2470-0029

Show all ︎Show first 5

Presentations 95

  1. Primordial black hole formation from the merger of oscillons

    Naoya Kitajima

    Dark matter and black holes　2025/12/04

  2. Multimodal axion emissions from Abelian-Higgs cosmic strings

    Naoya Kitajima

    Axions in Japan　2025/11/13

  3. ドメインウォールの非球対称崩壊による原始ブラックホール形成

    北嶋直弥

    日本物理学会第80回年次大会　2025/09/17

  4. Light particle emissions from cosmic strings Invited

    Naoya Kitajima

    The 4th International BSM Workshop: Building for Tomorrow　2025/08/25

  5. Primordial black hole formation from collapsing domain walls

    Naoya Kitajima

    New Perspectives on Cosmology 2025　2025/02/05

  6. Black hole formation from collapsing domain walls

    Naoya Kitajima

    Gravity 2025: New Horizon of Black Hole Physics　2025/01/31

  7. Gravitational relics from topological defects Invited

    Naoya Kitajima

    The International Joint Workshop on the Standard Model and Beyond 2024/3rd Gordon Godfrey Workshop on Astroparticle Physics　2024/12/09

  8. ドメインウォールによる原始ブラックホール形成

    北嶋直弥

    第13回 観測的宇宙論ワークショップ　2024/09/25

  9. ドメインウォールによる原始ブラックホール形成

    北嶋直弥

    日本物理学会 第79回年次大会　2024/09/17

  10. Cosmic string network and gravitational waves International-presentation

    Naoya Kitajima

    New Perspectives on Cosmology 2024　2024/01/12

  11. 原始ブラックホールによる非ガウス性密度揺らぎ

    北嶋直弥

    第12回 観測的宇宙論ワークショップ　2023/12/12

  12. Gravitational waves from early dark energy International-presentation

    Naoya Kitajima

    The 32nd Workshop on General Relativity and Gravitation in Japan　2023/11/30

  13. Nanohertz gravitational waves from axion domain walls coupled to QCD International-presentation Invited

    Naoya Kitajima

    KiPAS international workshop "From particle physics to supernovae and early Universe"　2023/11/23

  14. 中性子星磁場を用いたアクシオン探索 Invited

    北嶋直弥

    Aso workshop on Particle Physics and Cosmology 2023　2023/11/13

  15. Cosmic string network and dark photon dark matter International-presentation Invited

    Naoya Kitajima

    IBS CTPU-CGA 2023 Workshop on Topological Defects　2023/09/27

  16. Gravitational waves from early dark energy

    北嶋直弥

    日本物理学会 第78回年次大会　2023/09/18

  17. （一般シンポジウム講演）中性子星磁気圏におけるアクシオン暗黒物質の物理 Invited

    北嶋直弥

    日本物理学会 第78回年次大会　2023/09/16

  18. 宇宙初期のダークフォトン生成と重力波放射

    北嶋直弥

    素粒子物理学の進展２０２３　2023/09/01

  19. Cosmological production of dark photon dark matter International-presentation

    Naoya Kitajima

    XV International Conference on Gravitation, Astrophysics and Cosmology (ICGAC15)　2023/07/06

  20. Production of very light dark photon dark matter International-presentation

    Naoya Kitajima

    Workshop on Very Light Dark Matter 2023　2023/03/28

  21. 宇宙ひもによるダークフォトン生成と重力波放射

    北嶋直弥

    日本物理学会 2023年春季大会　2023/03/24

  22. 初期宇宙における軽いダークフォトンダークマターの生成

    北嶋直弥

    素粒子現象論研究会2022　2023/03/17

  23. Production of light dark photon dark matter in the early universe International-presentation

    Naoya Kitajima

    What is dark matter? - Comprehensive study of the huge discovery space in dark matter　2023/03/07

  24. Cosmic strings and dark photon dark matter International-presentation Invited

    Naoya Kitajima

    2023 Winter-I NRF-JSPS Workshop in particle physics, cosmology, and gravitation　2023/01/17

  25. 中性子星磁気圏のアクシオンダークマターによる電波放射

    北嶋直弥

    第11回観測的宇宙論ワークショップ　2022/12/12

  26. Axion dark matter in neutron star magnetosphere Invited

    Naoya Kitajima

    DM3 - Deep insights and Multiple strategies for Deciphering the Mystery of Dark Matter　2022/09/15

