DAMIC-M: Dark Matter In CCDs at Modane

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Precision measurement of Compton scattering in silicon with a skipper CCD for dark matter detection

We report a precision measurement of Compton scattering on silicon atomic shell electrons down to 23eV. A skipper charge-coupled device (CCD) with single-electron resolution, developed for the DAMIC-M experiment, was exposed to a 241Am γ-ray source over several months. Features associated with the silicon K, L1, and L2,3-shells are clearly identified, and scattering on valence electrons is detected for the first time below 100eV. 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 0.5keV. The data are in better agreement with ab initio calculations originally developed for X-ray absorption spectroscopy.

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DAMIC-MThe DAMIC-M (DArk Matter In CCDs at Modane) experiment employs a novel technique to search for the elusive particles that we think make up most of the matter in the universe—dark matter.

DAMIC-M detects nuclear and electronic recoils induced by dark matter particles in the silicon bulk of charge-coupled devices - the CCDs that have been used for many years in digital cameras and in the focal plane of astronomical telescopes for the digital imaging of faint astrophysical objects. Our unusually thick CCDs - almost a mm compared with the typical tens of microns – are extremely sensitive: they can detect signals as low as few electrons, as those expected from light dark matter interactions. In addition, the spatial resolution of these devices – the pixel size is 15 micron x 15 micron – results in the unique capability to characterize and reject backgrounds from radiogenic sources. This unconventional use of CCDs was pioneered by a precursor of DAMIC-M, the DAMIC experiment at the SNOLAB underground laboratory in Canada, where a 40-g prototype detector was operated.

DAMIC-M is being installed at the Laboratoire Souterrain de Modane in France, protected from cosmic rays by the rock overburden of the Alps. Its kg-size detector features the most massive CCDs ever built and a novel concept for signal readout – based on non-destructive, repetitive measurements of the pixel charge – resulting in the high-resolution detection of a single electron. With this unprecedented sensitivity DAMIC-M is taking a leap forward of several orders of magnitude in the exploration of the dark matter particle hypothesis, in particular of candidates pertaining to the so-called “hidden sector” which may have well so far escaped detection.