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Register a new userCosmological and astrophysical probes of dark baryons
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We examine the cosmological and astrophysical signatures of a dark baryon, a neutral fermion that mixes with the neutron. As the mixing is through a higher-dimensional operator at the quark level, production of the dark baryon at high energies is enhanced so that its abundance in the early universe may be significant. Treating its initial abundance as a free parameter, we derive new, powerful limits on the properties of the dark baryon. Primordial nucleosynthesis and the cosmic microwave background provide strong constraints due to the inter-conversion of neutrons to dark baryons through their induced transition dipole, and due to late decays of the dark baryon. Additionally, neutrons in a neutron star could decay slowly to dark baryons, providing a novel source of heat that is constrained by measurements of pulsar temperatures. Taking all the constraints into account, we identify parameter space where the dark baryon can be a viable dark matter candidate and discuss promising avenues for probing it.
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Traditional dark matter (DM) models, eg. WIMPs, assume dark matter is weakly coupled to the standard model so that elastic scattering between DM and baryons can be described perturbatively by Born approximation. Most direct detection experiments are analyzed according to that assumption. We show that when the interaction is attractive and strong, DM-nucleus scattering exhibits rich resonant behavior with a highly non-trivial dependence on atomic mass. The scattering is non-perturbative in much of the natural parameter range, and a full numerical calculation is needed. We also show that the extended rather than point-like nature of nuclei significantly impacts the cross sections and must therefore be properly taken into account. These effects are particularly important for dark matter with GeV-scale masses, near the boundary of exclusion regions from existing direct detection limits. They also affect the interpretation of CMB constraints, as we show. We report the corrected limits, which are in some respects weaker and in other respects stronger than previous bounds in the literature, which were based on perturbation theory and point-like sources and hence are now superceded. Sexaquark ($uuddss$) DM with mass $lesssim 2$ GeV, which exchanges QCD mesons with baryons, remains unconstrained for most of the parameter space of interest.
Starting from the evidence that dark matter indeed exists and permeates the entire cosmos, various bounds on its properties can be estimated. Beginning with the cosmic microwave background and large scale structure, we summarize bounds on the ultralight bosonic dark matter (UBDM) mass and cosmic density. These bounds are extended to larger masses by considering galaxy formation and evolution, and the phenomenon of black hole superradiance. We then discuss the formation of different classes of UBDM compact objects including solitons/axion stars and miniclusters. Next, we consider astrophysical constraints on the couplings of UBDM to Standard Model particles, from stellar cooling (production of UBDM) and indirect searches (decays or conversion of UBDM). Throughout, there are short discussions of hints and opportunities in searching for UBDM in each area.
The spatial and velocity distributions of dark matter particles in the Milky Way Halo affect the signals expected to be observed in searches for dark matter. Results from direct detection experiments are often analyzed assuming a simple isothermal distribution of dark matter, the Standard Halo Model (SHM). Yet there has been skepticism regarding the validity of this simple model due to the complicated gravitational collapse and merger history of actual galaxies. In this paper we compare the SHM to the results of cosmological hydrodynamical simulations of galaxy formation to investigate whether or not the SHM is a good representation of the true WIMP distribution in the analysis of direct detection data. We examine two Milky Way-like galaxies from the MaGICC cosmological simulations (a) with dark matter only and (b) with baryonic physics included. The inclusion of baryons drives the shape of the DM halo to become more spherical and makes the velocity distribution of dark matter particles less anisotropic especially at large heliocentric velocities, thereby making the SHM a better fit. We also note that we do not find a significant disk-like rotating dark matter component in either of the two galaxy halos with baryons that we examine, suggesting that dark disks are not a generic prediction of cosmological hydrodynamical simulations. We conclude that in the Solar neighborhood, the SHM is in fact a good approximation to the true dark matter distribution in these cosmological simulations (with baryons) which are reasonable representations of the Milky Way, and hence can also be used for the purpose of dark matter direct detection calculations.
Axions constituting dark matter (DM) are often considered to form a non-relativistic oscillating field. We explore bursts of relativistic axions from transient astrophysical sources, such as axion star explosions, where the sources are initially non-relativistic. For the QCD axion, bursts from collapsing axion stars lead to potentially detectable signals over a wide range of axion masses $10^{-15} , textrm{eV} lesssim m lesssim 10^{-7} , textrm{eV}$ in future experiments, such as ABRACADABRA, DMRadio and SHAFT. Unlike conventional cold axion DM searches, the sensitivity to axion bursts is not necessarily suppressed as $1/f$ for large decay constants $f$. The detection of axion bursts could provide new insights into the fundamental axion potential, which is challenging to probe otherwise. An ensemble of bursts in the distant past, in direct analogy with neutrinos, would give rise to a diffuse axion background distinct from the usual cold axion DM. Coincidence with other signatures, such as electromagnetic and gravitational-wave emission, would provide a new beyond-the-standard-model window into multi-messenger astronomy.
We study the possibility that dark radiation, sourced through the decay of dark matter in the late Universe, carries electromagnetic interactions. The relativistic flux of particles induces recoil signals in direct detection and neutrino experiments through its interaction with millicharge, electric/magnetic dipole moments, or anapole moment/charge radius. Taking the DM lifetime as 35 times the age of the Universe, as currently cosmologically allowed, we show that direct detection (neutrino) experiments have complementary sensitivity down to $epsilonsim 10^{-11}$ $(10^{-12})$, $d_chi/mu_chi sim 10^{-9},mu_B$ $(10^{-13}mu_B)$, and $a_chi/b_chi sim 10^{-2},{rm GeV}^{-2}$ $(10^{-8},{rm GeV}^{-2})$ on the respective couplings. Finally, we show that such dark radiation can lead to a satisfactory explanation of the recently observed XENON1T excess in the electron recoil signal without being in conflict with other bounds.
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