Search

The DArk Matter In CCDs at Modane (DAMIC-M) experiment is designed to search for light dark matter (m$_{\chi}$<10\,GeV/c$^2$) at the Laboratoire Souterrain de Modane (LSM) in France. DAMIC-M will use skipper charge-coupled devices (CCDs) as a kg-scale active detector target. Its single-electron resolution will enable eV-scale energy thresholds and thus world-leading sensitivity to a range of hidden sector dark matter candidates. A DAMIC-M prototype, the Low Background Chamber (LBC), has been taking data at LSM since 2022. The LBC provides a low-background environment, which has been used to characterize skipper CCDs, study dark current, and measure radiopurity of materials planned for DAMIC-M. It also allows testing of various subsystems like readout electronics, data acquisition software, and slow control. This paper describes the technical design and performance of the LBC.

Charge-coupled devices (CCDs) are a leading technology in direct dark matter searches because of their eV-scale energy threshold and high spatial resolution. The sensitivity of future CCD experiments could be enhanced by distinguishing nuclear recoil signals from electronic recoil backgrounds in the CCD silicon target. We present a technique for event-by-event identification of nuclear recoils based on the spatial correlation between the primary ionization event and the lattice defect left behind by the recoiling atom, later identified as a localized excess of leakage current under thermal stimulation. By irradiating a CCD with an $^{241}$Am$^{9}$Be neutron source, we demonstrate $>93\%$ identification efficiency for nuclear recoils with energies $>150$ keV, where the ionization events were confirmed to be nuclear recoils from topology. The technique remains fully efficient down to 90 keV, decreasing to 50$\%$ at 8 keV, and reaching ($6\pm2$)$\%$ at 1.5--3.5 keV. Irradiation with a $^{24}$Na $\gamma$-ray source shows no evidence of defect generation by electronic recoils, with the fraction of electronic recoils with energies $<85$ keV that are spatially correlated with defects $<0.1$$\%$., Comment: 9 pages, 7 figures

Dark Matter (DM) particles with sufficiently large cross sections may scatter as they travel through Earth's bulk. The corresponding changes in the DM flux give rise to a characteristic daily modulation signal in detectors sensitive to DM-electron interactions. Here, we report results obtained from the first underground operation of the DAMIC-M prototype detector searching for such a signal from DM with MeV-scale mass. A model-independent analysis finds no modulation in the rate of 1$e^-$ events with sidereal period, where a DM signal would appear. We then use these data to place exclusion limits on DM in the mass range [0.53, 2.7] MeV/c$^2$ interacting with electrons via a dark photon mediator. Taking advantage of the time-dependent signal we improve by $\sim$2 orders of magnitude on our previous limit obtained from the total rate of 1$e^-$ events, using the same data set. This daily modulation search represents the current strongest limit on DM-electron scattering via ultralight mediators for DM masses around 1 MeV/c$^2$.

We present results from a 3.25 kg-day target exposure of two silicon charge-coupled devices (CCDs), each with 24 megapixels and skipper readout, deployed in the DAMIC setup at SNOLAB. With a reduction in pixel readout noise of a factor of 10 relative to the previous detector, we investigate the excess population of low-energy events in the CCD bulk previously observed above expected backgrounds. We address the dominant systematic uncertainty of the previous analysis through a depth fiducialization designed to reject surface backgrounds on the CCDs. The measured bulk ionization spectrum confirms the presence of an excess population of low-energy events in the CCD target with characteristic rate of ${\sim}7$ events per kg-day and electron-equivalent energies of ${\sim}80~$eV, whose origin remains unknown., Comment: 8 pages, 4 figures

We report constraints on sub-GeV dark matter particles interacting with electrons from the first underground operation of DAMIC-M detectors. The search is performed with an integrated exposure of 85.23 g days, and exploits the subelectron charge resolution and low level of dark current of DAMIC-M charge-coupled devices (CCDs). Dark-matter-induced ionization signals above the detector dark current are searched for in CCD pixels with charge up to 7e$^-$. With this dataset we place limits on dark matter particles of mass between 0.53 and 1000 MeV/$c^2$, excluding unexplored regions of parameter space in the mass ranges [1.6,1000] MeV/$c^2$ and [1.5,15.1] MeV/$c^2$ for ultralight and heavy mediator interactions, respectively.

