What is Quark‑Gluon Plasma?
Quark‑gluon plasma (QGP) is a state of matter where quarks and gluons, normally confined within protons and neutrons, become free to move over a volume larger than a typical hadron. This extreme phase, often described as the “perfect liquid,” existed microseconds after the Big Bang and can be recreated in high‑energy heavy‑ion collisions.
Historical Discovery
The concept of QGP emerged from quantum chromodynamics (QCD) in the 1970s. Experimental evidence first appeared in the early 2000s at CERN’s Super‑Proton Synchrotron (SPS) and later at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory.
Key Properties
- Temperature: Over 4 trillion Kelvin, hotter than the core of the Sun.
- Viscosity: Near‑perfect fluid with a shear viscosity to entropy density ratio close to the quantum lower bound (ℏ/4πkB).
- Collective Flow: Exhibits strong anisotropic flow patterns (elliptic flow) indicating hydrodynamic behavior.
Major Experiments
The two primary facilities exploring QGP are:
- RHIC (Brookhaven, USA): Pioneered the discovery of a strongly coupled QGP.
- LHC – ALICE experiment (CERN, Switzerland): Extends the energy frontier, producing hotter and longer‑lived plasma.
Future Directions
Upcoming upgrades to RHIC and the High‑Luminosity LHC (HL‑LHC) will enable precision measurements of QGP transport coefficients. New facilities such as the Electron‑Ion Collider (EIC) will investigate how quarks and gluons distribute inside nucleons, complementing the heavy‑ion program.