Heavy ion collisions quickly form a droplet of quark-gluon plasma (QGP) with a remarkably small viscosity. We give an accessible introduction to how to study this smallest and hottest droplet of liquid made on earth and why it is so interesting. The physics of heavy ions ranges from highly energetic quarks and gluons described by perturbative QCD to a bath of strongly interacting gluons at lower energy scales. These gluons quickly thermalize and form QGP, while the energetic partons traverse this plasma and end in a shower of particles called jets. Analyzing the final particles in a variety of different ways allows us to study the properties of QGP and the complex dynamics of multi-scale processes in QCD which govern its formation and evolution, providing what is perhaps the simplest form of complex quantum matter that we know of. Much remains to be understood, and throughout the review big open questions will be encountered.

中文翻译：

---

重离子碰撞：大局和大问题

重离子碰撞会迅速形成一滴粘性非常小的夸克-胶子等离子体 (QGP)。我们提供了一个易于理解的介绍，介绍了如何研究地球上这种最小、最热的液滴，以及它为何如此有趣。重离子的物理学范围从微扰 QCD 描述的高能夸克和胶子到低能级的强相互作用胶子浴。这些胶子迅速热化并形成 QGP，而高能部分穿过该等离子体并以称为喷流的粒子雨结束。以各种不同的方式分析最终粒子使我们能够研究 QGP 的性质以及 QCD 中控制其形成和演化的多尺度过程的复杂动力学，提供可能是我们所知的最简单的复杂量子物质形式的。