Probing the Trillion-Degree Quark Soup

Quantum Chromodynamics (QCD) is our fundamental theory of the strong interaction, but calculating and visualizing the many-body phases of quark-gluon matter remain difficult. Collisions of heavy nuclei at the Relativistic Heavy Ion Collider and the Large Hadron Collider (LHC) create droplets of quark-gluon plasma (QGP) at temperatures above a trillion degrees, where quarks and gluons are deconfined. Measurements of collective flow and long-range correlations indicate that this plasma behaves as an almost perfect liquid with remarkably low viscosity.

To go beyond studies of the debris of the QGP, we can study the passage of color-charged particles through this fascinating medium. Heavy-ion collisions produce not only the QGP, but also heavy quarks and energetic quarks and gluons in rare hard-scattering processes. High-energy quarks and gluons lose energy by radiating gluons or by colliding with other quarks and gluons as they traverse the QGP, a phenomenon often referred to as “jet quenching.” New measurements with Z0-tagged jets reveal a negative wake in the medium, providing direct evidence that fast partons stir the QGP and leave behind a hydrodynamic response. Slow-moving heavy quarks, which interact strongly with the QGP, open a window into the in-medium color force and hadronization in a high-density environment. In this talk, I will review the most striking observations from recent data collected by the Compact Muon Solenoid detector at the LHC and discuss the properties of the QGP fluid extracted from these measurements.