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A<sc>bstract</sc> We study (multi) fermion - monopole bound states, many of which are the states that dyons adiabatically transition into as fermions become light. The properties of these bound states depend critically on the UV symmetries preserved by the fermion mass terms, their relative size, and the value ofθ. Depending on the relative size of the mass terms and the value ofθ, the bound states can undergo phase transitions as well as transition from being stable to unstable. In some simple situations, the bound state solution can be related to the Witten effect of another theory with fewer fermions and larger gauge coupling. These bound states are a result of mass terms and symmetry breaking boundary conditions at the monopole core and, consequently, these bound states do not necessarily have definite quantum numbers under accidental IR symmetries. Additionally, they have binding energies that are$$ \mathcal{O}(1) $$ $O\left(1\right)$times the fermion mass and bound state radii of order their inverse mass. As the massless limit is approached, the bound state radii approach infinity, and they become new asymptotic states with odd quantum numbers giving a dynamical understanding to the origin of semitons. 

A<sc>bstract</sc> We introduce a mechanism by which a misaligned ALP can be dynamically converted into a dark photon in the presence of a background magnetic field. An abundance of non-relativistic ALPs will convert to dark photons with momentum of order the inhomogeneities in the background field; therefore a highly homogeneous field will produce non-relativistic dark photons without relying on any redshifting of their momenta. Taking hidden sector magnetic fields produced by a first order phase transition, the mechanism can reproduce the relic abundance of dark matter for a wide range of dark photon masses down to 10⁻¹³eV. 

The near equality of the dark matter and baryon energy densities is a remarkable coincidence, especially when one realizes that the baryon mass is exponentially sensitive to UV parameters in the form of dimensional transmutation. We explore a new dynamical mechanism, where in the presence of an arbitrary number density of baryons and dark matter, a scalar adjusts the masses of dark matter and baryons until the two energy densities are comparable. In this manner, the coincidence is explained regardless of the microscopic identity of dark matter and how it was produced. This new scalar causes a variety of experimental effects such as a new force and a (dark) matter density-dependent proton mass. Published by the American Physical Society2024 

A<sc>bstract</sc> The presence of a plethora of light spin 0 and spin 1 fields is motivated in a number of BSM scenarios, such as the axiverse. The study of the interactions of such light bosonic fields with the Standard Model has focused mostly on interactions involving only one such field, such as the axion (ϕ) coupling to photons,$$\phi F\widetilde{F}$$, or the kinetic mixing between photon and the dark photon,FF_D. In this work, we continue the exploration of interactions involving two light BSM fields and the standard model, focusing on the mixed axion-photon-dark-photon interaction$$\phi F{\widetilde{F}}_{D}$$. If either the axion or dark photon are dark matter, we show that this interaction leads to conversion of the CMB photons into a dark sector particle, leading to a distortion in the CMB spectrum. We present the details of these unique distortion signatures and the resulting constraints on the$$\phi F{\widetilde{F}}_{D}$$coupling. In particular, we find that for a wide range of masses, the constraints from these effect are stronger than on the more widely studied axion-photon coupling. 

A<sc>bstract</sc> We argue that the striking similarity between the cosmic abundances of baryons and dark matter, despite their very different astrophysical behavior, strongly motivates the scenario in which dark matter resides within a rich dark sector parallel in structure to that of the standard model. The near cosmic coincidence is then explained by an approximateℤ₂exchange symmetry between the two sectors, where dark matter consists of stable dark neutrons, with matter and dark matter asymmetries arising via parallel WIMP baryogenesis mechanisms. Taking a top-down perspective, we point out that an adequateℤ₂symmetry necessitates solving the electroweak hierarchy problem in each sector, without our committing to a specific implementation. A higher-dimensional realization in the far UV is presented, in which the hierarchical couplings of the two sectors and the requisiteℤ₂-breaking structure arise naturally from extra-dimensional localization and gauge symmetries. We trace the cosmic history, paying attention to potential pitfalls not fully considered in previous literature. Residualℤ₂-breaking can very plausibly give rise to the asymmetric reheating of the two sectors, needed to keep the cosmological abundance of relativistic dark particles below tight bounds. We show that, despite the need to keep inter-sector couplings highly suppressed after asymmetric reheating, there can naturally be order-one couplings mediated by TeV scale particles which can allow experimental probes of the dark sector at high energy colliders. Massive mediators can also induce dark matter direct detection signals, but likely at or below the neutrino floor. 

A<sc>bstract</sc> Vector Dark Matter (VDM) that couples to lepton flavor (L_e,L_μ,L_τ) acts similarly to a chemical potential for the neutrino flavor eigenstates and modifies neutrino oscillations. VDM imparts unique signatures such as time and directional dependence with longer baselines giving better sensitivity. We use the non-observation of such a signal at Super-Kamiokande to rule out the existence of VDM in a region of parameter space several orders of magnitude beyond other constraints and show the projected reach of future experiments such as DUNE. 

A<sc>bstract</sc> The causal tail of stochastic gravitational waves can be used to probe the energy density in free streaming relativistic species as well as measureg_*(T) and beta functionsβ(T) as a function of temperature. In the event of the discovery of loud stochastic gravitational waves, we demonstrate that LISA can measure the free streaming fraction of the universe down to the the 10⁻³level, 100 times more sensitive than current constraints. Additionally, it would be sensitive to$$ \mathcal{O} $$ $O$(1) deviations ofg_*and the QCDβfunction from their Standard Model value at temperatures ~ 10⁵GeV. In this case, many motivated models such as split SUSY and other solutions to the Electroweak Hierarchy problem would be tested. Future detectors, such as DECIGO, would be 100 times more sensitive than LISA to these effects and be capable of testing other motivated scenarios such as WIMPs and axions. The amazing prospect of using precision gravitational wave measurements to test such well motivated theories provides a benchmark to aim for when developing a precise understanding of the gravitational wave spectrum both experimentally and theoretically.