Search

Oscura is a planned light-dark matter search experiment using Skipper-CCDs with a total active mass of 10 kg. As part of the detector development, the collaboration plans to build the Oscura Integration Test (OIT), an engineering test with 10% of the total mass. Here we discuss the early science opportunities with the OIT to search for millicharged particles (mCPs) using the NuMI beam at Fermilab. mCPs would be produced at low energies through photon-mediated processes from decays of scalar, pseudoscalar, and vector mesons, or direct Drell-Yan productions. Estimates show that the OIT would be a world-leading probe for mCPs in the MeV mass range., Comment: 13 pages, 10 figures

Oscura is a proposed multi-kg skipper-CCD experiment designed for a dark matter (DM) direct detection search that will reach unprecedented sensitivity to sub-GeV DM-electron interactions with its 10 kg detector array. Oscura is planning to operate at SNOLAB with 2070 m overburden, and aims to reach a background goal of less than one event in each electron bin in the 2-10 electron ionization-signal region for the full 30 kg-year exposure, with a radiation background rate of 0.01 dru. In order to achieve this goal, Oscura must address each potential source of background events, including instrumental backgrounds. In this work, we discuss the main instrumental background sources and the strategy to control them, establishing a set of constraints on the sensors' performance parameters. We present results from the tests of the first fabricated Oscura prototype sensors, evaluate their performance in the context of the established constraints and estimate the Oscura instrumental background based on these results.

High-energy physics detectors, like Low Gain Avalanche Detectors (LGADs) that will be used as fast timing detectors in the High Luminosity LHC experiments, have to exhibit a significant radiation tolerance. Thereby the impact of radiation on the highly boron-doped gain layer that enables the internal charge multiplication, is of special interest, since due to the so-called Acceptor Removal Effect (ARE) a radiation-induced deactivation of active boron dopants takes place. In this paper we present defect-spectroscopy measurements (Deep-Level Transient Spectroscopy and Thermally Stimulated Current technique) on neutron irradiated p-type silicon pad diodes of different resistivity as well as LGADs irradiated at fluences up to 1 x 10^15 neq/cm2. Thereby we show that while for the silicon pad diodes irradiated with electrons, neutrons or protons the determination of defect electronic properties and defect introduction rates is straightforward, DLTS and TSC measurements on LGADs are strongly influenced by the impact of the gain layer. It is shown that the measurability of the capacitance of the gain layer shows a strong frequency and temperature dependence leading to a capacitance drop in DLTS and non-reliable measurement results. With TSC defects formed in the LGADs can be very nicely observed and compared to the defects formed in the silicon pad diodes. However the exact assignment of defects to the gain layer or bulk region remains challenging and the charge amplification effect of the LGADs impacts the exact determination of defect concentrations. Additionally, we will demonstrate that depending on the TSC measurement conditions defect induced residual internal electric fields are built up in the irradiated LGADs that are influencing the current signal of carriers emitted from the defect states.

The Oscura experiment will lead the search for low-mass dark matter particles using a very large array of novel silicon Charge Coupled Devices (CCDs) with a threshold of two electrons and with a total exposure of 30 kg-yr. The R&D effort, which began in FY20, is currently entering the design phase with the goal of being ready to start construction in late 2024. Oscura will have unprecedented sensitivity to sub-GeV dark matter particles that interact with electrons, probing dark matter-electron scattering for masses down to 500 keV and dark matter being absorbed by electrons for masses down to 1 eV. The Oscura R&D effort has made some significant progress on the main technical challenges of the experiment, of which the most significant are engaging new foundries for the fabrication of the CCD sensors, developing a cold readout solution, and understanding the experimental backgrounds.

