We report direct-detection constraints on light dark matter particles interacting with electrons. The results are based on a method that exploits the extremely low levels of leakage current of the ...DAMIC detector at SNOLAB of 2–6×10−22 A cm−2. We evaluate the charge distribution of pixels that collect <10e− for contributions beyond the leakage current that may be attributed to dark matter interactions. Constraints are placed on so-far unexplored parameter space for dark matter masses between 0.6 and 100 MeV c−2. We also present new constraints on hidden-photon dark matter with masses in the range 1.2–30 eV c−2.
We present constraints on the existence of weakly interacting massive particles (WIMPs) from an 11 kg d target exposure of the DAMIC experiment at the SNOLAB underground laboratory. The observed ...energy spectrum and spatial distribution of ionization events with electron-equivalent energies >200 eV_{ee} in the DAMIC CCDs are consistent with backgrounds from natural radioactivity. An excess of ionization events is observed above the analysis threshold of 50 eV_{ee}. While the origin of this low-energy excess requires further investigation, our data exclude spin-independent WIMP-nucleon scattering cross sections σ_{χ-n} as low as 3×10^{-41} cm^{2} for WIMPs with masses m_{χ} from 7 to 10 GeV c^{-2}. These results are the strongest constraints from a silicon target on the existence of WIMPs with m_{χ}<9 GeV c^{-2} and are directly relevant to any dark matter interpretation of the excess of nuclear-recoil events observed by the CDMS silicon experiment in 2013.
Several years ago measurements at the Fermilab Tevatron proton-antiproton collider showed top quarks to be produced preferably in the proton direction and the antitop quarks in the antiproton ...direction. The size of such asymmetries was not expected in the standard model and this sparked intensive effort. This article reviews the current theoretical and experimental progress in understanding the asymmetries. The revised measurements and calculations are now in relatively good agreement. The LHC now produces large quantities of top quarks and can study related, but different, measures of asymmetric top quark production. The prospects for the LHC to extend our understanding and to probe for new physics beyond the standard model paradigm are also discussed. The asymmetries in top quark pair production at the Tevatron and the LHC are reviewed. The experimental measurements and the interpretations of a possible excess in terms of new physics are summarized. Other top quark properties are also reviewed, emphasizing effects related to the tt asymmetries, as well as other collider signals.
An important source of background in direct searches for low-mass dark matter particles are the energy deposits by small-angle scattering of environmental γ rays. We report detailed measurements of ...low-energy spectra from Compton scattering of γ rays in the bulk silicon of a charge-coupled device (CCD). Electron recoils produced by γ rays from Co57 and Am241 radioactive sources are measured between 60 eV and 4 keV. The observed spectra agree qualitatively with theoretical predictions, and characteristic spectral features associated with the atomic structure of the silicon target are accurately measured for the first time. A theoretically motivated parametrization of the data that describes the Compton spectrum at low energies for any incident γ-ray flux is derived. The result is directly applicable to background estimations for low-mass dark matter direct-detection experiments based on silicon detectors, in particular for the DAMIC experiment down to its current energy threshold.
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