PhD Defenses


Thu March 9th 2023, 12:30 - 1:30pm
PAB 102/103
Stanford Student Observatory

Linda Cicero

Ph.D. Candidate:  Jedidiah O. Thompson

Research Advisor: 
Savas Dimopoulos

Date: March 9, 2023
Time: 12:30pm

Location: PAB 102/103

Zoom Link:

Zoom Password: email nickswan [at] (nickswan[at]stanford[dot]edu) for password

The large-misalignment mechanism for the formation of compact axion structures

Axions are some of the best motivated particles beyond the Standard Model. In this talk I will show how attractive self-interactions of dark matter (DM) axions over a broad range of masses, from 10^-20 eV to 10^7 GeV, can lead to nongravitational growth of density fluctuations and the formation of bound objects. This structure formation enhancement is driven by parametric resonance when the initial field misalignment is large, and it affects axion density perturbations on length scales of order the Hubble horizon when the axion field starts oscillating, deep inside the radiation-dominated era. This effect can turn an otherwise nearly scale-invariant spectrum of adiabatic perturbations into one that has a spike at the aforementioned scales, producing objects ranging from dense DM halos to scalar-field configurations such as solitons and oscillons. I will discuss observational consequences of this effect for axions with masses up to 10 eV. For axions heavier than 10^-5 eV, the compact axion halos are numerous enough to significantly impact Earth-bound direct detection experiments, yielding intermittent but coherent signals with repetition rates exceeding one per decade and crossing times less than a day. These episodic increases in the axion density and kinematic coherence suggest new approaches for axion DM searches, including for the QCD axion. Dense structures made up of axions from 10^-20 eV to 10^-5 eV can be probed purely through their gravitational effects, and I will outline a few different techniques for doing so.