Workshop on Very Light Dark Matter 2023

Mario Royal Kaikan (Chino, Nagano) + Online

Mario Royal Kaikan (Chino, Nagano) + Online


Workshop on Very Light Dark Matter 2023

Date: March 28-30, 2023

Venue: Mario Royal Kaikan (Chino, Nagano) + Online (Zoom)

The present universe is filled with dark matter. Although it is still unclear what dark matter is made of, it is known to have played an extremely important role in the formation of the structure of the universe such as galaxies and galaxy clusters. Recently, dark matter with extremely low mass has attracted much attention. Characteristic signatures of some candidates (e.g. axions and dark photons) in the very early universe have also been vigorously discussed.

Following the success of VLDM2021, in this workshop, recent experimental results as well as future prospects, and theoretical progress of very light dark matter will be discussed. We will have invited talks by experts in the related fields, as well as contributed talks. One of the aims of this workshop is to deepen the understanding of recent developments and future prospects in this field through discussions among the participants. The following is a list of representative topics.

  • Axion and axion like particles
  • Dark photon and other light dark matter
  • Light dark matter search experiments
  • Black hole superradiance
  • Cosmic birefringence
  • Structure formation
  • Weak gravity conjecture


Invited Speakers:
Nancy Aggarwal (Northwestern University)
Hiroki Fujimoto (University of Tokyo)
Mariangela Lisanti (Princeton University)
Marta Monelli (Max-Planck-Institute for Astrophysics)
Toshifumi Noumi (Kobe University)
Yuka Oshima (University of Tokyo)
Alexandros Papageorgiou (IBS-CTPU)
Konstantin Springmann (Technical University of Munich)

Code of Conduct:
It is the policy of the Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU) that all members and visitors will conduct themselves in a professional manner that is welcoming and free from any form of discrimination, harassment, or retaliation. Members and visitors will avoid any inappropriate actions or statements based on individual characteristics such as age, race, ethnicity, gender, sexual orientation, marital status, nationality, or political affiliation. Disruptive or harassing behavior of any kind will not be tolerated. Harassment includes but is not limited to inappropriate or intimidating behavior and language, unwelcome jokes or comments, unwanted touching or attention, offensive images, photography without permission, and stalking. Members and visitors will treat each other with respect to create a collegial, inclusive, and professional environment at the Kavli IPMU.


Elisa Ferreira (Kavli IPMU)
Tomohiro Fujita (Waseda / RESCEU)
Motoko Fujiwara (TUM)
Nagisa Hiroshima (Toyama / RIKEN)
Naoya Kitajima (Tohoku)
Eiichiro Komatsu (MPA / Kavli IPMU)
Yuta Michimura (Caltech / RESCEU)
Ippei Obata (Kavli IPMU)
Maresuke Shiraishi (SUS)
Fuminobu Takahashi (Tohoku)
Yuko Urakawa (KEK)
Masaki Yamada (Tohoku)
Wen Yin (Tohoku)

Supported by:
JSPS KAKENHI No. JP20H05851 (PI: Fuminobu Takahashi)
JSPS KAKENHI No. JP20H05852 (PI: Kohta Murase)
JSPS KAKENHI No. JP20H05854 (PI: Yuta Michimura)
JSPS KAKENHI No. JP20H05859 (PI: Eiichiro Komatsu)
JSPS KAKENHI No. JP19K14718 (PI: Maresuke Shiraishi)
JSPS KAKENHI No. JP22K14035 (PI: Nagisa Hiroshima)

file file

Registration and abstract submission
    • 10:30 11:00
      Welcome reception
    • 11:00 11:05
      Opening remarks
      Convener: Fuminobu Takahashi (Tohoku Univesity)
    • 11:05 12:45
      Morning session-1
      Convener: Ippei Obata
      • 11:05
        Dark Matter and Quantum Gravity 40m

        Recent studies in the Swampland Program are pointing a possibility that quantum gravity constraints can be used to curve out the vast theory space of dark matters theoretically. In this talk, I will introduce our recent attempts to this direction based on unitarity of gravitational scattering amplitudes. In particular, I will discuss dark photon scenarios as an illustrative example and also provide prospects on applications to other dark matter scenarios.

        Speaker: Toshifumi Noumi
      • 11:45
        Fuzzy Vector Dark Matter 20m

        We will review the recent progress in the understanding of the non-relativistic weak-gravity dynamics of massive dark vector fields and whether they can play the role of dark matter in the late universe. We will also discuss different production mechanisms of vector dark matter in the primordial universe.

