Particle Physics

The inference of the present expansion rate from the Cosmic Microwave Background and other early-time probes (assuming standard
cosmology) is in significant tension with several direct measurements of the same quantity. If this discrepancy is not due to unresolved systematics, it could provide strong evidence for physics beyond the standard models of particle physics and cosmology. I will describe a few simple models that attempt to alleviate this Hubble tension. All of these scenarios contain additional energy density of new light particles at early times with interactions that modify the background cosmology and evolution of density perturbations in the early universe. First, I will motivate the existence of a decaying component of dark matter with a significant free-streaming length as a means of addressing this tension. Then I will consider models with self-interacting neutrino or dark radiation components. I will show that such scenarios face stringent cosmological and laboratory constraints. As a result, in these examples, the tension can be alleviated but not eliminated altogether.

Light dark photons are subject to various plasma effects, such as Debye screening and resonant oscillations, which can lead to a more complex cosmological evolution than is experienced by conventional cold dark matter candidates. Maintaining a consistent history of dark photon dark matter requires ensuring that the super-thermal abundance present in the early Universe (i) does not deviate significantly after the formation of the CMB, and (ii) does not excessively leak into the Standard Model plasma after BBN. In this talk, I will clarify the roles of resonant and non-resonant absorption and address some implications of plasma inhomogeneities on resonant transitions.

In this talk I will discuss ongoing efforts at UChicago to explore matter made of light. I will begin with a broad introduction to the challenges associated with making matter from photons, focusing specifically on (1) how to trap photons and imbue them with synthetic mass and charge; (2) how to induce photons to collide with one another; and (3) how to drive photons to order, by cooling or otherwise. I will then provide as examples two state-of-the-art photonic quantum matter platforms: microwave photons coupled to superconducting resonators and transmon qubits, and optical photons trapped in multimode optical cavities and made to interact through Rydberg-dressing. In each case I will describe a synthetic material created in that platform: a Mott insulator of microwave photons, stabilized by coupling to an engineered, non-Markovian reservoir, and a Laughlin molecule of optical photons prepared by scattering photons through the optical cavity. Indeed, building materials photon-by-photon will provide us with a unique opportunity to learn what all of the above words mean, and why they are important for quantum-materials science. Finally, I will conclude with my view of the broad prospects of photonic matter in particular, and of synthetic matter more generally.

Historically, new particles and forces in the Standard Model have most often revealed themselves at high-energy particle colliders. Certain phenomena beyond the Standard Model, however, are best studied by using carefully designed low-energy precision measurements, or via their imprints on astrophysical and cosmological observables. In this talk, I will provide a concise overview of some of the new experiments and searches devised to look for new physics beyond the Standard Model. In particular, I will discuss recent developments in the new experimental and theoretical program of cosmological collider physics and how we can use the cosmological collider as a tool to study the structure of the Higgs potential at very high energies.

Theories beyond the Standard Model of particle physics often predict new, light, feebly interacting particles whose discovery requires novel search strategies. A light particle, the QCD axion, elegantly solves the outstanding strong-CP problem of the Standard Model; cousins of the QCD axion can also appear, and are natural dark matter candidates.  First, I will discuss my experimental proposal based on thin films, in which dark matter can efficiently convert to detectable single photons. A prototype experiment is underway, and current techniques promise to reach significant new dark matter parameter space.

Second, I will show how rotating black holes turn into axionic and gravitational wave beacons, creating nature's laboratories for ultralight bosons. When an axion's Compton wavelength is comparable to a black hole size, energy and angular momentum from the black hole source exponentially-growing bound states of particles, forming `gravitational atoms'.  These `gravitational atoms' emit monochromatic gravitational wave signals, enabling current searches at gravitational wave observatories to discover ultralight axions. If the axions interact with one another, instead of gravitational waves, black holes populate the universe with axion waves.