In two dimensions, a system of self-gravitating particles collapses and forms a singularity in finite time below a critical temperature $T_c$. We investigate experimentally a quasi-two-dimensional cloud of cold neutral atoms in interaction with two pairs of perpendicular counterpropagating quasiresonant laser beams, in order to look for a signature of this ideal phase transition: indeed, the radiation pressure forces exerted by the laser beams can be viewed as an anisotropic, and nonpotential, generalization of two-dimensional self-gravity. We first show that our experiment operates in a parameter range which should be suitable to observe the collapse transition. However, the experiment unveils only a moderate compression instead of a phase transition between the two phases. A three-dimensional numerical simulation shows that both the finite small thickness of the cloud, which induces a competition between the effective gravity force and the repulsive force due to multiple scattering, and the atomic losses due to heating in the third dimension contribute to smearing the transition.

中文翻译：

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用冷原子研究各向异性的长程相互作用

在两个维度上，自重粒子系统崩溃并在低于临界温度的有限时间内形成奇点 ${Ť}_C$。我们通过实验研究冷中性原子的准二维云与两对垂直反向传播的准谐振激光束的相互作用，以寻找这种理想相变的特征：的确，激光束施加的辐射压力可以看作是二维自重的各向异性且非势能的概括。我们首先表明我们的实验在一个参数范围内进行，该参数范围应适合于观察坍塌过渡。但是，该实验仅显示了适度的压缩，而不显示两个相之间的相变。三维数值模拟表明，由于有限的云层厚度，由于多次散射，导致了有效重力和排斥力之间的竞争，