Search for very light WIMP's with gaseous spherical detector – NEWS

The main component is a large spherical proportional counter 1.4m in diameter able to reach 10 bar of high pressure gas. Such a detector consists of a spherical metallic vessel and a small metallic ball (about 2mm in diameter) located at the center of the vessel, also called the sensor. The sensor is maintained in the centre of the sphere by a rod and is set at high voltage. The electric field allows the ionisation electrons of a particle interaction inside the volume to drift to the sensor where an avalanche is formed, allowing an amplification of the signal. The detector will be installed at SNOLAB, which is 2 km underground in the Vale Creighton Mine near Sudbury Ontario/Canada. The detector will be surrounded by a adequate radio-pure shield to absorb gamma rays and neutrons. Radon free air will be used to flush the volume between the sphere and the lead shield when taking background data. A similar detector, 60 cm in diameter, will serve to validate the concept for the SNOLAB detector.

The NEWS experiment at Modane (LSM) sets new constraints on the spin-independent WIMP-nucleon scattering cross- section below 0.6 GeV/c2 and excludes at 90 % confidence level (C.L.) a cross-section of 4.8×10-37 cm2 for a 0.5 GeV/c2 WIMP mass. We thereby demonstrate the high potential of Spherical Proportional Counters for the search of low-mass WIMPs. The next phase of the experiment will build upon the knowledge acquired from the operation prototype at the LSM. Recent important development: A novel concept of proportional gas amplification for the read-out of the spherical proportional counter has been recently developed. The purpose is to improve performance for large diameter (1>m) spherical detector and for high pressure operation. We have developed a multi-ball read-out system which consists of several balls sitting at a fixed distance from the center of the spherical vessel. Such a module can tune the volume electric field at the desired value and can also provide detector segmentation with individual ball read-out. In the latter case the large volume of the vessel becomes a spherical time projection chamber with 3D capabilities.

The main characteristics of the detector are of great importance: sub-keV energy threshold, fiducialisation and background rejection by pulse shape analysis, ability to operate at pressures up to 10 bars, tens of kg of gas with various light targets such as H, He and Ne nuclei . Such a detector will have an unprecedented sensitivity to address the 0.1-10 GeV mass range of dark matter particles and the result would have an extraordinary scientific impact. The proposed concept of the spherical gaseous detector for the detection of very low mass particles of Dark Matter can also be used for other applications, but with very different sizing, constraints and running parameters.

Recent published papers [1] First results from the NEWS-G direct dark matter search experiment at the LSM, NEWS-G Collaboration, submitted to Astroparticle Physics Journal (arXiv:1706.04934). [2] Low energy recoil detection with a spherical proportional counter, Nucl. Instr. Meth. A (arXiv:1606.02146). [3] Neutron spectroscopy with the Spherical Proportional Counter based on nitrogen gas Nucl Instr. Meth. A847 (2017) 10. Patents 1. G. Gerbier, I. Giomataris, Th. Papaevangelou and I. Savvidis, BD14437SG 2. I. Giomataris, J. Derre, P. Magnier, CEA patent, BD15937SG ? 3. I. Giomataris, J. Derre, P. Magnier, CEA patent, BD16064SG

The NEWS project is dedicated to the direct search for very-low mass Dark Matter particles named WIMPs, from 0.1 to 10 GeV. Given the recent absence of evidence at LHC for SUSY and departure from the standard model of particle physics, the Dark Matter, an essential ingredient for understanding our Universe, appears as one of the only evidence for new physics. In particular, in a number of new models, the preferred particle candidates are less massive than anticipated. Searches for such light Dark Matter require new detection technology. The goal is to build a large (2 m diameter) radio-pure spherical gaseous detector that will operate at SNOLAB underground environment with the aim of reaching a much higher sensitivity for light Dark Matter search than any other experiment. The biggest part of the budget (2 M$) is already, thanks to a grant of excellence, assured by Queen’s University and it will be dedicated to the vessel and the infrastructure. The ANR request concerns the most critical parts required to reach the expected performance: the low-radioactivity sensor, the electronics, the DAQ and the calibration system. An existing detector at LSM underground laboratory will act as a prototype, where the upgrades for the SNOLAB detector will be validated. Indeed the very competitive background level reached by this detector, compared to existing experiments, makes it an ideal facility for testing key components of the SNOLAB project. The main characteristics of the detector are: sub-keV energy threshold, fiducialisation and background rejection by pulse shape analysis, ability to operate at pressures up to 10 bars, tens of kg of gas with various light targets such as H, He and Ne nuclei. Such a detector will have an unprecedented sensitivity to address the 0.1-10 GeV mass range of particles with spin independent nucleon cross section as low as 10-6 pb. We would like to point out that, for a modest cost, this project could have extraordinary scientific impact.