        Speaker: Kaloian Lozanov
      • 12:05
        Production of very light dark photon dark matter 20m

        In this talk, I will show several production mechanisms of light dark photon dark matter in the early universe. It contains (i) resonant dark production from axion oscillation, (ii) dark photon emission form the collapse of cosmic string loops, (iii) misalignment production of coherently oscillating dark photons. Observational implications will also be discussed.

        Speaker: Naoya Kitajima
      • 12:25
        Stability of domain wall network with initial inflationary fluctuations, and its implications for cosmic birefringence 20m

        We study the formation and evolution of domain walls with initial inflationary fluctuations by numerical lattice calculations, correctly taking into account correlations on superhorizon scales. We find that, contrary to the widely-held claim, the domain wall network exhibits remarkable stability even when the initial distribution is largely biased toward one of the minima. This is due to the fact that the domain wall network retains information about initial conditions on superhorizon scales, and the scaling solution is not a local attractor in this sense. Applying this result to the axion-like particle domain wall, we show that it not only explains the isotropic cosmic birefringence suggested by the recent analysis but also predicts anisotropic cosmic birefringence that is nearly scale-invariant on large scales and can be probed by future CMB observations.

        Speaker: Diego Gonzalez
    • 12:45 14:00
      Lunch 1h 15m
    • 14:00 15:00
      Afternoon session-1
      Convener: Tomohiro Fujita
      • 14:00
        The evolution of domain wall network with inflationary fluctuations under the asymmetric potential 20m

        Domain wall networks can induce cosmological problems. Previous work has shown that networks with initial inflationary fluctuations are long-lived despite being under population bias. This is due to large scale correlations, which cannot be seen in white noise case. However, the fate of these networks under potential bias is not well-understood. In our research, we investigate the time evolution of such networks under the linear potential bias using 2 dimensional lattice simulations. We show that the networks have a finite lifespan and collapse when the pressure becomes comparable to the tension they experience. Specifically, we find the relation between the lifetime and the bias strength. This implies that the cosmic domain wall problem is avoidable in certain scenarios.

        Speaker: Junseok Lee
      • 14:20
        Weak lensing and cosmic birefringence 20m

        Cosmic birefringence a newly reported signal from CMB and promising probe for new physics. In this talk, I am going to talk about weak lensing correction to Cosmic Birefringence signal. It will be important to use highly improved data by next generated CMB observations in the near future.

        Speaker: Fumihiro Naokawa
      • 14:40
        Cosmic birefringence tomography with late-time polarization signals 20m

        In this presentation, I consider polarization signals after the reionization epoch to constrain rotation angles of CMB polarization plane for the cosmic birefringence tomography. The late-time polarization signals, the so-called polarized SZ (pSZ) effect, arises in the process of the scattering of the cosmic microwave background (CMB) photons off the electrons. The pSZ signals can be reconstructed by combining CMB and galaxy surveys. I will present expected constraints on the rotation angles with future CMB experiments from pSZ signals and discuss potential application for constraining late-time cosmic birefringence.

        Speaker: Toshiya Namikawa
    • 15:00 15:20
      Coffee break 20m
    • 15:20 16:40
      Afternoon session-2
      Convener: Yuko Urakawa
      • 15:20
        Probing Axions through Tomography of Anisotropic Cosmic Birefringence 20m

        Cosmic birefringence is the in-vacuo rotation of the linear polarization plane experienced by photons of the Cosmic Microwave Background (CMB) radiation when theoretically well-motivated parity-violating extensions of Maxwell electromagnetism are considered. If the angle parametrizing such a rotation is dependent on the photon's direction, then this phenomenon is called Anisotropic Cosmic Birefringence (ACB). We have performed for the first time a tomographic treatment of the ACB, by considering photons emitted both at the recombination and reionization epoch. This allows one to extract additional and complementary information about the physical source of cosmic birefringence with respect to the isotropic case. We have focused on the case of an axion-like field, whose coupling with the electromagnetic sector induces such a phenomenon, by using an analytical and numerical approach (which involves a modification of the CLASS code). We have found that the anisotropic component of cosmic birefringence exhibits a peculiar behavior: an increase of the axion mass implies an enhancement of the anisotropic amplitude, allowing to probe a wider range of masses with respect to the purely isotropic case. Moreover, have we shown that at large angular scales, the interplay between the reionization and recombination contributions to ACB is sensitive to the axion mass, so that at sufficiently low multipoles, for sufficiently light masses, the reionization contribution overtakes the recombination one, making the tomographic approach to cosmic birefringence a promising tool for investigating the properties of this axion-like field.