  27. Parametric resonance instability of the QCD axion Invited

    Naoya Kitajima

    The 6th KEK-PH + 2nd KEK-COSMO joint mini-workshop (Online)　2022/03/09

  28. 重力波で探るアクシオン宇宙 Invited

    北嶋直弥

    若手による重力・宇宙論研究会2021　2021/03/17

  29. アクシオン暗黒物質のnHz重力波シグナル

    北嶋直弥

    日本物理学会 第76回年次大会　2021/03/13

  30. Primordial Black Holes from QCD Axion Bubbles

    Naoya Kitajima

    Online JGRG workshop 2020　2020/11/26

  31. アクシオン暗黒物質のnHz重力波シグナル

    北嶋直弥

    第９回観測的宇宙論ワークショップ　2020/11/10

  32. Primordial Black Holes from QCD Axion Bubbles

    Naoya Kitajima

    The 14th International Conference on Gravitation, Astrophysics and Cosmology　2020/08/18

  33. 宇宙初期のアクシオンの振動によるゲージ場生成および重力波生成

    北嶋直弥

    日本物理学会第75回年次大会　2020/03/19

  34. アクシオンの宇宙論と検出実験 Invited

    北嶋直弥

    第8回観測的宇宙論ワークショップ　2019/12/16

  35. Dark Matter in the Universe and Axion

    Kitajima Naoya

    4th FRIS retreat　2019/11/28

  36. Gravitational waves from axionic gauge field production International-presentation

    Kitajima Naoya

    JGRG29　2019/11/25

  37. Dark matter in the Universe and Axion

    Kitajima Naoya

    4th FRIS/DIARE Joint Workshop　2019/08/06

  38. AXION理論 Invited

    北嶋直弥

    ダークマターの懇談会2019　2019/07/05

  39. Gravitational wave forest from axons International-presentation

    Kitajima Naoya

    Axion Cosmology　2019/05/14

  40. 中性子星の電波観測によるアクシオン暗黒物質探索

    北嶋直弥

    日本物理学会第74回年次大会　2019/03/14

  41. Dark photon dark matter production from axion oscillations International-presentation

    Kitajima Naoya

    Accelerating Universe in the Dark　2019/03/04

  42. Axion dark matter in a neutron star magnetosphere International-presentation

    Kitajima Naoya

    FAPESP-JSPS Workshop　2019/02/18

  43. 中性子星の電波観測によるアクシオン暗黒物質探索

    北嶋直弥

    第３１回理論懇シンポジウム　2018/12/19

  44. Dark photon dark matter from axion oscillations International-presentation

    Kitajima Naoya

    KEK-PH　2018/12/04

  45. 中性子星の電波観測によるアクシオン暗黒物質探索

    北嶋直弥

    第７回観測的宇宙論ワークショップ　2018/11/12

  46. Distribution of primordial black holes and 21cm signature International-presentation

    Kitajima Naoya

    JGRG28　2018/11/05

  47. ストリングアクシオンによる重力波生成

    北嶋直弥

    日本物理学会2018年秋季大会　2018/09/14

  48. Gravitational wave forest from string axiverse International-presentation

    Kitajima Naoya

    COSMO18　2018/08/27

  49. Comology of the (QCD) axion coupled with hidden photons

    Kitajima Naoya

    2018/08/06

  50. 空洞共振器によるアクシオン暗黒物質探索

    北嶋直弥

    日本物理学会第73回年次大会　2018/03/22

  51. Gravitational wave emission and oscillon formation in string axiverse International-presentation

    Kitajima Naoya

    KEK-PH　2018/02/13

  52. Small-scale structure and 21cm fluctuations by prmordial Black holes International-presentation

    Kitajima Naoya

    CosPA 2017　2017/12/11

  53. Cosmological Higgs stabilization by a stochastic spectator field International-presentation

    Kitajima Naoya

    Workshop on Beyond Standard Model and the Early Universe　2017/10/25

  54. 原始ブラックホールによるミニハロー形成と21cm線観測

    北嶋直弥

    第6回観測的宇宙論ワークショップ　2017/10/23

  55. Cosmological stochastic Higgs stabilization International-presentation

    Kitajima Naoya

    COSMO-17　2017/09/01

  56. Small-scale structure and 21cm fluctuations by Primordlal Black Holes International-presentation Invited

    Kitajima Naoya

    13th Recontres du Vietnam, Cosmology　2017/07/09

  57. Study of Axion Dark Matter Detection Using Josephson Junctions International-presentation

    Kitajima Naoya

    The 3rd International Symposium on "Quest for the Origin of Particle and the Universe"　2017/01/05

