Heavy quark effective theory

{{short description|Effective field theory describing the physics of heavy quarks}}

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)[http://users.phys.psu.edu/~cteq/schools/summer09/talks/sullivan/sullivan1.pdf Heavy quarks], p. 10, CTEQ Summer School 2009, Illinois Institute of Technology, retrieved 2016-03-01. quarks. It is used in studying the properties of hadrons containing a single charm or bottom quark.{{cite book|author1=Aneesh V. Manohar|author2=Mark B. Wise|title=Heavy Quark Physics|url=https://books.google.com/books?id=codDQK5OQDIC|date=2 July 2007|publisher=Cambridge University Press|isbn=978-0-521-03757-0}}{{cite book|last1=Grozin|first1=Andrey|volume=201|year=2004|issn=0081-3869|doi=10.1007/b79301|title=Heavy Quark Effective Theory|series=Springer Tracts in Modern Physics |bibcode=2004STMP..201.....G|isbn=978-3-540-20692-7 }} The effective theory was formalised in 1990 by Howard Georgi,{{cite journal|last1=Georgi|first1=Howard|title=An effective field theory for heavy quarks at low energies|journal=Physics Letters B|volume=240|issue=3–4|year=1990|pages=447–450|issn=0370-2693|doi=10.1016/0370-2693(90)91128-X|bibcode=1990PhLB..240..447G}} Estia Eichten and Christopher Hill,{{cite journal|last1=Eichten|first1=Estia|last2=Hill|first2=Brian|title=An effective field theory for the calculation of matrix elements involving heavy quarks|journal=Physics Letters B|volume=234|issue=4|year=1990|pages=511–516|issn=0370-2693|doi=10.1016/0370-2693(90)92049-O|bibcode=1990PhLB..234..511E}} building upon the works of Nathan Isgur and Mark Wise,{{cite journal|last1=Isgur|first1=Nathan|last2=Wise|first2=Mark B.|title=Weak decays of heavy mesons in the static quark approximation|journal=Physics Letters B|volume=232|issue=1|year=1989|pages=113–117|issn=0370-2693|doi=10.1016/0370-2693(89)90566-2|bibcode=1989PhLB..232..113I}} 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). [http://www.people.fas.harvard.edu/~hgeorgi/tasi.pdf TASI Lectures: Heavy Quark Effective Field Theory], retrieved 2016-03-01.

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.[https://arxiv.org/abs/hep-ph/9805468 Heavy Quark Physics], Mark Wise, 1998 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.