        Speaker: Alessandro Greco
      • 15:40
        Search for vector dark matter in microwave cavities with Rydberg atoms 20m

        In this talk, I present a proposal of an experiment based on 1) the application of a strong electric field inside a microwave cavity and 2) electrometry using Rydberg atoms. This kind of experiment could be extremely useful at detecting dark photons through the stationary electric field filling the whole space it induces. The sensitivity of this experiment is quite constant over a significant frequency range, and looks overall independent of the mode of the cavity. We show that this experiment could improve the current constraint on the coupling constant of the dark photons to Standard Model photons around the tenth of μeV mass range. The main limiting factor on the sensitivity of the experiment is surprisingly not the quality factor of the cavity but mainly the amplitude stability of the applied field.

        Speaker: Jordan Gué
      • 16:00
        Impact of HWP systematics on the measurement of cosmic birefringence from CMB polarization 40m

        Employing an half-wave plate (HWP) as polarization modulator is a promising strategy to mitigate systematics in CMB experiments. However, if not calibrated well enough or accounted for correctly in the analysis, its non-idealities represent an additional source of systematics that can impact the science extracted from CMB. In this talk, I will show how their effect on the observed angular power spectra is partially degenerate with cosmic birefringence, according to both simulations and analytical computations.

        Speaker: Marta Monelli
    • 16:40 17:00
      Coffee break 20m
    • 17:00 18:20
      Afternoon session-3
      Convener: Motoko Fujiwara
      • 17:00
        How axions change stars 40m

        Lighter than expected QCD axions can get destabilized in sufficiently dense and large objects such as white dwarfs and neutron stars. Once the axion is sourced the mass of nucleons within the star is reduced, leading to a new ground state of nuclear matter. I will show that white dwarfs in this absolutely stable phase would look very different from what is observed, allowing us to set novel and strong constraints in unexplored axion parameter space. Furthermore, I will show how this new ground state changes the stellar composition of neutron stars.

        Speaker: Konstantin Springmann
      • 17:40
        Probing ultra-light dark matter with lensed gravitational waves 20m

        Probing cosmic structures with gravitational wave lensing Just like light, gravitational waves (GWs) are deflected and magnified by the large-scale structure of the Universe. Their low frequency, phase coherence and capacity to propagate with no absorption make GW lensing highly complementary to gravitational lensing of electromagnetic radiation. I will discuss the framework of lensing in the wave optics regime and describe some of the opportunities that strong lensing of GWs will open to probe the matter distribution, including novel tests for ultra-light dark matter. The rich phenomenology of GW lensing will greatly benefit with the rapidly increasing rate of GW detections.

        Speaker: Miguel Zumalacarregui
      • 18:00
        Motion of S2 and bounds on scalar clouds around SgrA* 20m

        The motion of S2, one of the stars closest to the Galactic Centre, has been measured accurately and used to study the compact object at the centre of the Milky Way. It is commonly accepted that this object is a supermassive black hole but the nature of its environment is open to discussion. Here, we investigate the possibility that dark matter in the form of an ultralight scalar field “cloud” clusters around Sgr A*. We use the available data for S2 to perform a Markov Chain Monte Carlo analysis and find the best-fit estimates for a scalar cloud structure. Our results show no substantial evidence for such structures. When the cloud size is of the order of the size of the orbit of S2, we are able to constrain its mass to be smaller than 0.1% of the central mass, setting a strong bound on the presence of new fields in the galactic centre.

        Speaker: Arianna Foschi
    • 18:20 18:30
      Information 10m
      Speaker: Maresuke Shiraishi
    • 09:00 10:20
      Morning session-1
      Convener: Elisa Ferreira
      • 09:00
        Stochastic Properties of Ultralight Scalar Fields 40m

        Ultralight axionlike particles are well-motivated dark matter candidates that are the target of numerous direct detection efforts. In the vicinity of the Solar System, such particles can be treated as oscillating scalar fields. The velocity dispersion of the Milky Way determines a coherence time of about
        oscillations, beyond which the amplitude of the axion field fluctuates stochastically. Any analysis of data from an axion direct detection experiment must carefully account for this stochastic behavior to properly interpret the results. This is especially true for experiments sensitive to the gradient of the axion field that are unable to collect data for many coherence times. Indeed, the direction, in addition to the amplitude, of the axion field gradient fluctuates stochastically. I will present a stochastic treatment for the gradient of the axion field, which can be applied to any axion signal, regardless of coherence time. Additionally, I will show that ignoring the stochastic behavior of the gradient of the axion field can potentially result in failure to discover a true axion signal.

