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) quarks.
It is used in studying the properties of hadrons
containing a single charm or bottom quark.
The effective theory was formalised in 1990
by Howard Georgi, Estia Eichten and Christopher
Hill, building upon the works of Nathan Isgur
and Mark Wise, Voloshin and Shifman, and others.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.
This approximation enables non-perturbative
(in the strong interaction coupling) treatment
of quarks that are much heavier than the QCD
mass scale.
The mass scale is of order 200 MeV.
Hence the heavy quarks include charm, bottom
and top quarks, whereas up, down and strange
quarks are considered light.
Since the top quark is extremely short-lived,
only the charm and bottom quarks are of significant
interest to HQET, of which only the latter
has mass sufficiently high that the effective
theory can be applied without major perturbative
corrections
