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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> 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> 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 bstract We present the supernova constraints on an axion-photon-dark photon coupling, which can be the leading coupling to dark sector models and can also lead to dramatic changes to axion cosmology. We show that the supernova bound on this coupling has two unusual features. One occurs because the scattering that leads to the trapping regime converts axions and dark photons into each other. Thus, if one of the two new particles is sufficiently massive, both production and scattering become suppressed and the bounds from bulk emission and trapped (area) emission both weaken exponentially and do not intersection The other unusual feature occurs because for light dark photons, longitudinal modes couple more weakly than transverse modes do. Since the longitudinal mode is more weakly coupled, it can still cause excessive cooling even if the transverse mode is trapped. Thus, the supernova constraints for massive dark photons look like two independent supernova bounds super-imposed on top of each other.