NEWS-G Search for Light Dark Matter with Spherical Proportional Counters

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Despite the overwhelming amount of astronomical and cosmological evidences for dark matter in the universe, its unknown nature is arguably the most pressing question of modern physics. WIMPs (Weakly Interacting Massive Particles) are a class of candidates emerging from beyond-the-Standard-Model frameworks that have been favoured so far. The 10 GeV to 10 TeV-mass WIMPs predicted by many well-motivated SUSY (supersymmetric) models have so far not been found by dark matter search experiments. This, combined with the lack of evidence for SUSY at the LHC (Large Hadron Collider), warrants for the extension of the search to lower WIMP masses. The motivation for this search is further strengthened by new theories predicting low-mass WIMPs such as asymmetric dark matter and dark sectors. The aim of the NEWS-G (New Experiment With Spheres - Gas) collaboration is the discovery of low-mass dark matter particles (< 1 GeV) using ultra-low background spherical proportional counters (SPC). SPCs are spherical gaseous detectors operated in proportional mode. A small sensor, held at high voltage at the centre of a grounded spherical shell, is drifting the primary ionization created by the interaction of a particle with the gaseous target. As the free charge approaches the sensor, the electric field becomes large enough to create an avalanche process. Sensitivity to single electrons, corresponding to an energy threshold of a few 10's of eV, is made possible by the high amplification gain and low intrinsic capacitance of the sensor. This, combined with the use of light target gases such as hydrogen, helium and neon which allows for the optimization of momentum transfer with low-mass WIMPs, makes the SPC a powerful tool in the search for WIMPs with masses below 1 GeV. The NEWS-G collaboration is comprised of 43 scientists from 11 institutions in Canada (Queen's University, SNOLAB/Laurentian University, TRIUMF, University of Alberta), France (LSM, CEA IRFU, LPSC Grenoble), Greece (Aristotle University of Thessaloniki), United Kingdom (University of Birmingham) and USA (PNNL). WIMP search results were recently reported by the collaboration using SEDINE, a 60-cm diameter prototype SPC installed at the Laboratoire Souterrain de Modane (LSM) in France. These results demonstrate the SPC operation at low energy threshold with a neon-methane mixture and at the time of publication were giving the most stringent limits on the WIMP-nucleus spin-independent cross-section for WIMP masses below 0.6 GeV. The competitive results obtained with SEDINE are promising for the next phase of the NEWS-G experiment: a 140-cm diameter SPC to be installed at SNOLAB in early 2020. Thanks to enhanced background control, sensor development and novel detector monitoring techniques, NEWS-G at SNOLAB is set to improve its sensitivity by a factor 1000 and is poised to obtain the world's best sensitivity to WIMP masses below 1 GeV.