We review dark matter (DM) candidates of a very low mass appearing in the window below the traditional weakly interacting massive particle (m χ ≲ 10 GeV) and extending down to m χ ≳ 1 meV, somewhat below the mass limit at which DM becomes wavelike. Such candidates are motivated by hidden sectors such as hidden valleys, which feature hidden forces and rich dynamics, but have evaded traditional accelerator searches for New Physics because of their relatively weak coupling to the Standard Model (SM). Such sectors can still be detected through dedicated low-energy colliders, which, through their intense beams, can have sensitivity to smaller couplings, or through astrophysical observations of the evolution of DM halos and stellar structures, which, through the Universe's epochs, can be sensitive to small DM interactions. We also consider mechanisms whereby the DM abundance is fixed through the interaction with the SM, which directly motivates the search for light DM in terrestrial experiments. The bulk of this review is dedicated to the new ideas that have been proposed for direct detection of such DM candidates of a low mass through nuclear recoils, electronic excitations, or collective modes such as phonons and magnons. The rich tapestry of materials and modes in the condensed matter landscape is reviewed along with specific prospects for detection.

中文翻译：

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质量极低的暗物质候选者


我们回顾了在传统弱相互作用大质量粒子 （m χ ≲ 10 GeV） 下方的窗口中出现的非常低质量的暗物质 （DM） 候选者，并向下延伸到 m χ ≳ 1 meV，略低于 DM 变成波状的质量极限。这些候选者受到隐藏部门的激励，例如隐藏的山谷，这些部门具有隐藏的力和丰富的动力学，但由于它们与标准模型 （SM） 的耦合相对较弱，因此避开了对新物理学的传统加速器搜索。这些扇区仍然可以通过专用的低能对撞机来探测，这些对撞机通过其强烈的光束，可以对较小的耦合敏感，或者通过对 DM 晕和恒星结构演化的天体物理观测来探测，在宇宙的时代中，这些对元对小的 DM 相互作用很敏感。我们还考虑了通过与 SM 相互作用固定 DM 丰度的机制，这直接激发了在地面实验中寻找轻 DM。本综述的大部分内容致力于通过核后坐力、电子激发或声子和磁振子等集体模式直接检测此类低质量 DM 候选者的新想法。综述了凝聚态景观中丰富的材料和模式以及探测的具体前景。