Searching for Ultralight Dark Matter with KAGRA
Gravitational wave detectors like LIGO, Virgo, and KAGRA are exquisitely sensitive instruments. While designed to detect astrophysical gravitational waves, they can also search for exotic physics beyond the Standard Model.
Our recent work (arXiv:2403.03004) uses KAGRA—the Japanese gravitational wave detector—to search for ultralight dark matter using innovative time-domain methods.
The Dark Matter Problem
Dark matter comprises ~85% of the universe's matter, yet we don't know what it is.
Evidence for Dark Matter
Multiple independent observations indicate dark matter exists:
- Galaxy rotation curves: Stars orbit too fast for visible matter alone
- Gravitational lensing: Light bends more than visible matter predicts
- Cosmic microwave background: Precise measurements require dark matter
- Structure formation: Galaxies and clusters need dark matter to form
What Could Dark Matter Be?
Candidates span enormous mass ranges:
- WIMPs (Weakly Interacting Massive Particles): ~10-1000 GeV
- Axions: ~10⁻⁶ to 10⁻³ eV
- Ultralight dark matter: less than 10⁻¹⁰ eV
Each mass range requires different detection methods.
Ultralight Dark Matter
Ultralight dark matter particles have:
- Extremely small mass: 10⁻¹⁶ to 10⁻¹⁰ eV
- Wave-like behavior: Coherence lengths of astronomical scales
- Oscillating fields: Act like classical wave fields, not particles
Theoretical Motivation
Ultralight dark matter candidates include:
- Ultralight axions: Extensions of the QCD axion
- Dark photons: Hidden sector gauge bosons
- Moduli fields: From string theory compactifications
These arise naturally in theories of fundamental physics beyond the Standard Model.
How KAGRA Can Detect Dark Matter
Ultralight dark matter would appear as an oscillating field permeating our galaxy. This field can couple to:
Fundamental Constants
Dark matter could cause oscillations in:
- Fine structure constant (α): Determines electromagnetic strength
- Electron mass (me): Fundamental particle property
- Strong coupling constant: Nuclear force strength
Length Scales
Variations in fundamental constants would cause:
- Mirror spacing changes: KAGRA's 3km arms would oscillate
- Laser frequency shifts: Wavelength of light changes
- Beam splitter properties: Optical properties modulate
These effects produce signals in gravitational wave detectors.
KAGRA: Japan's Gravitational Wave Observatory
KAGRA (Kamioka Gravitational-wave Detector) is unique among gravitational wave detectors:
Underground Location
Built 200 meters underground in the Kamioka mine—the same location as the Super-Kamiokande neutrino detector.
Advantages:
- Seismic isolation: Natural shielding from surface vibrations
- Temperature stability: Underground temperature is constant
- Lower ambient noise: Reduced environmental disturbances
Cryogenic Technology
KAGRA cools its mirror to ~20 Kelvin using liquid nitrogen.
Benefits:
- Reduced thermal noise: Atomic vibrations decrease
- Improved sensitivity: Lower noise floor in mid-frequency range
Arm Length
3-kilometer arms (shorter than LIGO's 4km) optimized for higher frequencies.
Our Search Method: Time-Domain Analysis
Previous dark matter searches with gravitational wave detectors used frequency-domain methods. We developed a novel time-domain approach.
Frequency-Domain Approach
Traditional method:
- Fourier transform data: Convert time series to frequency spectrum
- Look for narrow peaks: Dark matter produces monochromatic signals
- Statistical analysis: Determine if peaks are significant
Limitation: Assumes signal is perfectly monochromatic.
Time-Domain Approach
Our method:
- Model signal directly in time: Account for signal evolution
- Matched filtering: Template-based search
- Bayesian inference: Extract parameters and confidences
Advantage: Handles non-stationary signals, phase evolution, and transient effects.
Dark Matter Signal Characteristics
Ultralight dark matter produces distinctive signals:
Frequency
Dark matter mass determines oscillation frequency: f = m c² / h ≈ (2.4 × 10¹⁴ Hz) × (m / eV)
For masses around 10⁻¹³ eV, signals appear in KAGRA's sensitive band (10-1000 Hz).
Signal Morphology
Unlike gravitational waves from astrophysical sources:
- Continuous: Doesn't turn off
- Slowly evolving: Changes over months/years, not seconds
- Galactic origin: All-sky, not from specific direction
- Stochastic component: Random fluctuations from dark matter turbulence
Amplitude
Expected strain amplitude: h ~ 10⁻²³ × (ρDM / 0.3 GeV/cm³) × (coupling strength)
This is at the limit of current detector sensitivity, requiring long observation times.