  58. Topological defects in aligned axion models International-presentation

    Kitajima Naoya

    The 5th UTQuest workshop　2016/12/12

  59. Aligned QCD axion International-presentation

    Kitajima Naoya

    The 6th KIAS workshop on Particle Physics and Cosmology　2016/10/24

  60. Topological Defects and Gravitational Waves from Aligned Axions International-presentation

    Kitajima Naoya

    International Workshop for String theory and Cosmology 2016　2016/08/17

  61. Revisiting CDM isocurvature fluctuations in curvaton scenario International-presentation Invited

    Kitajima Naoya

    3rd Korea-Japan Workshop on Dark Energy　2016/04/04

  62. Cosmological implications of level crossing between QCD axion and ALP International-presentation

    Kitajima Naoya

    APCTP 2015 Workshop on Frontiers of Physics　2015/12/20

  63. Disappearing Inflaton Potential International-presentation

    Kitajima Naoya

    JGRG25　2015/12/07

  64. Cosmological implications of level crossing between QCD axion and ALP International-presentation Invited

    Kitajima Naoya

    IBS-PNU Joint Workshop on Particle Physics, String Theory and Cosmology　2015/12/02

  65. Cosmological implications of the level crossing between QCD axion and ALP International-presentation

    Kitajima Naoya

    KIAS-CFHEP Workshop and the 5th KIAS workshop on particle physics and cosmology　2015/11/10

  66. Disappearing Inflaton Potential International-presentation

    Kitajima Naoya

    The origin and evolution of the Universe　2015/10/23

  67. Domain wall formation and baryogenesis in the Axiverse International-presentation

    Kitajima Naoya

    CosPA 20175　2015/10/12

  68. Disappearing Inflaton Potential via Heavy Field Dynamics International-presentation

    Kitajima Naoya

    International Workshop on Particle Physics and Cosmology　2015/09/14

  69. Domain wall formation via “Axion Roulette” and axion domain wall baryogenesis International-presentation

    Kitajima Naoya

    COSMO-15　2015/09/07

  70. Domain wall in the Axiverse International-presentation Invited

    Kitajima Naoya

    APCTP-TUS Joint Workshop　2015/08/03

  71. Axion domain wall baryogenesis International-presentation

    Kitajima Naoya

    Gordon Research Conferences String Theory & Cosmology　2015/05/31

  72. Resonant conversion of QCD axions into hidden axions and suppressed isocurvature perturbations

    Kitajima Naoya

    2015/03/21

  73. Gravitational waves from Higgs domain walls International-presentation

    Kitajima Naoya

    B-mode Cosmology　2015/02/21

  74. Resonant conversion of QCD axions into hidden axions International-presentation

    Kitajima Naoya

    KEK Theory Workshop　2015/01/29

  75. Degenerate spectrum in the neutrino mass anarchy with Wishart matrices and implications for 0νββ & δCP

    Kitajima Naoya

    2015/01/10

  76. Resonant conversion of QCD axions into hidden axions International-presentation

    Kitajima Naoya

    JGRG24　2014/11/10

  77. Resonant conversion of QCD axions into hidden axions and suppressed isocurvature perturbations International-presentation

