Physics (Internal)

Discovering the QCD Axion with Black Holes and Gravitational Waves

This event is part of the HET Seminar Series.

When a particle’s Compton wavelength is comparable to the horizon size of a black hole, the particle can bind to the black hole, forming a “gravitational atom.” To form atoms with stellar black holes, the particle must be ultralight, with mass at or below 10^-11 eV. If such a particle exists, the levels of the atom can be populated by extracting energy and angular momentum from the black hole through a process known as black hole superradiance; for bosons, the occupation number of the levels grows exponentially and the black hole spins down. One candidate for such an ultralight boson is the QCD axion, proposed to solve the strong-CP problem. Black hole spin measurements result in new limits on the axion in the mass range 610^−13 eV < ma < 210^−11 eV. In addition, axions transitioning between levels of the gravitational atom and annihilating to gravitons can produce observable gravitational wave signals. These signals are coherent, monochromatic and last for many years. With its target sensitivity, Advanced LIGO has the chance to observe a transition event or thousands of annihilation events from systems in the Milky Way. With the help of black hole superradiance, Advanced LIGO could not only be the first experiment to observe gravitational waves, but may discover the QCD axion in the process.