        Speaker: Mariangela Lisanti
      • 09:40
        Gravitational impact of Supernovae in Ultra-light Axion Dark Matter halos 20m

        One of the viable dark matter models is the Ultra-light Scalar field Dark Matter where it assumes an ultra-light axion-like particle of mass 10^(-22)eV. Current cosmological (dark matter only) simulations reveal that the dark matter halos formed in this model have a unique wave-like structure and a characteristic soliton at their centers. In this talk, I will discuss how these inners solitons are modified when we account for some violent baryonic feedback effects using a semi-anlytical model to describe Supernovae feedback. I will then show how constraints from local dwarf galaxy observations may be affected.

        Speaker: Victor H. Robles
      • 10:00
        Tidal Love numbers for environmental black-hole mergers 20m

        Tidal Love numbers describe the deformability of compact objects under the presence of external tidal perturbations, and are found to be exactly zero for black holes. We discuss about the scenario of black holes dressed with an ultralight scalar field, addressing their detectability at future gravitational wave experiments.

        Speaker: Valerio De Luca
    • 10:20 11:10
      Coffee break 50m
    • 11:10 12:30
      Morning session-2
      Convener: Naoya Kitajima
      • 11:10
        The viability of ultralight bosonic dark matter in dwarf galaxies 20m

        The dark matter distribution in dwarf galaxies holds a wealth of information on the fundamental properties and interactions of the dark matter particle. We study whether ultralight boson dark matter is consistent with the gravitational potential extracted from stellar kinematics. We use velocity dispersion measurements from six classical dwarf galaxies to show that axion-like particles with masses of order m ~ 10^−22eV are inconsistent with the potential distribution in classical dwarf galaxies unless the hierarchical assembly of the Milky Way did not trace the mean evolution of Milky Way size halos.

        Speaker: Isabelle Goldstein
      • 11:30
        Continuous gravitational waves from self-interacting axion condensate 20m

        Ultra-light particles, such as axions, form a macroscopic condensate around a highly spinning black hole by the superradiant instability. The condensate emits characteristic gravitational waves, such as a monochromatic gravitational wave. The characteristic gravitational waves open the possibility of detecting the axion through gravitational wave search. However, the precise evolution of the condensate must be known for the actual detection. In this talk, we consider the effect of the self-interaction and the black hole spin-down on the evolution of the condensate and the resulting gravitational waves from the condensate. We discuss that the self-interaction will excite various modes, and thus, the condensate emits continuous gravitational waves in several frequencies.

        Speaker: Hidetoshi Omiya
      • 11:50
        Evolution of binary systems accompanying axion clouds in extreme mass ratio inspirals 20m

        Axions can form a cloud around a rotating black hole (BH) by the superradiant instability. We expect to be able to explore axions from gravitational waves (GWs) from binary BHs. In particular, when the BH with a cloud belongs to a binary system, the resonant phenomenon occurs during the inspiral phase. We study the evolution of binary systems accompanying the axion cloud around the resonance frequency, focusing on small mass ratio cases. We also discuss the implication to the observational signatures and show that the modification of the GW frequency evolution can be a signature of the presence of axions.

        Speaker: Takuya Takahashi
      • 12:10
        Quantum Nucleation of Chiral Soliton Lattice 20m

        The Chiral Soliton Lattice (CSL) is a lattice structure composed of pion or axion domain walls aligned in parallel at equal intervals, which is energetically stable in the presence of a background magnetic field and a finite (baryon) chemical potential due to the topological term originated from the chiral anomaly. In this talk, I will discuss how to describe its formation by the quantum tunneling. I will show that CSL formation is promoted when the magnetic field strength and the chemical potential of the system is slightly larger than the scale of the axion decay constant.