The DAMIC-M (DArk Matter In CCDs at Modane) experiment employs thick, fully depleted silicon charged-coupled devices (CCDs) to search for dark matter particles with a target exposure of 1 kg-year. A novel skipper readout implemented in the CCDs provides single electron resolution through multiple non-destructive measurements of the individual pixel charge, pushing the detection threshold to the eV-scale. DAMIC-M will advance by several orders of magnitude the exploration of the dark matter particle hypothesis, in particular of candidates pertaining to the so-called "hidden sector." A prototype, the Low Background Chamber (LBC), with 20g of low background Skipper CCDs, has been recently installed at Laboratoire Souterrain de Modane and is currently taking data. We will report the status of the DAMIC-M experiment and first results obtained with LBC commissioning data., Comment: 10 pages, 6 figures, Submission to SciPost Physics Proceedings: 14th International Conference on Identification of Dark Matter (IDM) 2022

Experiments aiming to directly detect dark matter through particle recoils can achieve energy thresholds of $\mathcal{O}(1\,\mathrm{eV})$. In this regime, ionization signals from small-angle Compton scatters of environmental $\gamma$-rays constitute a significant background. Monte Carlo simulations used to build background models have not been experimentally validated at these low energies. We report a precision measurement of Compton scattering on silicon atomic shell electrons down to 23$\,$eV. A skipper charge-coupled device (CCD) with single-electron resolution, developed for the DAMIC-M experiment, was exposed to a $^{241}$Am $\gamma$-ray source over several months. Features associated with the silicon K, L$_{1}$, and L$_{2,3}$-shells are clearly identified, and scattering on valence electrons is detected for the first time below 100$\,$eV. We find that the relativistic impulse approximation for Compton scattering, which is implemented in Monte Carlo simulations commonly used by direct detection experiments, does not reproduce the measured spectrum below 0.5$\,$keV. The data are in better agreement with $ab$ $initio$ calculations originally developed for X-ray absorption spectroscopy., Comment: 12 pages, 10 figures

This report presents an overview of the unconventional use of charge-coupled devices (CCDs) to search for Dark Matter (DM). The DArk Matter In CCDs (DAMIC Experiment) employs the bulk silicon of thick, fully-depleted CCDs as a target for ionization signals produced by interactions of particle dark matter from the galactic halo. The DAMIC collaboration has engaged in an extensive campaign of characterization efforts to understand the response of these CCDs to low-energy nuclear recoils and their unique capabilities, including the use of high spatial resolution for both the rejection and study of backgrounds. The preliminary results of DAMIC prove the performance of the detector, provide measurements of the background contamination and demonstrate the potentiality for DM searches, with only ~40 grams of detector mass. The next phase of the experiment, DAMIC-M (DArk Matter in CCDs at Modane), will consist of a kg-sized detector, implementing the most massive CCDs ever built. These CCDs will feature sub-electron noise and will be deployed in a low-radioactivity environment at the Laboratoire Souterrain de Modane in France., Comment: accepted by NIMA