    Kitajima Naoya

    4th KIAS Workshop on Particle Physics and Cosmology　2014/10/30

  78. Hidden axion dark matter (decaying through) mixing with QCD axion International-presentation

    Kitajima Naoya

    IBS MultiDark Joint Focus Program: WIMPs and Axions　2014/10/20

  79. Thermal inflation and isocurvature bound on string axion dark matter International-presentation

    Kitajima Naoya

    COSMO-14　2014/08/24

  80. Thermal inflation and isocurvature bound on string axion dark matter

    Kitajima Naoya

    2014/07/28

  81. 21cm signature from cosmic string-induced minihaloes

    Kitajima Naoya

    2013/12/25

  82. Gravitational waves from a curvaton model with blue spectrum International-presentation

    Kitajima Naoya

    PASCOS2013　2013/11/20

  83. Gravitational waves from curvaton International-presentation

    Kitajima Naoya

    CosPA 2013　2013/11/12

  84. 21cm signature of minihaloes from cosmic string wakes International-presentation

    Kitajima Naoya

    JGRG23　2013/11/05

  85. Gratational waves from a curvaton model with blue spectrum

    Kitajima Naoya

    2013/09/20

  86. Heavy gravitino in hybrid inflation

    Kitajima Naoya

    2013/08/05

  87. Gravitational waves from a curvaton model with blue spectrum International-presentation

    Kitajima Naoya

    YKIS2034　2013/06/02

  88. Smooth hybrid inflation in a supersymmetric axion model

    Kitajima Naoya

    2013/03/26

  89. Smooth hybrid inflation in a supersymmetric axion model International-presentation

    Kitajima Naoya

    KEK-PH2013　2013/03/04

  90. Primordial black hole formation from an axion-like curvaton model International-presentation

    Kitajima Naoya

    COSMO-12　2012/09/10

  91. Primordial black hole formation in an axion-like curvaton model International-presentation

    Kitajima Naoya

    SI2012　2012/08/18

  92. Primordial black hole formation from an axion-like curvaton model

    Kitajima Naoya

    2012/07/18

  93. Revising the cosmological coherent oscillation

    Kitajima Naoya

    2012/03/24

  94. Cosmological aspects of inflation in a supersymmetric axion model

    Kitajima Naoya

    2011/09/16

  95. Inflation from a Supersymmetric Axion Model International-presentation

    Kitajima Naoya

    KEK-PH 2011　2011/03/02

Show all Show first 5

Research Projects 14

  1. 宇宙初期における位相欠陥の一 般相対論的シミュレーション

    北嶋直弥, 神田行宏, 藤林翔, 滝沢 寛之

    Offer Organization: 学際大規模情報基盤共同利用・共同研究拠点

    System: 2025年度 学際大規模情報基盤共同利用・共同研究拠点(JHPCN)公募型共同研究課題

    Institution: 東北大学

    2025/04 - 2026/03

  2. 宇宙初期における位相欠陥の大規模格子シミュレーショ ン

    北嶋直弥

    Offer Organization: 一般財団法人高度情報科学技術研究機構

    System: 令和7年度HPCIシステム利用研究課題

    Category: 「富岳」一般課題（A期）

    Institution: 東北大学

    2025/04 - 2026/03

  3. 強磁場高密度天体で探るアクシオン暗黒物質

    北嶋 直弥, 木坂 将大

    Offer Organization: 日本学術振興会

    System: 科学研究費助成事業

    Category: 基盤研究(B)

    Institution: 東北大学

    2021/04/01 - 2025/03/31

    More details Close

    本研究では我々の宇宙における暗黒物質の有力候補であるアクシオンに焦点を当て、近傍宇宙に点在する強磁場高密度天体の電磁波観測によってアクシオン暗黒物質の間接検出が可能かどうかを理論的に検証することを目指している。特に中性子星に代表される天体周りの磁場構造、およびプラズマのダイナミクスに関する最先端の知見を取り入れ、それら天体の磁気圏でアクシオンが引き起こす特異な電磁気現象をシミュレーションを駆使して詳細に解析し、期待されるアクシオン由来の電磁波シグナルを定量的に評価することで観測可能性を明らかにすることを目的としている。以下、研究代表者の北嶋と研究分担者の木坂が2022年度に行なった研究の具体的内容を記す。 代表者の北嶋は、中性子星磁気圏におけるアクシオン・光子変換過程についての詳細な計算を行い、特に磁気圏における現実的なプラズマ粒子数分布を考慮した新たな効果を取り入れて、アクシオン暗黒物質の運動に起因する電波放射の角度依存性、シグナルの時間依存性を明らかにした。分担者の木坂は、中性子星磁気圏からのアクシオン起源の電波放射の検出に向け、その妨げとなる中性子星固有の電波放射が起こらないと想定される条件を満たす中性子星磁気圏のプラズマ状態に対して粒子シミュレーションを行い、そのパラメータを明らかにした。北嶋、木坂の各々の研究は相補的であり、双方の結果を組み合わせることで相乗効果が生じる。実際、代表者の北嶋と分担者の木坂は研究協力者である門田、當真とともに１、２ヶ月に一度程度の研究打ち合わせをリモートで行うことで緊密な連携を取り続けており、その結果、北嶋が担当するアクシオン・光子変換の解析と木坂が担当する中性子星磁気圏のプラズマのダイナミクスの効果を融合する段階まで達している。

