New Experimental Observables for the QCD Axion

Abstract

The QCD axion is one of the best motivated extensions to the Standard Model of particle physics that could also serve as the dark matter. The thesis will demonstrate new experimental observables that could be used to search for the axion. These observables are based on piezoelectric materials that spontaneously break parity symmetry, thereby enabling sensitivity to the axion's fundamental, model independent coupling to gluons.

The first observable explores how axion dark matter could generate an oscillating mechanical stress in a piezoelectric crystal. We call this new phenomenon ``the piezoaxionic effect". When the frequency of axion DM matches the natural frequency of a bulk acoustic normal mode of the piezoelectric crystal, the piezoaxionic effect is resonantly enhanced and can be read out electrically via the piezoelectric effect. We also point out another, subdominant phenomenon present in all dielectrics, namely the ``electroaxionic effect". An axion background can produce an electric displacement field in a crystal which in turn will give rise to a voltage across the crystal. We find that this model independent coupling of the QCD axion may be probed through the combination of the piezoaxionic and electroaxionic effects in piezoelectric crystals with aligned nuclear spins, with near-future experimental setups applicable for axion masses between $ 10^{-11}\text{eV}$ to $10^{-7}\text{eV}$, a challenging range for most other detection concepts.

The second observable, the ``piezoaxionic force" demonstrates how a piezoelectric crystal can be used to source virtual QCD axions in the laboratory, giving rise to a new axion-mediated force. The presence of parity violation in the piezoelectric crystal, combined with aligned nuclear spins, provides the necessary symmetry breaking to generate an effective in-medium scalar coupling of the axion to nucleons. We propose a detection scheme that uses the axion's model-dependent pseudoscalar coupling to nuclear spins, such that the new force can be detected by its effect on the precession of a sample of polarised nuclear spins. When the distance between the source crystal and the detector is modulated at the Larmor precession frequency of the nuclear spins, the signal is resonantly enhanced. We predict that near-future experimental setups should be sensitive to the axion in the unexplored mass range from $10^{-5} \text{eV}$ to $10^{-2} \text{eV}$.