Particle Physics

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.

We study the implications of precision measurements of light-element abundances, in combination with the Cosmic Microwave Background, for scenarios of physics beyond the Standard Model that generate large inhomogeneities in the baryon-to-photon ratio. We show that precision Big Bang Nucleosynthesis (BBN) places strong constraints on any mechanism that produces large-scale inhomogeneities at temperatures around or below the TeV scale. In particular, we find that fluctuations of order 25% on comoving length scales larger than the horizon at T ~ 3 TeV are incompatible with the observed light-element abundances. This sensitivity to early-universe physics arises because baryon-number inhomogeneities homogenize primarily through diffusion, a slow process. As a result, BBN serves as a novel probe of baryogenesis below the TeV scale, readily ruling out some proposed scenarios in the literature. We discuss the implications for electroweak baryogenesis, and further show that precision BBN provides a new probe of first-order phase transitions that generate gravitational waves in the pHz–mHz frequency range. This yields constraints on the electroweak phase transition, as well as first-order phase transitions that have been suggested as an explanation of the pulsar timing array signal. Finally, we comment on the future prospects for improving this probe

The QCD axion is not only a leading contender as a solution to the Strong CP problem, it is also a natural dark matter candidate. In particular, the post-inflationary axion is particularly attractive because of the possibility of a unique prediction of the axion mass if axion makes up all of dark matter. On the other hand, QCD axion models often suffers from the so-called axion quality problem, where the axion shift symmetry is unprotected from quantum gravity effects. In this talk I will discuss the tension between solutions to the quality problem and viable cosmology for post-inflationary QCD axions. I will start with a simple Z_N solution as an illustrative example of the close connection between the quality problem and the domain wall problem. I will then analyze proposals in the literature that involve more complex symmetry structure and show that they share a set of cosmological issues. Our study suggests that a viable post-inflationary QCD axion model is likely to have a non-standard cosmological history which could significantly impact the prediction of the "correct" axion mass.

The pseudo-Dirac Higgsino is one of the last surviving electroweak WIMPs. The LHC will not reach the 1.1 TeV target mass even with full luminosity and prospects for its indirect detection depend on a favorable dark matter density profile at the galactic center. Since it has only off-diagonal couplings at tree-level, its direct detection is possible only when the mass splitting is smaller than the initial center of mass kinetic energy. This direct detection loophole is actually more generic; going by the name 'inelastic dark matter'. In this talk I will talk about my recent efforts to reach mass splittings larger than what has been thought to be possible thus far, by invoking interesting astrophysics, elements from the end of the periodic table, and large volume neutrino detectors. 

I describe a new solution to the Higgs hierarchy problem based on a dynamical vacuum selection mechanism in a landscape which scans the Higgs mass. The Higgs potential only admits a stable minimum when its mass is less than a critical value, cosmologically crunching away patches with a heavier Higgs. This critical value is set by the instability scale where the Higgs quartic turns negative. I consider the phenomenology of two explicit models that address the hierarchy problem in this context.

In this talk I will outline a fundamentally quantum description of bosonic dark matter from which the conventional classical-wave picture emerges when the mass is far below 10 eV. Exploiting fundamental results from quantum optics I will argue that the density matrix for dark matter is explicitly mixed. The formalism provides a continuous description of DM through the wave-particle transition, and using this I show how density fluctuations over various physical scales evolve between the two limits, with a unique behavior for DM emerging near the boundary of the wave and particle descriptions. If time permits, I will briefly describe how these effects would appear in axion haloscopes.

We introduce 'Lattice Cosmology’ as an emergent field encompassing a set of techniques capable of solving the non-linear dynamics of interactive fields in an expanding Universe. As a demonstration of the power of these techniques, we solve two very different problems of early Universe cosmology: i) the non-linear dynamics of axion inflation, and ii) the dynamics and gravitational wave emission of cosmic string loops.

I will introduce how to incorporate loop contributions into positivity bounds in a massless scalar theory with shift symmetry. The allowed region of Wilson coefficients is called the EFT-hedron. With loop contributions, the n=4 null constraint, which plays an important role in determining the allowed region for g3–g4 (the Wilson coefficients of dimension-10 and dimension-12 operators, respectively), is modified. This modification of the null constraint reveals new features of the EFT-hedron. We obtained a much stronger bound for g2 (the Wilson coefficient of dimension-8 operators) without using the full unitarity condition. Furthermore, we found a negative g4, which is not possible under the tree-level bound. It would be interesting to generalize our method to more complicated theory and test our results.

The measurement of the electron magnetic moment (g-2)_e has reached the spectacular precision of sub-parts-per-trillion, and is currently the highest-precision measurement of a property of a fundamental particle. The next iteration of the measurement is expected to improve the precision by almost an order of magnitude, at which point the leading systematic uncertainty will be the “cavity shift”: the modification of the electron energy levels due to the conducting walls of the cavity in which the electron is trapped. This quantity cannot be directly measured at the required precision, and must be calculated. I will review the setup of the (g-2)_e experiment and present a new calculation of the cavity shift using the full machinery of quantum mechanics and quantum electrodynamics, which improves upon previous classical calculations with a consistent renormalization scheme while allowing for individual quality factors for each cavity mode.