Discovering the QCD Axion with Black Holes and Gravitational Waves (1411.2263v3)

Abstract: Advanced LIGO may be the first experiment to detect gravitational waves. Through superradiance of stellar black holes, it may also be the first experiment to discover the QCD axion with decay constant above the GUT scale. When an axion's Compton wavelength is comparable to the size of a black hole, the axion binds to the black hole, forming a "gravitational atom." Through the superradiance process, the number of axions occupying the bound levels grows exponentially, extracting energy and angular momentum from the black hole. Axions transitioning between levels of the gravitational atom and axions annihilating to gravitons can produce observable gravitational wave signals. The signals are long-lasting, monochromatic, and can be distinguished from ordinary astrophysical sources. We estimate up to O(1) transition events at aLIGO for an axion between 10^-11 and 10^-10 eV and up to 10^4 annihilation events for an axion between 10^-13 and 10^-11 eV. In the event of a null search, aLIGO can constrain the axion mass for a range of rapidly spinning black hole formation rates. Axion annihilations are also promising for much lighter masses at future lower-frequency gravitational wave observatories; the rates have large uncertainties, dominated by supermassive black hole spin distributions. Our projections for aLIGO are robust against perturbations from the black hole environment and account for our updated exclusion on the QCD axion of 6*10^-13 eV < ma < 2*10^-11 eV suggested by stellar black hole spin measurements.

• The paper demonstrates that axions can be detected through black hole superradiance, opening a novel observational window into QCD axion physics.
• It shows that gravitational wave transitions and annihilations generate distinct monochromatic signals that Advanced LIGO may capture.
• The study sets potential constraints on axion masses and paves the way for using astrophysical observatories to probe physics beyond the Standard Model.

Discovering the QCD Axion with Black Holes and Gravitational Waves

The paper "Discovering the QCD Axion with Black Holes and Gravitational Waves" investigates the potential of detecting quantum chromodynamics (QCD) axions using astrophysical black holes and gravitational wave measurements. The research taps into a process known as black hole superradiance, which may allow current and future gravitational wave observatories, such as Advanced LIGO and VIRGO, to detect or constrain the parameter space of the QCD axion.

Overview of Black Hole Superradiance

Superradiance refers to the amplification of waves scattering off a rotating black hole, an effect that affects both light and matter waves under certain conditions. The process can lead to the exponential growth of occupation numbers for bosonic fields under specific circumstances, which can be conceptualized as bound states with the black hole. Notably, an axion field that meets the criteria for superradiance could form a gravitational "atom" around the black hole, leading to a cloud of axions that efficiently extract angular momentum from the black hole and generate long-lasting gravitational wave emissions.

Methodology and Results

The authors propose that when the Compton wavelength of axions is comparable to the size of a black hole, the resulting bound states will undergo superradiance, generating detectable gravitational wave signals. These waves, resultant from axionic transitions between energy levels or annihilations into gravitons, display monochromatic features that could be distinguishable by gravitational wave detectors.

The authors estimate that Advanced LIGO could observe up to $\mathcal{O}(1)$ transition events for axions with masses in the range of $10^{-11}$ to $10^{-10}$ eV, and up to $10^4$ annihilation events for axions between $10^{-13}$ and $10^{-11}$ eV. Even if no signals are detected, these observations could place constraints on axion masses across a spectrum of rapidly spinning black hole formation rates.

Implications and Future Prospects

Significantly, this paper opens a unique observational channel for QCD axions, whose high decay constants place them out of reach of terrestrial experiments. With Advanced LIGO and potentially future low-frequency detectors, such as eLISA, the parameter space for axions could be probed up to unexplored ranges, exceeding those accessible through traditional particle physics methodologies.

The results also suggest that black hole superradiance serves as a viable pathway to explore physics beyond the Standard Model. It underscores the value of gravitational wave astronomy in potentially revealing new particles and interactions, casting black holes as cosmic accelerators and observatories for new physics.

Conclusion

The development outlined in this paper significantly enhances the potential for detecting light bosonic fields and specifically the QCD axion, leveraging the interplay of gravitational wave physics and black hole astrophysics. The findings extend the reach of current gravitational wave observatories and suggest a concerted future effort to optimize and expand their capabilities may yield groundbreaking discoveries in fundamental physics. The theoretical paper serves as a foundation for experimental efforts, indicating a robust methodology that could soon provide evidence or constraints critical to our understanding of the universe's building blocks.