Accretion onto SMBH is believed to occur mostly in obscured AGN. Such objects are proving rather elusive in surveys of distant galaxies, including those at X-ray energies. Our main goal is to determine whether the revised IRAC criteria of Donley et al. (2012) (objects with an IR power-law spectral shape), are effective at selecting X-ray type-2 AGN. We present the results from the X-ray spectral analysis of 147 AGN selected by cross-correlating the highest spectral quality ultra-deep XMM-Newton and the Spitzer/IRAC catalogues in the CDF-S. Consequently it is biased towards sources with high S/N X-ray spectra. In order to measure the amount of intrinsic absorption in these sources, we adopt a simple X-ray spectral model that includes a power-law modified by intrinsic absorption and a possible soft X-ray component. We find 21/147 sources to be heavily absorbed but the uncertainties in their obscuring column densities do not allow us to confirm their Compton-Thick nature without resorting to additional criteria. Although IR power-law galaxies are less numerous in our sample than IR non-power-law galaxies (60 versus 87 respectively), we find that the fraction of absorbed (N_{H} > 10^{22} cm^{-2}) AGN is significantly higher (at about 3 sigma level) for IR-power-law sources ($\sim$2/3) than for those sources that do not meet this IR selection criteria ($\sim$1/2). This behaviour is particularly notable at low luminosities, but it appears to be present, although with a marginal significance, at all luminosities. We therefore conclude that the IR power-law method is efficient in finding X-ray-absorbed sources. We would then expect that the long-sought dominant population of absorbed AGN is abundant among IR power-law spectral shape sources not detected in X-rays., Comment: 13 pages, 6 figures, 5 tables, accepted to A&A (01/07/2013)

The Chandra Deep Field is the region of the sky with the highest concentration of X-ray data available: 4Ms of Chandra and 3Ms of XMM data, allowing excellent quality spectra to be extracted even for faint sources. We take advantage of this in order to compile a sample of heavily obscured Active Galactic Nuclei (AGN) using X-ray spectroscopy. We select our sample among the 176 brightest XMM sources, searching for either a) flat X-ray spectra (Photon index<1.4 at the 90% confidence level) suggestive of a reflection dominated continuum or b) an absorption turn-over suggestive of a column density higher than ~10^{24} cm-2. We find a sample of nine candidate heavily obscured sources satisfying the above criteria. Four of these show statistically significant FeKalpha lines with large equivalent widths (three out of four have EW consistent with 1 keV) suggesting that these are the most secure Compton-thick AGN candidates. Two of these sources are transmission dominated while the other two are most probably reflection dominated Compton-thick AGN. Although this sample of four sources is by no means statistically complete, it represents the best example of Compton-thick sources found at moderate-to-high redshift with three sources at z=1.2-1.5 and one source at z=3.7. Using Spitzer and Herschel observations, we estimate with good accuracy the X-ray to mid-IR (12 micron) luminosity ratio of our sources. These are well below the average AGN relation, independently suggesting that these four sources are heavily obscured., Comment: To appear in A&A. 14 pages

We aim to characterize the nature of galaxies whose optical emission line diagnostics are consistent with star formation, but whose X-ray properties strongly point towards the presence of an AGN. Understanding these sources is of particular importance in assessing the completeness of AGN samples derived from large galaxy surveys, selected solely on the basis of their optical spectral properties.We construct a large sample of 211 NELGs, which have FWHMs Hb emission line <1200 km/s from the SDSS-DR7 galaxy spectroscopic catalogue, for which we are able to construct a classical diagnostic diagram, [OIII]/Hb versus [NII]/Ha (hence z<0.4), and that are also detected in the hard energy band and present in the 2XMM catalogue. This sample offers a large database by which to investigate potential mismatches between optical diagnostics and X-ray emission. Among these 211 objects, which based on our selection criteria are all at z<0.4, we find that 145 galaxies are diagnosed as AGNs, having 2-10 keV X-ray luminosities that span a wide range, from 10^40 erg/s to above 10^44 erg/s. Out of the remaining 66 galaxies, which are instead diagnosed as SF, we find a bimodal distribution in which 28 have X-ray luminosities in excess of 10^42 erg/s, large T (>1), and large X/O ratio (>0.1), while the rest are consistent with being simply SF galaxies. Those 28 galaxies exhibit the broadest Hb line FWHMs, from ~300 to 1200 km/s, and their X-ray spectrum is steeper than average and often displays a soft excess. We therefore conclude that the population of X-ray luminous NELGs with optical lines consistent with those of a starforming galaxy (which represent 19% of our whole sample) is largely dominated by NLS1s. The occurrence of such sources in the overall optically selected sample is small (<2%), hence the contamination of optically selected galaxies by NLS1s is very small., Comment: 16 pages, 8 figures, 4 tables, accepted for publication in A&A