  4. 宇宙初期における位相欠陥の大規模格子シミュレーショ ン

    北嶋直弥

    Offer Organization: 一般財団法人高度情報科学技術研究機構

    System: 令和6年度HPCIシステム利用研究課題

    Category: 「富岳」一般課題（A期）

    Institution: 東北大学

    2024/04 - 2025/03

  5. Axion dark matter search with the world's largest radio telescope SKA

    Offer Organization: Japan Society for the Promotion of Science

    System: Grants-in-Aid for Scientific Research

    Category: Fund for the Promotion of Joint International Research (Fostering Joint International Research (B))

    Institution: High Energy Accelerator Research Organization

    2021/10 - 2025/03

  6. 強磁場高密度天体で探るアクシオン暗黒物質

    北嶋 直弥, 木坂 将大

    Offer Organization: 日本学術振興会

    System: 科学研究費助成事業

    Category: 基盤研究(B)

    Institution: 東北大学

    2021/04 - 2025/03

    More details Close

    本研究では我々の宇宙における暗黒物質の有力候補であるアクシオンに焦点を当て、近傍宇宙に点在する強磁場高密度天体の電磁波観測によってアクシオン暗黒物質の間接検出が可能かどうかを理論的に検証することを目指している。特に中性子星、白色矮星といった天体周りの磁場構造およびプラズマのダイナミクスに関する最先端の知見を取り入れ、それら天体の磁気圏でアクシオンが引き起こす特異な電磁気現象をシミュレーションを駆使して詳細に解析し、期待されるアクシオン由来の電磁波信号を定量的に評価することで観測可能性を明らかにすることを目的としている。以下、研究代表者の北嶋と研究分担者の木坂が2021年度に行なった研究の具体的内容を記す。 代表者の北嶋はアクシオンと電磁場の相互作用系をコンピュータ上の格子空間で解く数値シミューションコードの開発を行なった。当初の計画であった、曲がった背景時空の効果や、現実に則したアクシオンの速度分布を取り入れたコード開発は、テストシミュレーションも含め、成功している。また、アクシオンの自己重力相互作用の効果も取り入れ、ソリトンを形成している場合についてのシミュレーションにも成功した。 分担者の木坂はアクシオン-光子の変換率に対して中性子星磁気圏の現実的なプラズマの状態を考慮した結果、粒子数だけでなくこれまで考えられていないその温度が重要であることを特定した。また、実際の電波観測を想定して観測の研究者との打ち合わせを通して変換率の制限として重要となるターゲットの選定を行なった。 代表者の北嶋と分担者の木坂は１、２ヶ月に一度程度の研究打ち合わせをリモートで行い、緊密な連携を取り続けている。その結果、北嶋が開発したシミュレーションと木坂が指摘する中性子星磁気圏の粒子数・温度の効果を融合する段階まで達した。本成果は速やかに論文にまとめ、発表する予定である。

  7. A theoretical study of the generation and evolution of light dark matter

    Offer Organization: Japan Society for the Promotion of Science

    System: Grants-in-Aid for Scientific Research

    Category: Grant-in-Aid for Transformative Research Areas (A)

    Institution: Tohoku University

    2020/11 - 2025/03

  8. 極弱相互作用を持つ軽い新粒子の描く新宇宙描像とその実験的検証

    高橋 史宜, 石渡 弘治, 北嶋 直弥

    Offer Organization: 日本学術振興会

    System: 科学研究費助成事業

    Category: 基盤研究(B)

