Search for dark matter axions

在低温谐振腔中利用里德伯原子寻找质量在11 μeV范围内的暗物质轴子。在搜索的同时，人们还努力通过修改实验装置的一些部分来提高检测效率。具体来说，为了提高探测效率，得到了以下改进：1)减少了腔体和场电离探测器区域周围的杂散电场；2)通过消除腔体和探测器区域内的磁场，减少了运动Stark效应对腔体中里德堡态共振宽度展宽的影响。此外，还研究了里德伯原子与基态Rb原子碰撞对选择性场电离性能的影响，以提高选择性。为了这些改进，对腔体内部和探测器系统进行了进一步的清洁，并通过加入一些由NbTi-Nb-Cu络合物制成的薄片，特别改善了场电离探测区域周围的磁场屏蔽。同时改进了脉冲场电离性能，提高了探测效率。我们还对腔内金属柱系统的结构进行了一定的修改，使腔内电场模式更加均匀，从而提高了检测效率。通过安装六极磁体增加原子束强度，选择和极化原子超精细态，提高了整体探测效率。通过这些努力，整体检测效率得到了提高，这些改进使我们能够缩短搜索轴子所需的时间。在所研究的质量区域没有明确观察到轴子信号。

Search for dark matter axions over the mass range of 11 μeV is in progress with Rydberg atoms in a resonant cavity at low temperature. Along with the search, efforts have been devoted to improve the detection efficiency by modifying a number of parts of the experimental apparatus. Specifically the following improvements were obtained for increasing the detection efficiency: 1) Reduction of the stray electric field around the cavity and the field ionization detector regions, 2) Reduction of the motional Stark effect on the broadening of the resonant width for the Rydberg states in a cavity by eliminating the magnetic field in the region of the cavity and the detector. In addition the effect of the collisions between the Rydberg atoms and the ground state Rb atoms on the selective field ionization performance was carefully examined to improve the selectivity. For these improvements, further cleaning of the inside of the cavity and of the detector system were carefully tried and shielding of the magnetic field around the field ionization detection region was especially improved by incorporating a number of sheets made of NbTi-Nb-Cu complex. Also we improved the pulsed field ionization performance to get better detection efficiency.We also somewhat modified the structure of metal post system in the cavity to get more uniform electric field mode in the cavity, thus improving the detection efficiency. Installing a sextupole magnet to increase the intensity of the atomic beam and to select and polarize the atomic hyperfine state was also examined to increase the overall detection efficiencies. With these efforts, the overall detection efficiency was increased and these improvements enabled us to shorten the necessary time to search for axions. No axion signals have been definitely observed at the mass region investigated.