Title: Axion minivoids and implications for direct detection Show Chinese title

Title: 轴子小空洞及其直接探测的影响

Abstract: In the scenario in which QCD axion dark matter is produced after inflation, the Universe is populated by large inhomogeneities on very small scales. Eventually, these fluctuations will collapse gravitationally to form dense axion miniclusters that trap up to $\sim$75% of the dark matter within asteroid-mass clumps. Axion miniclusters are physically tiny however, so haloscope experiments searching for axions directly on Earth are much more likely to be probing ``minivoids'' -- the space in between miniclusters. This scenario seems like it ought to spell doom for haloscopes, but while these minivoids might be underdense, they are not totally devoid of axions. Using Schr\"odinger-Poisson and N-body simulations to evolve from realistic initial field configurations, we quantify the extent to which the local ambient dark matter density is suppressed in the post-inflationary scenario. We find that a typical experimental measurement will sample an axion density that is only around 10% of the expected galactic dark matter density. Our results also have implications for experimental campaigns lasting longer than a few years, as well as broadband haloscopes that have sensitivity to transient signatures. We show that for a $\mathcal{O}$(year)-long integration times, the measured dark matter density should be expected to vary by 20--30%. Show Chinese abstract

Abstract: 在量子色动力学（QCD）轴子暗物质在暴胀后产生的场景下，宇宙中充满了非常小尺度上的大不均匀性。最终，这些涨落会引力坍缩形成密集的轴子小型团块，这些小型团块在小行星质量的团块内捕获多达 $\sim$ 75% 的暗物质。然而，轴子小型团块在物理上非常微小，因此在地球上直接寻找轴子的腔体实验更有可能探测到的是“小型空洞”——即小型团块之间的空间。 这种场景似乎对腔体实验来说是灾难性的，但尽管这些小型空洞可能稀疏，它们并不完全缺乏轴子。 使用薛定谔-泊松和N体模拟从真实的初始场构型演化，我们量化了暴胀后场景中局地环境暗物质密度被抑制的程度。我们发现，典型的实验测量将采样一个仅为预期星系暗物质密度约10%的轴子密度。我们的结果还对持续时间超过几年的实验活动以及对瞬态信号敏感的宽带腔体实验有影响。我们表明，对于 $\mathcal{O}$（年）长的积分时间，预计测得的暗物质密度会变化20–30%。