Data Analysis
We analyzed KAGRA data from observing run O3 (2019-2020).
Data Selection
Quality criteria:
- Low noise periods: Excluding times with instrumental issues
- Science mode operation: Detector in stable configuration
- Vetoes applied: Removing known glitches and artifacts
This yielded ~50 days of usable data.
Signal Processing
Steps:
- Whitening: Normalize detector noise spectrum
- Template generation: Create expected dark matter waveforms
- Matched filtering: Cross-correlate data with templates
- Statistical analysis: Assess significance of candidates
Systematics
Careful handling of systematic effects:
- Calibration uncertainties: ±5% amplitude uncertainty
- Noise non-stationarity: Time-varying detector characteristics
- Environmental correlations: Temperature, humidity, seismic activity
Results
We found no evidence for ultralight dark matter in the search mass range.
Upper Limits
Our null result constrains dark matter-Standard Model couplings:
- Photon coupling: |gγγ| less than 10⁻¹² GeV⁻¹
- Baryon coupling: |gb| less than 10⁻⁹
- Lepton coupling: |gl| less than 10⁻¹⁰
These are among the strongest laboratory constraints in this mass range.
Complementarity
Our results complement other searches:
- Fifth force experiments: Different coupling combinations
- Atomic clocks: Higher frequency range
- Astrophysical observations: Model-dependent constraints
The combination narrows the parameter space where ultralight dark matter could hide.
Broader Impact
This search demonstrates:
Multi-Use Detectors
Gravitational wave detectors serve multiple purposes:
- Astrophysical observations: Primary mission
- Fundamental physics: Dark matter, extra dimensions
- Quantum mechanics: Macroscopic quantum states
- Technology development: Precision measurement techniques
The infrastructure investment yields diverse scientific returns.
Collaboration Across Fields
Our team included:
- Gravitational wave physicists
- Particle physicists
- Cosmologists
- Data scientists
This interdisciplinary collaboration brought diverse expertise to bear on a challenging problem.
Novel Methods
The time-domain analysis technique we developed has applications beyond dark matter:
- Continuous wave searches: For pulsar-driven gravitational waves
- Stochastic backgrounds: Cosmological gravitational waves
- Detector characterization: Understanding instrumental effects
Future Prospects
Next-Generation Detectors
Einstein Telescope and Cosmic Explorer will have:
- 10× better sensitivity: Accessing weaker couplings
- Broader frequency range: Larger mass window
- Longer observation time: Accumulated data over years
This will enable searches 100× more sensitive than current limits.
KAGRA Upgrades
KAGRA is continuously improving:
- Enhanced cryogenic systems: Lower thermal noise
- Better seismic isolation: Reduced low-frequency noise
- Quantum squeezing: Beating quantum noise limits
Each upgrade improves dark matter search sensitivity.
Multi-Messenger Dark Matter
Combining different experimental approaches:
- Direct detection: XENON, LUX, PandaX
- Indirect detection: Fermi, HESS, IceCube
- Collider searches: LHC
- Gravitational wave detectors: LIGO-Virgo-KAGRA
The combination constrains dark matter models more powerfully than any single experiment.
Personal Reflections
Working on this search connected me with Japan's gravitational wave community. KAGRA's underground location, cryogenic technology, and collaboration culture offered different perspectives from LIGO and Virgo.
The search for dark matter represents physics at its most fundamental: we know something exists, but not what it is. Using gravitational wave detectors to address this mystery exemplifies the creativity and versatility of modern experimental physics.
Why This Matters
Dark matter is one of the deepest mysteries in physics. Its nature will likely require new fundamental physics beyond our current theories.
Ultralight dark matter represents a theoretically well-motivated possibility. Our search constrains where it could hide, narrowing the parameter space for future searches.
Even null results advance science. By ruling out possibilities, we guide theory and inform future experiments.
Conclusion
Our search for ultralight dark matter with KAGRA using time-domain methods found no signal but set stringent constraints on dark matter-Standard Model couplings.
This work demonstrates the versatility of gravitational wave detectors, the power of interdisciplinary collaboration, and the importance of exploring diverse dark matter candidates.
The search continues. Perhaps ultralight dark matter exists at slightly different masses, with weaker couplings, or in forms we haven't yet imagined.
Science progresses through both discoveries and constraints. Our work contributes to the latter, making the ultimate discovery—whatever dark matter is—more likely.
Based on arXiv:2403.03004: Search for ultralight dark matter with KAGRA using time-domain method