The quark–gluon plasma (QGP) is the high-temperature phase of QCD in which quarks and gluons are no longer confined into hadronic bound states but form a hot, colored medium. It is realized in ultra-relativistic heavy-ion collisions and existed in the early universe up to a few microseconds after the Big Bang.
As \(T\) increases through \(T_c\approx 155\) MeV (a crossover at zero baryon density), QCD passes from the confined, chiral-symmetry-broken hadronic phase to the QGP. Two aspects change:
- Deconfinement: color charges are liberated and Debye-screened (see Debye mass, deconfinement); the temporal string tension vanishes.
- Chiral symmetry restoration: the chiral condensate melts.
The QGP is not a simple gas of free partons. Its physics separates by scale:
the electric sector (\(gT\)) being Debye-screened while the magnetic sector (\(g^2T\)) remains non-perturbative (the Linde problem). Dimensional reduction maps these onto the 3D effective theories EQCD (electric) and MQCD (magnetic).
Even in the deconfined QGP the magnetic fields stay confining: a spatial Wilson loop obeys an area law with a non-vanishing spatial string tension \(\sigma_s(T)\) that grows with \(T\). Measuring \(\sigma_s(T)\) and comparing with the MQCD prediction (Cheng et al., arXiv:0806.3264) is a precise probe of the plasma's magnetic sector; see Why spatial string tension and dimensional reduction.