The Dark Axion Portal provides a model for Dark Matter (DM) in which both Dark Photons $\gamma^\prime$ and Axions $a$ can contribute to the present day abundance of DM. We study the parameter space of the Dark Axion Portal to pinpoint regions of the parameter space where $\gamma^\prime$ and $a$ can be produced with sufficient abundance to account for the cosmic DM density, while still being detectable in planned direct detection and axion haloscope experiments. In particular, we explore the production of eV-scale Dark Photons in the Dark Axion Portal, taking into account a possible kinetic mixing between the dark and visible photons, which is essential for the detection of dark photons through absorption in direct searches. We show that a non-zero kinetic mixing does not generally spoil the phenomenology of the model, leaving both the axion and dark photon stable. Viable production mechanisms point to a sub-dominant population of dark photons making up $\lesssim 10\%$ of the DM, with the remainder consisting of axion DM. Dark photons in the mass range $m_{\gamma^\prime} \sim 20-200\,\mathrm{eV}$ and axions in the mass range $m_a \sim 30 - 400\,\mu\mathrm{eV}$ may be produced with these abundances self-consistently in the Dark Axion Portal and are within the reach of future direct searches., Comment: 19 pages, 9 figures. Code can be found online at https://github.com/bradkav/DarkAxionPortal (archived at https://doi.org/10.5281/zenodo.5794543 ). v2: Updated axion-photon coupling constraints

In this work, which is a continuation of Castell\'o-Mor et al. (2016), we present new X-ray and infrared (IR) data for a sample of active galactic nuclei (AGN) covering a wide range in Eddington ratio over a small luminosity range. In particular, we rigorously explore the dependence of the optical-to-X-ray spectral index $\alpha_{OX}$ and the IR-to-optical spectral index on the dimensionless accretion rate, $\dot{\mathcal{M}}=\dot{m}/\eta$ where $\dot{m}=L_{AGN}/L_{Edd}$ and $\eta$ is the mass-to-radiation conversion efficiency, in low and high accretion rate sources. We find that the SED of the faster accreting sources are surprisingly similar to those from the comparison sample of sources with lower accretion rate. In particular: i) the optical-to-UV AGN SED of slow and fast accreting AGN can be fitted with thin AD models. ii) The value of $\alpha_{OX}$ is very similar in slow and fast accreting systems up to a dimensionless accretion rate $\dot{\mathcal{M}}_{c}\sim$10. We only find a correlation between $\alpha_{OX}$ and $\dot{\mathcal{M}}$ for sources with $\dot{\mathcal{M}} > \dot{\mathcal{M}}_{c}$. In such cases, the faster accreting sources appear to have systematically larger $\alpha_{OX}$ values. iii) We also find that the torus in the faster accreting systems seems to be less efficient in reprocessing the primary AGN radiation having lower IR-to-optical spectral slopes. These findings, failing to recover the predicted differences between the SEDs of slim and thin ADs within the observed spectral window, suggest that additional physical processes or very special geometry act to reduce the extreme UV radiation in fast accreting AGN. This may be related to photon trapping, strong winds, and perhaps other yet unknown physical processes., Comment: Accepted for publication on MNRAS, 14 pages, 7 figures

We employ optical and UV observations to present SEDs for two reverberation-mapped samples of super-Eddington and sub-Eddington AGN with similar luminosity distributions. The samples are fitted with accretion disc models in order to look for SED differences that depend on the Eddington ratio. The fitting takes into account measured BH mass and accretion rates, BH spin and intrinsic reddening of the sources. All objects in both groups can be fitted by thin AD models over the range 0.2-1$\,\mu$m with reddening as a free parameter. The intrinsic reddening required to fit the data are relatively small, $E(B-V)\leq0.2$~mag, except for one source. Super-Eddington AGN seem to require more reddening. The distribution of $E(B-V)$ is similar to what is observed in larger AGN samples. The best fit disc models recover very well the BH mass and accretion for the two groups. However, the SEDs are very different, with super-Eddington sources requiring much more luminous far-UV continuum. The exact amount depends on the possible saturation of the UV radiation in slim discs. In particular, we derive for the super-Eddington sources a typical bolometric correction at 5100\AA{} of 60-150 compared with a median of $\sim$20 for the sub-Eddington AGN. The measured torus luminosity relative to $\lambda L_{\lambda}(5100\AA{}$) are similar in both groups. The $\alpha_{OX}$ distribution is similar too. However, we find extremely small torus covering factors for super-Eddington sources, an order of magnitude smaller than those of sub-Eddington AGN. The small differences between the groups regarding the spectral range 0.2-22$\,\mu$m, and the significant differences related to the part of the SED that we cannot observed may be consistent with some slim disc models. An alternative explanation is that present day slim-disc models over-estimate the far UV luminosity of such objects by a large amount., Comment: 21 pages, 10 figures, 4 tables, Accepted for publication in MNRAS