        Speaker: Kohei Kamada
    • 12:30 12:35
      Group photos
    • 12:35 14:00
      Lunch 1h 25m
    • 14:00 16:30
      Free discussion
    • 16:30 17:30
      Afternoon session-1
      Convener: Wen Yin
      • 16:30
        Axion Dark Matter from Frictional Misalignment 40m

        We study the impact of sphaleron-induced thermal friction on the axion dark-matter abundance due to the interaction of an axion-like particle (ALP) with a dark non-abelian gauge sector in a secluded thermal bath. Thermal friction can either enhance the axion relic density by delaying the onset of oscillations or suppress it by damping them. We derive an analytical formula for the frictional adiabatic invariant, which remains constant along the axion evolution and which allows us to compute the axion relic density in a general set-up. Even in the most minimal scenario, in which a single gauge group is responsible for both the generation of the ALP mass and the friction force, we find that the resulting dark-matter abundance from the misalignment mechanism deviates from the standard scenario for axion masses m≳ 100 eV. We also generalize our analysis to the case where the gauge field that induces friction and the gauge sector responsible for the ALP mass are distinct and their couplings to the axion have a large hierarchy as can be justified by means of alignment or clockwork scenarios. We find that it is easy to open up the ALP parameter space where the resulting axion abundance matches the observed dark-matter relic density both in the traditionally over- and underabundant regimes. This conclusion also holds for the QCD axion.

        Speaker: Alexandros Papageorgiou
      • 17:10
        Hybrid inflation driven by QCD axion 20m

        Inflation plays a central role in modern cosmology. In order to reheat the Universe, inflaton must couple to the Standard Model particles, including gluons. Meanwhile, the QCD axion may couple to gluons to solve the strong CP problem. In this study, we investigate the inflationary scenario involving a QCD axion and a scalar field coupled to gluons. We find that this scenario can solve the isocurvature and quality problems associated with the QCD axion. We also discuss the possibility of QCD axion dark matter. Interestingly, the QCD axion can serve as the inflaton.

        Speaker: Yuma Narita
    • 17:30 18:00
      Coffee break 30m
    • 09:00 10:20
      Morning session-1
      Convener: Nagisa Hiroshima
      • 09:00
        Multi-prong searches for light dark matter 40m
        Speaker: Nancy Aggarwal
      • 09:40
        Bounds on Long-lived Dark Matter Mediators from Neutron Stars 20m

        Neutron stars close to the Galactic center are expected to swim in a dense background of dark matter. For models in which the dark matter has efficient interactions with neutrons, they are expected to accumulate their own local cloud of dark matter, making them appealing targets for observations seeking signs of dark matter annihilation. For theories with very light mediators, the dark matter may annihilate into pairs of mediators which are sufficiently long-lived to escape the star and decay outside it into neutrinos. We examine the bounds on the parameter space of heavy (\sim∼TeV to \sim∼PeV) dark matter theories with long-lived mediators decaying into neutrinos based on observations of high energy neutrino observatories, and make projections for the future. We find that these observations provide information that is complementary to terrestrial searches, and probe otherwise inaccessible regimes of dark matter parameter space.

        Speaker: Thong Nguyen
    • 10:20 10:50
      Coffee break 30m
    • 10:50 11:50
      Morning session-2
      Convener: Yuta Michimura
      • 10:50
        First Results of DANCE from Long-Term Observation 20m

        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 search for axion dark matter. We aim to detect the rotation and oscillation of optical linear polarization caused by axion-photon coupling with a bow-tie cavity. DANCE can improve the sensitivity to the axion-photon coupling constant in the axion mass range of 10^{-17} eV < m_a < 10^{-11} eV by several orders of magnitude compared to the best upper limits at present. A prototype experiment DANCE Act-1 is ongoing to demonstrate the feasibility of our method. We will report the first results of DANCE Act-1 from 24-hour observation in this workshop. We found no evidence for axions and set 95% confidence level upper limits on the axion-photon coupling g_ag ≾ 8 x 10^{-4} GeV^{-1} in 10^{-14} eV < m_a < 10^{-13} eV. Although the bounds did not exceed the current best limits, this work is the first demonstration of axion dark matter search with an optical ring cavity.

        Speaker: Yuka Oshima
      • 11:10
        Recent Upgrades of Optical System and Data Analysis in DANCE 20m
        Speaker: Hiroki Fujimoto
      • 11:30
        Light dark matter around eV mass range 20m

        I will talk about dark matter in eV mass range such as the experimental hints, future reaches, and dark matter theory.

        Speaker: Wen Yin
    • 11:50 11:55
      Closing remarks
      Convener: Fuminobu Takahashi (Tohoku Univesity)