quantum chromodynamics In theoretical physics, quantum chromodynamics (QCD) is the theory 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 ...

, 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 In physics, a coupling constant or gauge coupling parameter (or, more simply, a coupling), is a number that determines the strength of the force exerted in an interaction. Originally, the coupling constant related the force acting between two ...

)

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 nuclei. All commonly o ...

s. It is used in studying the properties of

hadron In particle physics, a hadron (; grc, ἁδρός, hadrós; "stout, thick") is a composite subatomic particle made of two or more quarks held together by the strong interaction. They are analogous to molecules that are held together by the ele ...

s containing a single

charm Charm may refer to: Social science * Charisma, a person or thing's pronounced ability to attract others * Superficial charm, flattery, telling people what they want to hear Science and technology * Charm quark, a type of elementary particle * Ch ...

bottom Bottom may refer to: Anatomy and sex * Bottom (BDSM), the partner in a BDSM who takes the passive, receiving, or obedient role, to that of the top or dominant * Bottom (sex), a term used by gay couples and BDSM * Buttocks or bottom, part of th ...

quark. The effective theory was formalised in 1990 by

Howard Georgi Howard Mason Georgi III (born January 6, 1947) 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 Physics. He was Co-M ...

Estia Eichten Estia Joseph Eichten (born 1946), is an American theoretical physicist, of the Fermi National Accelerator Laboratory (Fermilab). He received his Ph.D. in 1972 from the MIT Center for Theoretical Physics, where he was a student of Roman Jackiw's, ...

and Christopher Hill, building upon the works of Nathan Isgur and Mark Wise,

Voloshin Voloshin, Woloshin,Wolloshin, Voloshyn or Woloshyn (Cyrillic: Волошин) is a Ukrainian and Russian masculine surname. It comes from the dated exonym Volokh ("Vlach", "Romanian"). Its feminine forms are Voloshina, Woloshina, Voloshyna or Wolos ...

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 The strong interaction or strong force is a fundamental interaction that confines quarks into proton, neutron, and other hadron particles. The strong interaction also binds neutrons and protons to create atomic nuclei, where it is called the n ...

, through which quarks and

gluon A gluon ( ) is an elementary particle that acts as the exchange particle (or gauge boson) for the strong force between quarks. It is analogous to the exchange of photons in the electromagnetic force between two charged particles. Gluons bind q ...

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 or process is one that cannot be described by perturbation theory. An example is the function : f(x) = e^, which does not have a Taylor series at ''x'' = 0. Every coefficient of the Taylor ...

(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 A spinning top, or simply a top, is a toy with a squat body and a sharp point at the bottom, designed to be spun on its vertical axis, balancing on the tip due to the gyroscopic effect. Once set in motion, a top will usually wobble for a few ...

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 char ...

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 whi ...

corrections.

References

Further reading