Quantum Chromodynamics (QCD) predicts a transition from normal hadronic matter to a phase where the quarks and gluons are no longer bound together and can move freely. Quark gluon plasma is now produced regularly in collisions of heavy nuclei at very high energy at both the Relativistic Heavy Ion Collider (RHIC) in the U.S. and at the LHC in Europe.
Quark gluon plasma exhibits remarkable properties. Its vanishingly small shear viscosity to entropy density ratio means that it ﬂows essentially without internal friction, making it one of the most "perfect" liquids known. It is also very opaque to transiting particles. Determining the transport properties of quark gluon plasma is a key goal of current research, and I will show how we can access this by measuring the features of hadron jets transiting the plasma. Recent data suggest that even very small colliding systems may produce a droplet of quark gluon plasma, deepening the mystery of how plasma emerges from cold, dense gluonic matter deep inside nuclei within 1 fm/c. I will discuss how a future electron-ion collider can help address this question.