    Institution: 東北大学

    2020/04 - 2025/03

    More details Close

    高橋は，現在の宇宙のバリオン非対称性を説明するひとつのシナリオとして知られていた，インフレーションが生じる前に非常に大きなバリオン非対称性を生成するシナリオに着目し，このようなシナリオにおいては断熱揺らぎと相関を持つバリオン等曲率揺らぎが大きくなりすぎることを発見した。これまでは原理的にはインフレーション以前に十分大きなバリオン数を用意すれば現在の宇宙に存在するバリオン非対称性を説明できると考えられていた。この結果，インフレーション以前にバリオン生成するシナリオは完全に棄却され，インフレーション後にバリオン非対称性を生成しなくてはならないことが初めて論理的に示すことができた。更にアクシオンの振動エネルギーがモノポールのWitten効果とシュウィンガー効果によって散逸することを指摘し，アクシオンダークマターに関するシナリオに示唆を与えた。石渡は宇宙観測と整合性の高い超共形ハイブリッドインフレーションモデルを拡張し、インフレーション後にレプトン数を生成する機構を考えた。保存量子数の伝搬の解析により、残存するバリオン数が現在の観測量と整合することを明らかにした。北嶋はHubble tensionに動機づけられたEarly dark energyのモデルとして極弱粒子であるアクシオンに着目し、自己共鳴現象による重力波生成を格子シミュレーションにより明らかにした。特に、宇宙背景放射の観測によりモデルの検証が可能であることを明らかにした。

  9. Deciphering string axiverse from multi-wavelength observations of gravitational waves

    Offer Organization: Japan Society for the Promotion of Science

    System: Grants-in-Aid for Scientific Research

    Category: Grant-in-Aid for Scientific Research (B)

    2019/04 - 2023/03

  10. Theoretical study towards the indirect detection of the axion dark matter by the observation of neutron star

    Naoya Kitajima

    Offer Organization: Japan Society for the Promotion of Science

    System: Grants-in-Aid for Scientific Research

    Category: Grant-in-Aid for Early-Career Scientists

    Institution: Tohoku University

    2019/04 - 2022/03

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    In this research project, I have focused on a hypothetical particle “axion” as one of the plausible candidates for the dark matter in our Universe and performed theoretical analysis towards the possibility of indirect detection of the axion with the observation of the neutron star. Particularly focusing on the tiny interaction of the axion with the electromagnetic fields around the neutron star, I have developed numerical simulation code to follow the dynamics of the system. This code has also been applied for the analysis of resonance phenomena of the axion and gauge fields in the early Universe and consequently we have pointed out a new possibility of the production of the dark photon dark matter and gravitational waves.

  11. Search for dark matter axions using superconducting junctions

    Kashiwaya Satoshi

    Offer Organization: Japan Society for the Promotion of Science

    System: Grants-in-Aid for Scientific Research

    Category: Grant-in-Aid for Scientific Research (B)

    Institution: Nagoya University

    2018/04 - 2021/03

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    We aim to establish the basic concept of the interaction between Josephson junctions and axion, and to establish a method for searching for axion by using only laboratory-based devices. Initially, the experiment was carried out by detecting the signal related to the axion from the conductance of the Josephson junctions, but the structure originated by the axion was not observed. Therefore, we proceeded with theoretical analysis and experiments based on the resonantly generated magnetic field from the rf current. It was shown that the generated magnetic field resonantly increases by matching the rf frequency and the axion frequency, and can be measured by high-sensitivity magnetic field detection such as SQUID. Currently, we are developing a device with a unique structure that combines SQUID and Josephson junctions, and are now proceeding with measurements.

