Since the universe is not transparent to gamma rays with energies above around one hundred GeV, it is necessary to account for the interaction of high-energy photons with intergalactic radiation fields in order to model gamma- ray propagation. Here, we present a public numerical software for the modeling of gamma-ray observables. This code computes the effects on gamma-ray spectra from the development of electromagnetic cascades and cosmological redshifting. The code introduced here is based on the original y-Cascade, and builds on it by improving its performance at high redshifts, introducing new propagation modules, and adding many more extragalactic radiation field models, which enables the ability to estimate the uncertainties inherent to EBL modeling. We compare the results of this new code to existing Monte Carlo electromagnetic transport models, finding good agreement within EBL uncertainties.

Optically dense clouds in the interstellar medium composed predominantly of molecular hydrogen, known as molecular clouds, are sensitive to energy injection in the form of photon absorption, cosmic-ray scattering, and dark matter (DM) scattering. The ionization rates in dense molecular clouds are heavily constrained by observations of abundances of various molecular tracers. Recent studies have set constraints on the DM-electron scattering cross section using measurements of ionization rates in dense molecular clouds. Here we calculate the analogous bounds on the DM-proton cross section using the molecular Migdal effect, recently adapted from the neutron scattering literature to the DM context. These bounds may be the strongest limits on a strongly coupled DM subfraction, and represent the first application of the Migdal effect to astrophysical systems.

Recent studies indicate that thermally produced dark matter will form highly concentrated, low-mass cusps in the early universe that often survive until the present. While these cusps contain a small fraction of the dark matter, their high density significantly increases the expected 𝛾-ray flux from dark matter annihilation, particularly in searches of large angular regions. We utilize 14 years of Fermi-LAT data to set strong constraints on dark matter annihilation through a detailed study of the isotropic 𝛾-ray background, excluding with 95% confidence dark matter annihilation to 𝑏⁢¯𝑏 final states for dark matter masses below 120 GeV.

We present a new search for dark matter (DM) using planetary atmospheres. We point out that annihilating DM in planets can produce ionizing radiation, which can lead to excess production of ionospheric H+3. We apply this search strategy to the night side of Jupiter near the equator. The night side has zero solar irradiation, and low latitudes are sufficiently far from ionizing auroras, leading to a low-background search. We use Cassini data on ionospheric emission collected three hours either side of Jovian midnight, during its flyby in 2000, and set novel constraints on the DM-nucleon scattering cross section down to about 10⁻³⁸  cm². We also highlight that DM atmospheric ionization may be detected in Jovian exoplanets using future high-precision measurements of planetary spectra.

Blazars are among the most powerful accelerators in the Universe and are expected to produce a strong TeV gamma-ray flux. The gamma-rays from these blazars may produce cascades by inverse-Compton scattering off intergalactic radiation. The GeV gamma-rays produced by these interactions, make up a large contribution to the IGRB. In a previous paper, Blancos et al. showed that this contribution overproduces the IGRB, indicating that something is effectively quenching the cascade component of the IGRB or that blazars have an intrinsic spectral cutoff that is in tension with observations of local blazars.

The nature of dark matter and properties of neutrinos are among the most pressing issues in contemporary particle physics. The dual-phase xenon time-projection chamber is the leading technology to cover the available parameter space for weakly interacting massive particles, while featuring extensive sensitivity to many alternative dark matter candidates. These detectors can also study neutrinos through neutrinoless double-beta decay and through a variety of astrophysical sources. A next-generation xenon-based detector will therefore be a true multi-purpose observatory to significantly advance particle physics, nuclear physics, astrophysics, solar physics, and cosmology. This review article presents the science cases for such a detector.

We demonstrate that ionization of H₂ by dark matter in dense molecular clouds can provide strong constraints on the scattering strength of dark matter with electrons. Molecular clouds have high UV-optical attenuation, shielding them from ultraviolet and x-ray photons. Their chemical and thermal evolution are governed by low-energy cosmic rays. Dark matter with mass ≳4 MeV can ionize H₂, contributing to the observed ionization rate. We require that the dark matter-induced ionization rate of H₂ not exceed the observed cosmic-ray ionization rate ζ^H₂, in diffuse molecular clouds as well as dense molecular clouds such as L1551 in the Taurus cloud complex. This allows us to place strong constraints on the dark matter-electron cross section σ¯_e that complement existing astrophysical constraints and probe the strongly interacting parameter space where terrestrial and underground direct-detection experiments lose sensitivity. We show that constraints from molecular clouds combined with planned balloon and satellite-based experiments would strongly constrain the fractional abundance of dark matter that interacts strongly with electrons. We comment on future modeling and observational efforts that may improve our bounds.

We propose using quantum dots as novel targets to probe sub-GeV dark matter-electron interactions. Quantum dots are nanocrystals of semiconducting material, which are commercially available, with gramscale quantities suspended in liter-scale volumes of solvent. Quantum dots can be efficient scintillators, with near unity single-photon quantum yields, and their band-edge electronic properties are determined by their characteristic size, which can be precisely tuned. Examples include lead sulfide and lead selenide quantum dots, which can be tuned to have sub-eV optical gaps. A dark-matter interaction can generate one or more electron-hole pairs (excitons), with the multiexciton state decaying via the emission of two photons with an efficiency of about 10% of the single-photon quantum yield. An experimental setup using commercially available quantum dots and two photomultiplier-tubes for detecting the coincident twophoton signal can already improve on existing dark-matter bounds, while using photodetectors with lower dark-count rates can improve on current constraints by orders of magnitude.

We outline the unique opportunities and challenges in the search for "ultraheavy" dark matter candidates with masses between roughly 10 TeV and the Planck scale m_pl≈10¹⁶ TeV. This mass range presents a wide and relatively unexplored dark matter parameter space, with a rich space of possible models and cosmic histories. We emphasize that both current detectors and new, targeted search techniques, via both direct and indirect detection, are poised to contribute to searches for ultraheavy particle dark matter in the coming decade. We highlight the need for new developments in this space, including new analyses of current and imminent direct and indirect experiments targeting ultraheavy dark matter and development of new, ultra-sensitive detector technologies like next-generation liquid noble detectors, neutrino experiments, and specialized quantum sensing techniques.

The total extragalactic γ-ray flux provides a powerful probe into the origin and evolution of the highest energy processes in our universe. An important component of this emission is the isotropic γ-ray background (IGRB), composed of sources that cannot be individually resolved by current experiments. Previous studies have determined that the IGRB can be dominated by either misaligned active galactic nuclei (mAGN) or star-forming galaxies (SFGs). However, these analyses are limited, because they have utilized binary source classifications and examined only one source class at a time. We perform the first combined joint-likelihood analysis that simultaneously correlates the γ-ray luminosity of extragalactic objects with both star-formation and mAGN activity. We find that SFGs produce 48⁺³³_-20% of the total IGRB at 1 GeV and 56⁺⁴⁰_-23% of the total IGRB at 10 GeV. The contribution of mAGN is more uncertain, but can also be significant. Future work to quantify the radio and infrared properties of nearby galaxies could significantly improve these constraints.