quantum field theory In theoretical physics, quantum field theory (QFT) is a theoretical framework that combines classical field theory, special relativity, and quantum mechanics. QFT is used in particle physics to construct physical models of subatomic particles and ...

, soft-collinear

effective theory In science, an effective theory is a scientific theory which proposes to describe a certain set of observations, but explicitly without the claim or implication that the mechanism employed in the theory has a direct counterpart in the actual causes ...

(or SCET) is a theoretical framework for doing calculations that involve interacting particles carrying widely different energies. The motivation for developing SCET was to control the

infrared divergence In physics, an infrared divergence (also IR divergence or infrared catastrophe) is a situation in which an integral, for example a Feynman diagram, diverges because of contributions of objects with very small energy approaching zero, or equivalent ...

s that occur in

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 ...

(QCD) calculations that involve particles that are ''soft''—carrying much lower energy or momentum than other particles in the process—or ''collinear''—traveling in the same direction as another particle in the process. SCET is an effective theory for highly energetic

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 interacting with collinear and/or soft

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. It has been used for calculations of the decays of

B meson In particle physics, B mesons are mesons composed of a bottom antiquark and either an up (), down (), strange () or charm quark (). The combination of a bottom antiquark and a top quark is not thought to be possible because of the top quark' ...

s (quark-antiquark bound states involving a

bottom quark The bottom quark or b quark, also known as the beauty quark, is a third-generation heavy quark with a charge of − ''e''. All quarks are described in a similar way by electroweak and quantum chromodynamics, but the bottom quark has exc ...

) and the properties of jets (sprays of

hadrons 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 ...

that emerge from particle collisions when a quark or gluon is produced). SCET has also been used to calculate electroweak interactions in Higgs boson production. The new feature of SCET is its ability to handle more than one soft energy scale. For example, processes involving quarks carrying a high energy ''Q'' interacting with gluons have two soft scales: the transverse momentum ''p_T'' of the collinear particles, plus the even softer scale ''p_T²/Q''. SCET provides a power-counting formalism for doing

perturbation theory In mathematics and applied mathematics, perturbation theory comprises methods for finding an approximate solution to a problem, by starting from the exact solution of a related, simpler problem. A critical feature of the technique is a middle ...

in the small parameter ''Λ_QCD/Q''.

External links

See the original papers were by Christian Bauer, Sean Fleming, Michael Luke, Dan Pirjol, and Iain Stewart: * * * * *

References

Quantum chromodynamics {{particle-stub