quantum chromodynamics In theoretical physics, quantum chromodynamics (QCD) is the study of the strong interaction between quarks mediated by gluons. Quarks are fundamental particles that make up composite hadrons such as the proton, neutron and pion. QCD is a type of ...

, heavy quark effective theory (HQET) is an

effective field theory In physics, an effective field theory is a type of approximation, or effective theory, for an underlying physical theory, such as a quantum field theory or a statistical mechanics model. An effective field theory includes the appropriate degrees ...

describing the physics of heavy (that is, of mass far greater than the QCD scale)

quark A quark () is a type of elementary particle and a fundamental constituent of matter. Quarks combine to form composite particles called hadrons, the most stable of which are protons and neutrons, the components of atomic nucleus, atomic nuclei ...

s. It is used in studying the properties of

hadron In particle physics, a hadron is a composite subatomic particle made of two or more quarks held together by the strong nuclear force. Pronounced , the name is derived . They are analogous to molecules, which are held together by the electri ...

s containing a single

charm Charm or Charms may refer to: Arts and entertainment * The Charms, an American garage rock band * Otis Williams and the Charms, an American doo-wop group * The Charm (Bubba Sparxxx album), ''The Charm'' (Bubba Sparxxx album), 2006 * Charm (Danny! ...

or bottom quark. The effective theory was formalised in 1990 by

Howard Georgi Howard Mason Georgi III (born January 6, 1947 in San Bernardino) is an American theoretical physicist and the Mallinckrodt Professor of Physics and Harvard College Professor at Harvard University. He is also director of undergraduate studies in ...

, Estia Eichten and Christopher Hill, building upon the works of Nathan Isgur and Mark Wise, Voloshin and Shifman,Shifman, M.A., & Voloshin, M.V. (1987). On production of D and D^* mesons in B-meson decays (ITEP--64(1987)). USSR and others. Georgi, H. (1991)
TASI Lectures: Heavy Quark Effective Field Theory
retrieved 2016-03-01. Quantum chromodynamics (QCD) is the theory of

strong force In nuclear physics and particle physics, the strong interaction, also called the strong force or strong nuclear force, is one of the four known fundamental interactions. It confines quarks into protons, neutrons, and other hadron particles, an ...

, through which quarks and

gluon A gluon ( ) is a type of Massless particle, massless elementary particle that mediates the strong interaction between quarks, acting as the exchange particle for the interaction. Gluons are massless vector bosons, thereby having a Spin (physi ...

s interact. HQET is the limit of QCD with the quark mass taken to infinity while its four-velocity is held fixed.Heavy Quark Physics
Mark Wise, 1998 This approximation enables

non-perturbative In mathematics and physics, a non-perturbative function (mathematics), function or process is one that cannot be described by perturbation theory. An example is the function : f(x) = e^, which does not equal its own Taylor series in any neighbo ...

(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 Top most commonly refers to: * Top, a basic term of orientation, distinguished from bottom, front, back, and sides * Spinning top, a ubiquitous traditional toy * Top (clothing), clothing designed to be worn over the torso * Mountain top, a moun ...

quarks, whereas up, down and

strange Strange may refer to: Fiction * Strange (comic book), a comic book limited series by Marvel Comics * Strange (Marvel Comics), one of a pair of Marvel Comics characters known as The Strangers * Adam Strange, a DC Comics superhero * The title c ...

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 In quantum mechanics, perturbation theory is a set of approximation schemes directly related to mathematical perturbation for describing a complicated quantum system in terms of a simpler one. The idea is to start with a simple system for which ...

corrections.

References

Further reading