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

尽管有大量的天文学和宇宙学证据表明宇宙中存在暗物质，但其未知的性质可以说是现代物理学中最紧迫的问题。wimp（弱相互作用大质量粒子）是迄今为止备受青睐的标准模型框架之外出现的一类候选者。许多良好的超对称（SUSY）模型预测的10 GeV到10 tev质量的wimp到目前为止还没有在暗物质搜索实验中发现。这一点，再加上LHC（大型强子对撞机）缺乏SUSY的证据，保证了将搜索扩展到更低的WIMP质量。预测低质量wimp的新理论，如不对称暗物质和暗扇区，进一步加强了这项研究的动机。NEWS-G（球体-气体新实验）合作的目的是使用超低背景球形比例计数器（SPC）发现低质量暗物质粒子（< 1 GeV）。SPCs是按比例模式操作的球形气体探测器。一个小的传感器，在一个接地的球形壳的中心，以高电压保持，正在漂移由粒子与气体目标相互作用产生的初级电离。当自由电荷接近传感器时，电场变得足够大，足以产生雪崩过程。传感器的高放大增益和低本征电容使其对单个电子的灵敏度达到10 eV的能量阈值。这一点，再加上使用轻目标气体，如氢、氦和氖，可以优化低质量wimp的动量传递，使SPC成为寻找质量低于1 GeV的wimp的有力工具。NEWS-G合作项目由43名科学家组成，他们分别来自加拿大（女王大学、斯诺拉布/劳伦森大学、TRIUMF、阿尔伯塔大学）、法国（LSM、CEA IRFU、格勒诺布尔LPSC）、希腊（塞萨洛尼基亚里士多德大学）、英国（伯明翰大学）和美国（PNNL）的11个机构。WIMP搜索结果最近由SEDINE合作报告，SEDINE是安装在法国Souterrain de Modane实验室（LSM）的直径60厘米的原型SPC。这些结果证明了在氖-甲烷混合物的低能量阈值下的SPC操作，并且在发表时对WIMP质量低于0.6 GeV的WIMP-核自旋无关截面给出了最严格的限制。SEDINE获得的竞争性结果为NEWS-G实验的下一阶段提供了希望：2020年初将在SNOLAB安装一个直径140厘米的SPC。得益于增强的背景控制、传感器开发和新型探测器监测技术，SNOLAB的NEWS-G将其灵敏度提高1000倍，并准备获得世界上对低于1 GeV的WIMP质量的最佳灵敏度。

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.