In quantum chromodynamics, heavy quark effective theory (HQET) is an effective field theory describing the physics of heavy (that is, of mass far greater than the QCD scale)[1] quarks.
[2][3] The effective theory was formalised in 1990 by Howard Georgi,[4] Estia Eichten and Christopher Hill,[5] building upon the works of Nathan Isgur and Mark Wise,[6] Voloshin and Shifman,[7] and others.
[8] Quantum chromodynamics (QCD) is the theory of strong force, through which quarks and gluons interact.
HQET is the limit of QCD with the quark mass taken to infinity while its four-velocity is held fixed.
[9] This approximation enables non-perturbative (in the strong interaction coupling) treatment of quarks that are much heavier than the QCD mass scale.