  12. アクシオンと初期宇宙シナリオに関する研究

    北嶋 直弥

    Offer Organization: 日本学術振興会

    System: 科学研究費助成事業

    Category: 特別研究員奨励費

    Institution: 名古屋大学

    2017/04 - 2020/03

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    平成30年度における私の研究として、宇宙初期におけるアクシオンの共鳴現象から重力波が生成される可能性を指摘し、数値シミュレーションを駆使して生成される重力波強度を定量的に評価した。特にアクシオンの自己相互作用によるパラメター共鳴とそれに伴う重力波生成を解析するための３次元格子シミュレーションコードの独自開発に成功し、新しい結果を出せたことが成果として挙げられる。また、将来の多波長重力波観測を見据え、アクシオン宇宙が予言する重力波フォレストという概念を新たに提唱したことも成果の一つである。 また、アクシオンとゲージ場が相互作用するモデルについて、宇宙初期のゲージ場の共鳴的生成現象を詳細に解析した研究も実績として挙げられる。本研究ではゲージ場の指数関数的成長という極めて非線形な発展を追うために、アクシオンとゲージ場の相互作用を組み込んだ格子シミュレーションを独自開発し、最終的にアクシオンやゲージ場（ダークフォトン）の暗黒物質としての残存量などを計算した。特に本研究はダークフォトンで現在の暗黒物質の存在量を説明する新しい初期宇宙シナリオを提案したものであり、将来のダークフォトン検出実験に対し、理論から新たな指針を与えることに成功した。 また、原始ブラックホールが暗黒物質として寄与するシナリオにも着目し、原始ブラックホール形成に伴って新たに生じる密度ゆらぎに関する詳細な解析も研究実績の一つである。特に先行研究ではゆらぎはポアソン分布に従うものと考えられてきたが、本研究では対数正規分布に従うゆらぎが生じ得ることを指摘した。研究手法は格子シミュレーションを用いた数値解析で、形成された原始ブラックホールの数を計測する新たな手法を確立させた。また、この新しいタイプのゆらぎが後の宇宙の小規模構造形成に影響し、将来の21cm線観測によってそのシグナルが検出される可能性も議論した。

  13. Fundamental Study of Dark Matter Axion Detection using Quantum Interference in the Josephson Device

    Naka Tatsuhiro, Takeuchi Yuji, KITAJIMA Naoya, SHIRAISHI Takuya

    Offer Organization: Japan Society for the Promotion of Science

    System: Grants-in-Aid for Scientific Research

    Category: Grant-in-Aid for Challenging Exploratory Research

    Institution: Nagoya University

    2016/04 - 2018/03

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    The motivation of this research is to make new methodology for the dark matter axion, specially to study about the detection method using phase synchronize effect between cooper-pair in a superconducting device and the axion. In this term, we made the Josephson device with Nb/Al/Nb and measured the data of dV/dI-V character. As result, noise level was high, but we could see the signal that is expected as the Andreev reflection. By that, we clarified the subject about future task, and it was progress as new collaboration with some scientists. Also, we started new topics as new collaboration with material scientists beginning of this study.

  14. 超対称性アクシオン模型の宇宙論

    北嶋 直弥

    Offer Organization: 日本学術振興会

    System: 科学研究費助成事業

    Category: 特別研究員奨励費

    Institution: 東京大学

    2012 - 2013

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    平成25年度における私の研究として、カーバトン模型という一つの初期宇宙模型に着目し、この模型を将来の重力波観測によって検出する可能性などを議論した。 本研究ではインフレーション中にインフラトンに加えて「カーバトン」というスカラー場が存在するシナリオに着目した。このような状況は超重力理論の枠組みにおいて自然と実現され、特に質量がインフレーション中のハッブルスケールと同じくらいであるようなカーバトンが予言されることが多々ある。私はこのようなカーバトンにおいて曲率ゆらぎのパワースペクトルが小スケールで大きくなるという特徴に着目し、インフレーション後にスカラーゆらぎの2次の効果から重力波が生成されるという先行研究を応用して、このシナリオで予言される重力波が将来の重力波観測によって検出されうるかどうかを議論した。特に具体的なカーバトン模型として2次のポテンシャルをもつシンプルな模型と超対称性アクシオン模型に基づく模型において計算を行った。結果として、LISA, DECIGOなどの将来の直接検出実験や大型電波望遠鏡SKAによるパルサータイミング実験においてこのシナリオの痕跡を見ることが出来ることを示した。宇宙背景放射の観測では実証することの難しい初期宇宙模型を重力波観測で捉えるという点が本研究の特徴であり、非常に意義深いと言える。特に興味深いのは、この模型において原始ブラックホールが形成され、これによって銀河中心の超大質量ブラックホールを説明出来るようなパラメターを選んだときに、将来の重力波観測による検出が予言されるということである。

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