Fairbairn Malcolm, Marsh David J E, Quevillon Jérémie
Kings College London, Strand, London, WC2R 2LS, United Kingdom.
Phys Rev Lett. 2017 Jul 14;119(2):021101. doi: 10.1103/PhysRevLett.119.021101. Epub 2017 Jul 11.
The phase transition responsible for axion dark matter (DM) production can create large amplitude isocurvature perturbations, which collapse into dense objects known as axion miniclusters. We use microlensing data from the EROS survey and from recent observations with the Subaru Hyper Suprime Cam to place constraints on the minicluster scenario. We compute the microlensing event rate for miniclusters, treating them as spatially extended objects. Using the published bounds on the number of microlensing events, we bound the fraction of DM collapsed into miniclusters f_{MC}. For an axion with temperature-dependent mass consistent with the QCD axion, we find f_{MC}<0.083(m_{a}/100 μeV)^{0.12}, which represents the first observational constraint on the minicluster fraction. We forecast that a high-efficiency observation of around ten nights with Subaru would be sufficient to constrain f_{MC}≲0.004 over the entire QCD axion mass range. We make various approximations to derive these constraints, and dedicated analyses by the observing teams of EROS and Subaru are necessary to confirm our results. If accurate theoretical predictions for f_{MC} can be made in the future, then microlensing can be used to exclude or discover the QCD axion. Further details of our computations are presented in a companion paper [M. Fairbairn, D. J. E. Marsh, J. Quevillon, and S. Rozier (to be published)].
负责轴子暗物质（DM）产生的相变可以产生大幅等曲率微扰，这些微扰会坍缩成被称为轴子微团簇的致密物体。我们使用来自EROS巡天的微引力透镜数据以及近期斯巴鲁超新星相机的观测数据来对微团簇模型施加限制。我们将微团簇视为空间扩展物体，计算其微引力透镜事件率。利用已发表的微引力透镜事件数量的限制，我们对坍缩成微团簇的暗物质比例(f_{MC})进行了限制。对于质量与量子色动力学轴子一致且与温度相关的轴子，我们发现(f_{MC}<0.083(m_{a}/100 μeV)^{0.12})，这代表了对微团簇比例的首个观测限制。我们预测，斯巴鲁进行大约十个晚上的高效观测足以在整个量子色动力学轴子质量范围内将(f_{MC}≲0.004)限制住。我们进行了各种近似来得出这些限制，EROS和斯巴鲁的观测团队需要进行专门分析来确认我们的结果。如果未来能够对(f_{MC})做出准确的理论预测，那么微引力透镜可用于排除或发现量子色动力学轴子。我们计算的更多细节在一篇配套论文[M. Fairbairn, D. J. E. Marsh, J. Quevillon, and S. Rozier（待发表）]中给出。
