Particle Physics

The range of possible masses of particle dark matter spans over many orders of magnitude. The extremes of the mass range -- so-called ''superheavy'' and ''ultralight'' dark matter candidates -- are increasingly attracting attention due to their novel phenomenology and new developments in production mechanisms. In this talk I will discuss the complementary nature of superheavy and ultralight dark matter, starting from the production during cosmic inflation, using examples from the string theory axiverse and from a QCD-like dark sector.

We derive a cutting rule for equal-time in-in correlators, including cosmological correlators based on Keldysh r/a basis, which decomposes diagrams into fully retarded functions and cut-propagators consisting of Wightman functions. Our derivation relies only on basic assumptions such as unitarity, locality, and the causal structure of the in-in formalism. We apply the cutting rule to simplify the extraction of non-local cosmological collider signals, and prove the absence of one-loop scale-invariant corrections from enhanced small-scale perturbations to superhorizon curvature and tensor perturbations.

We showcase cosmology's ability to constrain long-range forces between dark matter particles. Specifically, we consider a fermionic dark matter interacting via a Yukawa-coupled light scalar, focusing on regimes where the dark forces are stronger than gravitational and yet unconstrained. We show that the dark sector dynamics, both at the background and perturbation levels, is far richer than what can be captured with just the static interparticle Yukawa potential. The background dynamics includes an attractor that funnels a wide range of initial conditions onto an evolution unique to each parameter space. In a large swath of parameter space beyond existing limits, the dark sector deviates drastically from cold dark matter in observable epochs. We rule out this parameter space using existing constraints on dark-sector equation of state and small-scale cosmic perturbations, thus setting the strongest constraints yet on dark matter self-interactions at length scales shorter than 100 kpc. In addition, we briefly discuss repulsive dark forces and place cosmological limits that are stricter than in the attractive case.

Gravitational wave signals provide a unique insight into the dynamics of the early universe. Notably, secondary gravitational waves sourced by curvature perturbations from inflation are a generic feature. I will outline the details of this mechanism and discuss several scenarios where a significant signal may arise.

Inflaton couplings during warm inflation result in the production of a thermal bath. Thermal fluctuations can dominate the standard de Sitter analogues, resulting in a modified slow-roll scenario with a new source of density fluctuations. It is generally difficult to reliably calculate the predictions of warm inflation, and therefore assess its viability as a model of the early universe. We will present a fully calculable perturbative model of warm inflation, and discuss its main features. We will see that the thermal bath develops a chemical potential. The chemical potential has a rather large impact, and in extreme cases the chemical potential can stop warm inflation from happening all together.

I discuss particle physics models for dark matter that can leave a non-trivial imprint in the dark matter density perturbations and serve as targets of precise cosmological measurements. Scenarios include sterile neutrino dark matter enabled by novel neutrino self-interaction, thermal relic via an entropy dilution mechanism, and dark matter mass generated by a first-order phase transition in the early universe. Interplay with other probes will also be discussed when applicable.

Horizon-scale images from the Event Horizon Telescope (EHT) now reveal two stable features of M87⋆: a dark inner shadow and a bright, polarized ring. These two features provide clean and robust standing points for new-physics searches. In the shadow region, any model that adds microwave emission would partially illuminate it, so the observed darkness becomes a background-free intensity test that yields strong bounds, which we showcased with dark matter annihilation. On the ring, the differential linear polarization angle between observing days, predicted and observed to be nearly zero, cancels accretion flow systematics; its stability leads to leading limits on the axion–photon coupling in the ultralight mass range, assuming a superradiant origin. I will also highlight recent progress on controlling observational uncertainties, strengthening which parts of these conclusions can be regarded as robust.

Gravitational waves (GW) offer a unique window into primordial density perturbations that may differ significantly from the adiabatic fluctuations observed in the CMB and large-scale structure. This raises an exciting prospect of seeing fluctuations of a completely new field from inflation in the anisotropy patterns of GW maps. However, CMB constraints impose an apparent tradeoff between distinctive anisotropy and sizable isotropic signal of such a GW background. I will show that rotating axion-like fields can naturally circumvent this limitation, producing both a strong GW background and large anisotropies within it. I will present the key characteristics of this signal and explore its detection prospects with future GW experiments.

The past decade has seen rapid advancement in electron excitation-based direct detection experiments, which target light, MeV-GeV mass dark matter. Recently, the DAMIC-M collaboration began to probe the benchmark light dark matter models, renewing questions about the broader range of dark matter models these experiments can detect, and the necessity of new experiments to search beyond. In this talk we'll discuss a novel mechanism to cosmologically produce light, keV-GeV mass, dark matter coupled to electrons via a magnetic dipole moment. We consider a minimally extended dark sector in which only heavier states are charged under the kinetically mixed dark photon. These states are produced via freeze-in and subsequently thermalize the rest of the dark sector, efficiently populating the lighter, neutral, magnetic dipole-coupled dark matter. Finally, we'll discuss how magnons, collective excitations of spin waves, can be used to search for keV-MeV mass magnetic dipole-coupled dark matter, outside the reach of any semiconductor based direct detection technology.

The QCD axion may solve the strong CP problem and constitute the dark matter (DM) abundance in our Universe. Peccei-Quinn (PQ) axions may form axion strings if the PQ phase transition occurs after inflation. I will discuss recent advances in the computation of the QCD axion DM mass in this scenario from the most precise and accurate lattice simulations to-date of axion-string networks, leading to a predicted mass range of 40 - 300 µeV. On the other hand, string theory axions, which are compelling solutions to the PQ quality problem, do not generically form strings - except in special inflationary paradigms such as brane inflation - meaning that there is no unique predicted value for the QCD axion DM mass. Nonetheless, if a Grand Unified Theory is assumed, I will explain why in broad regions of the string landscape, the mass of a stringy QCD axion is expected to be ~ 0.01 to 10 neV, independently of its DM fraction.

Primordial black holes (PBHs) are a well-motivated candidate for dark matter that may constitute a sub-fraction of the dark sector in the Earth-mass range. The strongest observational probe of this population is through gravitational microlensing, an effect in which the bending of light by a massive object results in the apparent transient magnification of a distant source. While ground-based observatories have produced tantalizing hints of a PBH population in this mass range, our understanding of this potential signal will be transformed with the launch of the Roman Space Telescope in 2026, whose Galactic Bulge Time Domain Survey will usher in a new era of discovery in this field. In this talk, I will discuss how by leveraging the high-statistics observations that Roman will make, we will be able to discern multiple subpopulations of non-luminous lenses and provide a new window into the existence of macroscopic dark matter.