arXiv subjects
Publications and source records attributed to Amalia Madden.
Five-dimensional 't Hooft-Polyakov monopole strings whose cores restore a non-abelian gauge symmetry become axion strings for extra-dimensional axions. The parametric separation between the string tension and the scale of quantum gravity admits a viable post-inflationary cosmology for high-quality axion dark matter.
We present a concept for a tabletop-scale detector with an amorphous target designed to search for dark matter absorption into phonon excitations. In crystalline materials, absorption occurs only at narrow resonances where the dark matter mass matches a zero momentum optical phonon mode, whereas amorphous targets provide a broadband response that can substantially enhance the absorption rate away from these resonances. The predicted backgrounds arise from the relaxation of disorder-induced metastable defects in the amorphous target, as well as from low-energy noise intrinsic to superconducting phonon sensors. A prototype detector with a target mass of only a few $\mu$g could provide broadband sensitivity to dark photon absorption across the 50 meV-200 meV mass range, probing up to two orders of magnitude beyond existing constraints.
We present the FERMIACC, a scaffolded reasoning model built on OpenAI agents designed to autonomously generate and quantitatively validate theory hypotheses for high energy physics data at scale.
We show that axion dark matter in the range meV $\lesssim m_a\lesssim$ 100 meV can incoherently excite phonons in crystal targets with unpolarised nuclear spins. This can occur through its coupling to nuclear spins and/or through its induced time-dependent electric dipole moment in nuclei. Due to the random orientation of the nuclear spins, translation symmetry is broken in the phonon effective theory, allowing axion absorption to create phonons with unrestricted momentum. The absorption rate is therefore proportional to the phonon density of states, which generically has support across a wide range of energies, allowing for a broadband detection scheme. We calculate the absorption rate for solid $\text{H}_2$, $\text{D}_2$, $\text{Al}_2\text{O}_3$, $\text{GaAs}$, $\text{H}_2\text{O}$, $\text{D}_2\text{O}$, $\text{Be}$ and $\text{Li}_2 \text{O}$, and find that materials containing light, non-zero spin nuclei are the most promising. The predicted rates for the QCD axion are of the order of a few events / 10 kg-year exposure, setting an ambitious target for the required exposure and background suppression.
We show that piezoelectric materials can be used to source virtual QCD axions, generating a new axion-mediated force. Spontaneous parity violation within the piezoelectric crystal combined with time-reversal violation from aligned spins provide the necessary symmetry breaking to produce an effective in-medium scalar coupling of the axion to nucleons up to 7 orders of magnitude larger than that in vacuum. We propose a detection scheme based on nuclear spin precession caused by the axion's pseudoscalar coupling to nuclear spins. This signal is resonantly enhanced when the distance between the source crystal and the spin sample is modulated at the spin precession frequency. Using this effect, future experimental setups can be sensitive to the QCD axion in the unexplored mass range from $10^{-5}\,\mathrm{eV}$ to $10^{-2}\,\mathrm{eV}$.
Axion dark matter (DM) constitutes an oscillating background that violates parity and time-reversal symmtries. Inside piezoelectric crystals, where parity is broken spontaneously, this axion background can result in a stress. 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 explore all axion couplings that can give rise to the piezoaxionic effect -- the most promising one is the defining coupling of the QCD axion, through the anomaly of the strong sector. We also point our 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. The electroaxionic effect is again largest for the axion coupling to gluons. 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 $10^{-11}\,\mathrm{eV}$ and $10^{-7}\,\mathrm{eV}$, a challenging range for most other detection concepts.
The QCD axion is one of the best motivated dark matter candidates. The misalignment mechanism is well known to produce an abundance of the QCD axion consistent with dark matter for an axion decay constant of order $10^{12}$ GeV. For a smaller decay constant, the QCD axion, with Peccei-Quinn symmetry broken during inflation, makes up only a fraction of dark matter unless the axion field starts oscillating very close to the top of its potential, in a scenario called "large-misalignment". In this scenario, QCD axion dark matter with a small axion decay constant is partially comprised of very dense structures. We present a simple dynamical model realising the large-misalignment mechanism. During inflation, the axion classically rolls down its potential approaching its minimum. After inflation, the Universe reheats to a high temperature and a modulus (real scalar field) changes the sign of its minimum dynamically, which changes the sign of the mass of a vector-like fermion charged under QCD. As a result, the minimum of the axion potential during inflation becomes the maximum of the potential after the Universe has cooled through the QCD phase transition and the axion starts oscillating. In this model, we can produce QCD axion dark matter with a decay constant as low as $6\times 10^9\,{\rm GeV}$ and an axion mass up to 1 meV. We also summarise the phenomenological implications of this mechanism for dark matter experiments and colliders.